Impact

Wasmtime's filesystem sandbox implementation on Windows blocks access to special device filenames such as "COM1", "COM2", "LPT0", "LPT1", and so on, however it did not block access to the special device filenames which use superscript digits, such as "COM¹", "COM²", "LPT⁰", "LPT¹", and so on. Untrusted Wasm programs that are given access to any filesystem directory could bypass the sandbox and access devices through those special device filenames with superscript digits, and through them gain access peripheral devices connected to the computer, or network resources mapped to those devices. This can include modems, printers, network printers, and any other device connected to a serial or parallel port, including emulated USB serial ports.

Patches

Patch releases for Wasmtime have been issued as 24.0.2, 25.0.3, and 26.0.1. Users of Wasmtime 23.0.x and prior are recommended to upgrade to one of these patched versions.

Workarounds

There are no known workarounds for this issue. Affected Windows users are recommended to upgrade.

References

- Microsoft's documentation of the special device filenames - ISO-8859-1 - The original PR reporting the issue

Impact

Wasmtime's code generator, Cranelift, has a bug on x8664 targets where address-mode computation mistakenly would calculate a 35-bit effective address instead of WebAssembly's defined 33-bit effective address. This bug means that, with default codegen settings, a wasm-controlled load/store operation could read/write addresses up to 35 bits away from the base of linear memory. Wasmtime's default sandbox settings provide up to 6G of protection from the base of linear memory to guarantee that any memory access in that range will be semantically correct. Due to this bug, however, addresses up to 0xffffffff 8 + 0x7ffffffc = 36507222004 = ~34G bytes away from the base of linear memory are possible from guest code. This means that the virtual memory 6G away from the base of linear memory up to ~34G away can be read/written by a malicious module.

This out of bounds read/write is not semantically correct and poses a threat as an arbitrary read/write within ~34G of linear memory away from the base of a wasm module's linear memory. A guest module can, without the knowledge of the embedder, read/write memory in this region. The memory may belong to other WebAssembly instances when using the pooling allocator, for example. The memory may also belong to the embedder, depending on address layout.

Embedders do not have a necessarily reliable means of detecting when this happens. Wasm loads/stores are allowed to cause machine segfaults meaning that an invalid read/write would be translated to a nominal WebAssembly trap. This means that a malicious module in the worst case silently reads/writes memory outside its bounds and in the "best" case looks like a normal "something trapped here" during its execution. This makes it difficult to retroactively determine whether this bug has been exploited on hosts. Affected embedders are recommended to analyze preexisting wasm modules to see if they're affected by the incorrect codegen rules and possibly correlate that with an anomalous number of traps during historical execution to locate possibly suspicious modules.

The specific bug in Cranelift's x8664 backend is that a WebAssembly address which is left-shifted by a constant amount from 1 to 3 will get folded into x8664's addressing modes which perform shifts. For example (i32.load (i32.shl (local.get 0) (i32.const 3))) loads from the WebAssembly address $local0 << 3. When translated to Cranelift the $local0 << 3 computation, a 32-bit value, is zero-extended to a 64-bit value and then added to the base address of linear memory. Cranelift would generate an instruction of the form movl (%base, %local0, 8), %dst which calculates %base + %local0 << 3. The bug here, however, is that the address computation happens with 64-bit values, where the $local0 << 3 computation was supposed to be truncated to a 32-bit value. This means that %local0, which can use up to 32-bits for an address, gets 3 extra bits of address space to be accessible via this movl instruction.

The fix in Cranelift is to remove the erroneous lowering rules in the backend which handle these zero-extended expressions. The above example is then translated to movl %local0, %temp; shl $3, %temp; movl (%base, %temp), %dst which correctly truncates the intermediate computation of %local0 << 3 to 32-bits inside the %temp register which is then added to the %base value.

Patches

Wasmtime version 4.0.1, 5.0.1, and 6.0.1 have been released and have all been patched to no longer contain the erroneous lowering rules.

Workarounds

While updating Wasmtime is recommended, there are a number of possible workarounds that embedders can employ to mitigate this issue if updating is not possible. Note that none of these workarounds are on-by-default and require explicit configuration:

The Config::staticmemorymaximumsize(0) option can be used to force all accesses to linear memory to be explicitly bounds-checked. This will perform a bounds check separately from the address-mode computation which correctly calculates the effective address of a load/store. Note that this can have a large impact on the execution performance of WebAssembly modules. The Config::staticmemoryguardsize(1 << 36) option can be used to greatly increase the guard pages placed after linear memory. This will guarantee that memory accesses up-to-34G away are guaranteed to be semantically correct by reserving unmapped memory for the instance. Note that this reserves a very large amount of virtual memory per-instances and can greatly reduce the maximum number of concurrent instances being run. If using a non-x8664 host is possible, then that will also work around this bug. This bug does not affect Wasmtime's or Cranelift's AArch64 backend, for example.

References

Config::staticmemorymaximumsize Config::staticmemoryguardsize Mailing list announcement GitHub advisory Commit to fix this issue on Wasmtime's main branch

For more information

If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

There is a use after free vulnerability in Wasmtime when both running Wasm that uses externrefs and enabling epoch interruption in Wasmtime. If you are not explicitly enabling epoch interruption (it is disabled by default) then you are not affected. If you are explicitly disabling the Wasm reference types proposal (it is enabled by default) then you are also not affected.

The use after free is caused by Cranelift failing to emit stack maps when there are safepoints inside cold blocks. Cold blocks occur when epoch interruption is enabled. Cold blocks are emitted at the end of compiled functions, and change the order blocks are emitted versus defined. This reordering accidentally caused Cranelift to skip emitting some stack maps because it expected to emit the stack maps in block definition order, rather than block emission order. When Wasmtime would eventually collect garbage, it would fail to find live references on the stack because of the missing stack maps, think that they were unreferenced garbage, and therefore reclaim them. Then after the collection ended, the Wasm code could use the reclaimed-too-early references, which is a use after free.

This bug was discovered while extending our fuzz targets for externrefs and GC in Wasmtime. The updated fuzz target thoroughly exercises these code paths and feature combinations now. We have also added a regression test for this bug. Released versions 0.34.2 and 0.35.2, which fix the vulnerability. We recommend all Wasmtime users upgrade to these patched versions. If upgrading is not an option for you at this time, you can avoid the vulnerability by either disabling the Wasm reference types proposal or by disabling epoch interruption if you were previously enabling it.

Impact

There is a bug in Wasmtime's C API implementation where the definition of the wasmtimetrapcode does not match its declared signature in the wasmtime/trap.h header file. This discrepancy causes the function implementation to perform a 4-byte write into a 1-byte buffer provided by the caller. This can lead to three zero bytes being written beyond the 1-byte location provided by the caller.

Patches

This bug has been patched and users should upgrade to Wasmtime 2.0.2.

Workarounds

This can be worked around by providing a 4-byte buffer casted to a 1-byte buffer when calling wasmtimetrapcode. Users of the wasmtime crate are not affected by this issue, only users of the C API function wasmtimetrapcode are affected.

References

Definition of wasmtimetrapcode Mailing list announcement Patch to fix for main branch

For more information

If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

Impact

Wasmtime's Cranelift compilation backend contains a bug on aarch64 when performing a certain shape of heap accesses which means that the wrong address is accessed. When combined with explicit bounds checks a guest WebAssembly module this can create a situation where there are two diverging computations for the same address: one for the address to bounds-check and one for the address to load. This difference in address being operated on means that a guest module can pass a bounds check but then load a different address. Combined together this enables an arbitrary read/write primitive for guest WebAssembly when accesssing host memory. This is a sandbox escape as guests are able to read/write arbitrary host memory.

This vulnerability has a few ingredients, all of which must be met, for this situation to occur and bypass the sandbox restrictions:

This miscompiled shape of load only occurs on 64-bit WebAssembly linear memories, or when Config::wasmmemory64 is enabled. 32-bit WebAssembly is not affected. Spectre mitigations or signals-based-traps must be disabled. When spectre mitigations are enabled then the offending shape of load is not generated. When signals-based-traps are disabled then spectre mitigations are also automatically disabled.

The specific bug in Cranelift is a miscompile of a load of the shape load(iadd(base, ishl(index, amt))) where amt is a constant. The amt value is masked incorrectly to test if it's a certain value, and this incorrect mask means that Cranelift can pattern-match this lowering rule during instruction selection erroneously, diverging from WebAssembly's and Cranelift's semantics. This incorrect lowering would, for example, load an address much further away than intended as the correct address's computation would have wrapped around to a smaller value insetad.

Patches

Wasmtime 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime.

Workarounds

This bug only affects users of Cranelift on aarch64. Cranelift on other platforms is not affected. Additionally this only affects 64-bit WebAssembly linear memories, so if Config::wasmmemory64 is disabled then hosts are not affected. Note that Config::wasmmemory64 is enabled by default. If spectre mitigations are enabled, which are enabled by default, then hosts are not affected by this issue.

Impact

Wasmtime with its Winch (baseline) non-default compiler backend may allow properly constructed guest Wasm to access host memory outside of its linear-memory sandbox.

This vulnerability requires use of the Winch compiler (-Ccompiler=winch). By default, Wasmtime uses its Cranelift backend, not Winch. With Winch, the same incorrect assumption is present in theory on both aarch64 and x86-64. The aarch64 case has an observed-working proof of concept, while the x86-64 case is theoretical and may not be reachable in practice.

This Winch compiler bug can allow the Wasm guest to access memory before or after the linear-memory region, independently of whether pre- or post-guard regions are configured. The accessible range in the initial bug proof-of-concept is up to 32KiB before the start of memory, or ~4GiB after the start of memory, independently of the size of pre- or post-guard regions or the use of explicit or guard-region-based bounds checking. However, the underlying bug assumes a 32-bit memory offset stored in a 64-bit register has its upper bits cleared when it may not, and so closely related variants of the initial proof-of-concept may be able to access truly arbitrary memory in-process. This could result in a host process segmentation fault (DoS), an arbitrary data leak from the host process, or with a write, potentially an arbitrary RCE.

Patches

Wasmtime 43.0.1, 42.0.2, and 36.0.7 have been released with fixes for this issue.

Workaround

There are no workarounds within the Winch compiler backend while using the affected versions. Users of Wasmtime are encouraged either to upgrade to patched versions or, if that is not possible, use the Cranelift compiler backend.

An issue in bytecodealliance wasm-micro-runtime before v.b3f728c and fixed in commit 06df58f allows a remote attacker to escalate privileges via a crafted file to the checkwasabicompatibility function.

Cranelift is an open-source code generator maintained by Bytecode Alliance. It translates a target-independent intermediate representation into executable machine code. There is a bug in 0.73 of the Cranelift x64 backend that can create a scenario that could result in a potential sandbox escape in a Wasm program. This bug was introduced in the new backend on 2020-09-08 and first included in a release on 2020-09-30, but the new backend was not the default prior to 0.73. The recently-released version 0.73 with default settings, and prior versions with an explicit build flag to select the new backend, are vulnerable. The bug in question performs a sign-extend instead of a zero-extend on a value loaded from the stack, under a specific set of circumstances. If those circumstances occur, the bug could allow access to memory addresses upto 2GiB before the start of the Wasm program heap. If the heap bound is larger than 2GiB, then it would be possible to read memory from a computable range dependent on the size of the heaps bound. The impact of this bug is highly dependent on heap implementation, specifically: if the heap has bounds checks, and does not rely exclusively on guard pages, and the heap bound is 2GiB or smaller then this bug cannot be used to reach memory from another Wasm program heap. The impact of the vulnerability is mitigated if there is no memory mapped in the range accessible using this bug, for example, if there is a 2 GiB guard region before the Wasm program heap. The bug in question performs a sign-extend instead of a zero-extend on a value loaded from the stack, when the register allocator reloads a spilled integer value narrower than 64 bits. This interacts poorly with another optimization: the instruction selector elides a 32-to-64-bit zero-extend operator when we know that an instruction producing a 32-bit value actually zeros the upper 32 bits of its destination register. Hence, we rely on these zeroed bits, but the type of the value is still i32, and the spill/reload reconstitutes those bits as the sign extension of the i32’s MSB. The issue would thus occur when: An i32 value in a Wasm program is greater than or equal to 0x80000000; The value is spilled and reloaded by the register allocator due to high register pressure in the program between the value’s definition and its use; The value is produced by an instruction that we know to be “special” in that it zeroes the upper 32 bits of its destination: add, sub, mul, and, or; The value is then zero-extended to 64 bits in the Wasm program; The resulting 64-bit value is used. Under these circumstances there is a potential sandbox escape when the i32 value is a pointer. The usual code emitted for heap accesses zero-extends the Wasm heap address, adds it to a 64-bit heap base, and accesses the resulting address. If the zero-extend becomes a sign-extend, the program could reach backward and access memory up to 2GiB before the start of its heap. In addition to assessing the nature of the code generation bug in Cranelift, we have also determined that under specific circumstances, both Lucet and Wasmtime using this version of Cranelift may be exploitable. See referenced GitHub Advisory for more details.

There is a bug in Wasmtime's code generator, Cranelift, where functions using reference types may be incorrectly missing metadata required for runtime garbage collection (GC). This means that if a GC happens at runtime then the collector will mistakenly think some Wasm stack frames do not have live references to garbage collected values and therefore reclaim and deallocate them. The function can then subsequently continue to use the values, leading later to use-after-free bugs. This bug was introduced in Cranelift's migration to the regalloc2 register allocator in the Wasmtime 0.37.0 release on 2022-05-20. This bug has been patched and users should upgrade to Wasmtime version 0.38.2.

Mitigations for this issue can be achieved by doing one of:

Disabling the reference types proposal by passing false to wasmtime::Config::wasmreferencetypes. Downgrading to Wasmtime 0.36.0 or prior.

Impact

Wasmtime's implementation of managing per-instance state, such as tables and memories, contains LLVM-level undefined behavior. This undefined behavior was found to cause runtime-level issues when compiled with LLVM 16 which causes some writes, which are critical for correctness, to be optimized away. Vulnerable versions of Wasmtime compiled with Rust 1.70, which is currently in beta, or later are known to have incorrectly compiled functions. Versions of Wasmtime compiled with the current Rust stable release, 1.69, and prior are not known at this time to have any issues, but can theoretically exhibit potential issues.

The underlying problem is that Wasmtime's runtime state for an instance involves a Rust-defined structure called Instance which has a trailing VMContext structure after it. This VMContext structure has a runtime-defined layout that is unique per-module. This representation cannot be expressed with safe code in Rust so unsafe code is required to maintain this state. The code doing this, however, has methods which take &self as an argument but modify data in the VMContext part of the allocation. This means that pointers derived from &self are mutated. This is typically not allowed, except in the presence of UnsafeCell, in Rust. When compiled to LLVM these functions have noalias readonly parameters which means it's UB to write through the pointers.

Wasmtime's internal representation and management of VMContext has been updated to use &mut self methods where appropriate. Additionally verification tools for unsafe code in Rust, such as cargo miri, are planned to be executed on the main branch soon to fix any Rust-level issues that may be exploited in future compiler versions.

Precomplied binaries available for Wasmtime from GitHub releases have been compiled with at most LLVM 15 so are not known to be vulnerable. As mentioned above, however, it's still recommended to update.

Patches

Wasmtime version 6.0.2, 7.0.1, and 8.0.1 have been issued which contain the patch necessary to work correctly on LLVM 16 and have no known UB on LLVM 15 and earlier.

Workarounds

If Wasmtime is compiled with Rust 1.69 and prior, which use LLVM 15, then there are no known issues. There is a theoretical possibility for UB to exploited, however, so it's recommended that users upgrade to a patched version of Wasmtime. Users using beta Rust (1.70 at this time) or nightly Rust (1.71 at this time) must update to a patched version to work correctly.

References

GitHub Advisory Mailing list announcement

For more information

If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

Impact

There is a bug in Wasmtime's implementation of it's pooling instance allocator where when a linear memory is reused for another instance the initial heap snapshot of the prior instance can be visible, erroneously to the next instance. The pooling instance allocator in Wasmtime works by preallocating virtual memory for a fixed number of instances to reside in and then new instantiations pick a slot to use. Most conventional modules additionally have an initial copy-on-write "heap image" which is mapped in Wasmtime into the linear memory slot. When a heap slot is deallocated Wasmtime resets all of its contents back to the initial state but it does not unmap the image in case the next instance is an instantiation of the same module.

The bug in Wasmtime occurs when a slot in the pooling allocator previously was used for a module with a heap image, meaning that its current state of memory contains the initial heap contents of that module. If the next instantiation within that slot does not itself contain a heap image then Wasmtime would leave the old heap image in place erroneously and continue with instantiation. This means that instantiations of modules without a heap image can see the initial heap image of the prior instantiation within that slot.

Heap images in Wasmtime are created by precomputing WebAssembly data segments into one large mapping to be placed into linear memory at a particular offset. Most modules produced by toolchains today will have a heap image and an initialization snapshot. Creating a module without a heap image would require a hand-crafted .wat file or a specially crafted source program. This consequence means that this bug is highly unlikely to be accidentally triggered and would otherwise require an intentional trigger with a hand-crafted module.

One important part of this vulnerability is Wasmtime is highly likely to segfault when the slot is reused again with a module that itself has an initialization image. For example if module A has a heap initialization image and module B does not have a heap initialization image, then the following sequence of events could happen if they all are instantiated into the same instance slot:

Module A is instantiated, and then deallocated. This leaves A's heap image in place, reset to its initial contents. Module B is instantiated and erroneously can see the initial heap contents of A. Module B is then deallocated and the entire heap is unmapped and reset back to zero. Module A is instantiated again, but the state tracking the slot did not account for module B so it thinks the module image is still mapped and proceeds with instantiation. Any action on A's part to access linear memory will then trap and if the host accesses A's memory it will segfault because the data that's supposed to be mapped is all unmapped.

Adding this all together this means that in practice modules must be deliberately crafted to not have an initial heap image to view the contents of a prior image. If this module is instantiated though then when the slot is reused the next, likely image-using, module will believe its memory is mapped when it isn't, causing the host to segfault on unmapped memory it believed was mapped.

Patches

This bug has been patched and users should upgrade to Wasmtime 2.0.2.

Workarounds

Trigging this bug requires the pooling allocator to be configured and for copy-on-write heap images to also be enabled. Pooling allocation is not enabled by default but copy-on-write heap images are. Mitigations for this bug include:

Disabling the pooling allocator - note that pooling allocation is not enabled by default in Wasmtime Disabling the memory-init-cow feature or with Config::memoryinitcow

References

Config::allocationstrategy - configuration required to enable the pooling allocator. Config::memoryinitcow - configuration required to enable or disable copy-on-write (this is enabled by default). Mailing list announcement Patch for release-2.0.0 branch Patch for main

For more information

If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

Lucet is a native WebAssembly compiler and runtime. There is a bug in the main branch of lucet-runtime affecting all versions published to crates.io that allows a use-after-free in an Instance object that could result in memory corruption, data race, or other related issues. This bug was introduced early in the development of Lucet and is present in all releases. As a result of this bug, and dependent on the memory backing for the Instance objects, it is possible to trigger a use-after-free when the Instance is dropped. Users should upgrade to the main branch of the Lucet repository. Lucet no longer provides versioned releases on crates.io. There is no way to remediate this vulnerability without upgrading.

Impact

There exists a bug in the pooling instance allocator in Wasmtime's runtime where a failure to instantiate an instance for a module that defines an externref global will result in an invalid drop of a VMExternRef via an uninitialized pointer.

As instance slots may be reused between consecutive instantiations, the value of the uninitialized pointer may be from a previous instantiation and therefore under the control of an attacker via a module's initial values for its globals. If the attacker can somehow determine an address under their control inside the mapped memory representing the instance pool, it is possible to trick the runtime to call dropinplace on a trait object under the attacker's control and therefore cause remote code execution.

Exploiting the bug to cause remote code execution would be very difficult as attackers cannot determine the addresses of globals from code executing within the WebAssembly VM and the memory space for the instance pool cannot be statically determined. Operating system mitigations, such as address space layout randomization, would additionally increase the difficulty for attackers to determine useful executable code to target with an exploit. It is also very unlikely that attackers will be able to directly influence the conditions that trigger the bug as described below.

When the conditions to trigger the bug are met, however, it is much easier to exploit this bug to cause a denial of service by crashing the host with an invalid memory read.

The following engine configuration (via Config) is required to be impacted by this bug:

support for the reference types proposal must be enabled (this is the default for Config). a pooling allocation strategy must be configured via Config::allocationstrategy, which is not the default allocation strategy.

A module must be instantiated with all the following characteristics:

The module defines at least one table or memory. The module defines at least one externref global.

During instantiation, one of the following must occur to cause the instantiation to fail:

a call to mprotect or VirtualAlloc fails (e.g. out-of-memory conditions). a resource limiter was configured in the associated Store (via Store::limiter or Store::limiterasync) and the limiter returns false from the initial call to memorygrowing or tablegrowing. Stores do not have a resource limiter set by default.

This results in a partially-initialized instance being dropped and that attempts to drop the uninitialized VMExternRef representing the defined externref global.

We have reason to believe that the effective impact of this bug is relatively small because the usage of externref is still uncommon and without a resource limiter configured on the Store, which is not the default configuration, it is only possible to trigger the bug from an error returned by mprotect or VirtualAlloc.

Note that on Linux with the uffd feature enabled, it is only possible to trigger the bug from a resource limiter as the call to mprotect is skipped; if no resource limiter is used, then this configuration is not vulnerable.

Patches

The bug has been fixed in 0.34.1 and 0.33.1; users are encouraged to upgrade as soon as possible.

Workarounds

If it is not possible to upgrade to 0.34.1 or 0.33.1 of the wasmtime crate, it is recommend that support for the reference types proposal be disabled by passing false to Config::wasmreferencetypes.

Doing so will prevent modules that use externref from being loaded entirely.

For more information

If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

An out-of-bound memory read vulnerability was discovered in Bytecode Alliance wasm-micro-runtime v2.0.0 which allows a remote attacker to cause a denial of service via the "blocktypegetarity" function in core/iwasm/interpreter/wasm.h.

wasm-micro-runtime (aka WebAssembly Micro Runtime or WAMR) 06df58f is vulnerable to NULL Pointer Dereference in function blocktypegetresulttypes.

The affected versions of Wasmtime can panic if the host embedder drops the future returned by wasmtime::component::[Typed]Func::callasync before it resolves.

Details

Starting with Wasmtime 39.0.0, the component-model-async feature became the default, which brought with it a new implementation of [Typed]Func::callasync which made it capable of calling async-typed guest export functions. However, that implementation had a bug leading to a panic under certain circumstances:

1. The host embedding calls [Typed]Func::callasync on a function exported by a component, polling the returned Future once. 2. The component function yields control to the async runtime (e.g. Tokio), e.g. due to a call to host function registered using LinkerInstance::funcwrapasync which yields, or due an epoch interruption. 3. The host embedding drops the Future after polling it once. This leaves the component instance in a non-reenterable state since the call never had a chance to complete. 4. The host embedding calls [Typed]Func::callasync again, polling the returned Future. Since the component instance cannot be entered at this point, the call traps, but not before allocating a task and thread for the call. 5. The host embedding ignores the trap and drops the Future. This panics due to the runtime attempting to dispose of the task created above, which panics since the thread has not yet exited.

Impact When a host embedder using the affected versions of Wasmtime calls wasmtime::component::[Typed]Func::callasync on a guest export and then drops the returned future without waiting for it to resolve, and then does so again with the same component instance, Wasmtime will panic. Embeddings that have the component-model-async compile-time feature disabled are unaffected.

Patches Wasmtime 40.0.4 and 41.0.4 have been patched to fix this issue. Versions 42.0.0 and later are not affected.

Workarounds If an embedding is not actually using any component-model-async features then disabling the component-model-async Cargo feature can work around this issue. This issue can also be worked around by either ensuring every callasync future is awaited until it completes or refraining from using the Store again after dropping a not-yet-resolved callasync future.

Resources This was first reported in https://bytecodealliance.zulipchat.com/#narrow/channel/206238-general/topic/Panic.20in.20Wasmtime.2041.2E0.2E3.20.28runtime.2Fconcurrent.2Fcomponent.29

Impact

Wasmtime's implementation of the wasi:http/types.fields resource is susceptible to panics when too many fields are added to the set of headers. Wasmtime's implementation in the wasmtime-wasi-http crate is backed by a data structure which panics when it reaches excessive capacity and this condition was not handled gracefully in Wasmtime. Panicking in a WASI implementation is a Denial of Service vector for embedders and is treated as a security vulnerability in Wasmtime.

Patches

Wasmtime 24.0.6, 36.0.6, 40.0.4, 41.0.4, and 42.0.0 patch this vulnerability and return a trap to the guest instead of panicking.

Workarounds

There are no known workarounds at this time, embedders are encouraged to update to a patched version of Wasmtime.

Resources

Limitations of http::HeaderMap

Summary

In wasmtime-wasi, when a filesystem preopen is given DirPerms::all() and FilePerms::READ without FilePerms::WRITE, this wasmtime-wasi enforced access control mechanism can be bypassed by using the wasip2 descriptor.open-at or wasip1 pathopen interfaces by opening a file with OpenFlags::TRUNCATE oflag only, for example:

rust dirdescriptor.openat( PathFlags::empty(), FILENAME, OpenFlags::TRUNCATE, DescriptorFlags::READ, )

rust wasip1::pathopen( dirfd, 0, FILENAME, wasip1::OFLAGSTRUNC, wasip1::RIGHTSFDREAD, 0, 0 )

The root cause is that the clause that considered OpenFlags::TRUNCATE did not set openmode |= OpenMode::WRITE;, used later in that function for the access control check against FilePerms for whether opening that file is permitted. With the bug corrected, these calls to open-at and pathopen fail with error-code.not-permitted and ERRNOPERM respectively.

The bug in crates/wasi/src/filesystem.rs, Dir::openat, lines 967–969:

rust if oflags.contains(OpenFlags::TRUNCATE) { opts.truncate(true).write(true); } and the single line fix is: rust if oflags.contains(OpenFlags::TRUNCATE) { opts.truncate(true).write(true); openmode |= OpenMode::WRITE; }

Only wasmtime-wasi embeddings that use a combination of DirPerms::MUTATE with FilePerms::READ are affected by this bug, e.g. those that use in the WasiCtxBuilder: rust builder.preopeneddir("readonly", "readonly", DirPerms::READ | DirPerms::MUTATE, FilePerms::READ);

In particular, the Wasmtime project's wasmtime-cli's use of wasmtime-wasi is not affected, because it always sets FilePerms::all() for all preopens.

Impact

There was a bug in Wasmtime's code generator, Cranelift, for AArch64 targets where constant divisors could result in incorrect division results at runtime. The translation rules for constants did not take into account whether sign- or zero-extension should happen, which resulted in an incorrect value being placed into a register when a division was encountered. For example, a constant 32-bit unsigned divisor of 0xfffffffe would be incorrectly sign-extended to 64-bits to 0xfffffffffffffffe. Any kind of division of operands smaller than 64 bits is implemented with a 64-bit division instruction which would then result in an incorrect result because the divisor was larger than expected.

The impact of this bug is that programs executing within the WebAssembly sandbox would not behave according to the WebAssembly specification. This means that it is hypothetically possible for execution within the sandbox to go awry and WebAssembly programs could produce unexpected results. This should not impact hosts executing WebAssembly, but does affect the correctness of guest programs.

This bug was found with differential fuzzing of Wasmtime against other engines on the AArch64 platform. Fuzzing on AArch64 is not regularly performed at this time and the Wasmtime team is investigating how best to continuously fuzz AArch64 in the same manner as x8664.

Patches

This bug has been patched and users should upgrade to Wasmtime version 0.38.2.

Workarounds

If upgrading is not an option at this time, direct users of Cranelift that control the exact Cranelift instructions being compiled can avoid the vulnerability by explicitly extending constant divisors to 64 bits using either the sextend.i64 or the uextend.i64 operation.

Note, though, that this issue only affects the AArch64 targets. Other platforms are not affected.

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If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

An heap overflow vulnerability was discovered in Bytecode alliance wasm-micro-runtime v.1.2.3 allows a remote attacker to cause a denial of service via the wasmloaderpreparebytecode function in core/iwasm/interpreter/wasmloader.c.

WebAssembly Micro Runtime (WAMR) is a lightweight standalone WebAssembly (Wasm) runtime. Prior to version 2.4.4, an out-of-bounds array access issue exists in WAMR's fast interpreter mode during WASM bytecode loading. When framerefbottom and frameoffsetbottom arrays are at capacity and a GETGLOBAL(I32) opcode is encountered, framerefbottom is expanded but frameoffsetbottom may not be. If this is immediately followed by an if opcode that triggers preservelocalforblock, the function traverses arrays using stackcellnum as the upper bound, causing out-of-bounds access to frameoffsetbottom since it wasn't expanded to match the increased stackcellnum. This issue has been patched in version 2.4.4.

Impact

There is a bug in Wasmtime's implementation of its pooling instance allocator when the allocator is configured to give WebAssembly instances a maximum of zero pages of memory. In this configuration the virtual memory mapping for WebAssembly memories did not meet the compiler-required configuration requirements for safely executing WebAssembly modules. Wasmtime's default settings require virtual memory page faults to indicate that wasm reads/writes are out-of-bounds, but the pooling allocator's configuration would not create an appropriate virtual memory mapping for this meaning out of bounds reads/writes can successfully read/write memory unrelated to the wasm sandbox within range of the base address of the memory mapping created by the pooling allocator.

This bug can only be triggered by setting InstanceLimits::memorypages to zero. This is expected to be a very rare configuration since this means that wasm modules cannot allocate any pages of linear memory. All wasm modules produced by all current toolchains are highly likely to use linear memory, so it's expected to be unlikely that this configuration is set to zero by any production embedding of Wasmtime, hence the low severity of this bug despite the critical consequences.

Patches

This bug has been patched and users should upgrade to Wasmtime 2.0.2.

Workarounds

One way to mitigate this issue is to disable usage of the pooling allocator. Note that the pooling allocator is not enabled by default.

This bug can also only be worked around by increasing the memorypages allotment when configuring the pooling allocator to a value greater than zero. If an embedding wishes to still prevent memory from actually being used then the Store::limiter method can be used to dynamically disallow growth of memory beyond 0 bytes large. Note that the default memorypages value is greater than zero.

This bug is not applicable with the default settings of the wasmtime crate.

References

Config::allocationstrategy - configuration required to enable the pooling allocator. InstanceLimits::memorypages - configuration field that, when zero, exhibits this bug. Store::limiter - means of limiting memory without using memorypages Mailing list announcement Patch for the release-2.0.0 branch

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The WebAssembly Micro Runtime's (WAMR) iwasm package is the executable binary built with WAMR VMcore which supports WebAssembly System Interface (WASI) and command line interface. Anyone running WAMR up to and including version 2.2.0 or WAMR built with libc-uvwasi on Windows is affected by a symlink following vulnerability. On WAMR running in Windows, creating a symlink pointing outside of the preopened directory and subsequently opening it with create flag will create a file on host outside of the sandbox. If the symlink points to an existing host file, it's also possible to open it and read its content. Version 2.3.0 fixes the issue.

The WebAssembly Micro Runtime's (WAMR) iwasm package is the executable binary built with WAMR VMcore which supports WebAssembly System Interface (WASI) and command line interface. In versions 2.4.0 and below, iwasm uses --addr-pool with an IPv4 address that lacks a subnet mask, allowing the system to accept all IP addresses. This can unintentionally expose the service to all incoming connections and bypass intended access restrictions. Services relying on --addr-pool for restricting access by IP may unintentionally become open to all external connections. This may lead to unauthorized access in production deployments, especially when users assume that specifying an IP without a subnet mask implies a default secure configuration. This is fixed in version 2.4.1.

Impact

Wasmtime's implementation of WASI host interfaces are susceptible to guest-controlled resource exhaustion on the host. Wasmtime did not appropriately place limits on resource allocations requested by the guests. This serves as a Denial of Service vector where a guest can induce a range of crashing behaviors on the host such as:

Allocating arbitrarily large amounts of host memory. Causing an allocation failure on the host, which in Rust defaults to aborting the process. Causing a panic on the host due to over-large allocations being performed. Cause degredation in performance of the host by holding excessive host memory alive.

Wasmtime's security bug policy considers all of these behaviors a security vulnerability. Wasmtime's implementation of WASI has a number of different ways that resource exhaustion could happen, and fixing any one of them is insufficient from solving this vulnerability. A number of individual issues are grouped within this advisory and as a whole represent the known ways that guests can exhaust resources on the host.

An example of guest-controlled resource exhaustion within Wasmtime's implementation of WASI is guests could repeatedly allocate handles to themselves without limit. Some APIs also caused the host to perform a guest-controlled-sized allocation of a buffer on the host for I/O operations. Other APIs could force the host to buffer arbitrary amounts of data for the guest. Finally the guest could hand arbitrarily large allocations from itself to the host which could cause the host to perform an arbitrarily sized copy of memory which in some situations could result in quadratically sized allocations.

Wasmtime's implementations of WASIp1 and WASIp2 are affected by this vulnerability. Any host API modeled with the Component Model (or WIT) which operates on a string or list<T> type is also affected. Not all WIT and WASI APIs are affected by this issue, but that's more of an exception so it's recommended for all embedders to consider themselves affected.

To address this issue a number of mitigations are being applied to limit the behavior of a guest in WASI. All of these mitigations manifest in the form of a limit of some kind applied to various situations, and as such all of these mitigations are backwards-incompatible as they run the risk of breaking preexisting programs. To address this all backports to previous stable releases have these limits tuned to overly-large values. This ensures that preexisting guests do not break while still providing embedders the knobs to prevent this DoS vector as well. The limits added to Wasmtime are:

-Smax-resources=N or ResourceTable::setmaxcapacity - the maximum number of resources that a guest is allowed to allocate for itself. -Shostcall-fuel=N or Store::sethostcallfuel - the maximum amount of data that the guest may copy to the host in a single function call. -Smax-random-size=N or WasiCtxBuilder::maxrandomsize - the maximum size of the return value of get-random-bytes and get-insecure-random-bytes in the wasi:random implementations. -Smax-http-fields-size=N or WasiHttpCtx::setmaxfieldssize - the maximum size of headers for an HTTP request/response.

These settings are equally applicable to both WASIp1 and WASIp2. Wasmtime 41.0.x and prior previously did not limit these settings and the knobs being released are set to very large values by default to avoid any breaking behavior. Embedders will need to proactively tune these knobs as appropriate for their embeddings. The default settings in the unreleased Wasmtime 42.0.0 are 1M for max resources, 128MiB for hostcall fuel, 64MiB for max-random-size, and 32KiB for http fields size. Tuning is not expected for Wasmtime 42.0.0+.

Hosts/embedders affected by this issue are encouraged to audit and double-check their own host APIs they have implemented to see whether they are affected by this issue as well. The -Shostcall-fuel setting is intended to be a relatively coarse fix for many possible issues by limiting the amount of data for all host APIs at once, so many embedders may not need to take further action beyond updating Wasmtime and configuring it appropriately (if not updating to 42.0.0). Embedders should audit to see, however, if the guest is able to force the host to allocate on its behalf and ensure that the allocation is limited or tracked somehow.

Patches

Wasmtime 24.0.6, 36.0.6, 40.0.4, 41.0.4, and 42.0.0 have all been released with the fix for this issue. These versions do not prevent this issue in their default configuration to avoid breaking preexisting behaviors. All versions of Wasmtime have appropriate knobs to prevent this behavior, and Wasmtime 42.0.0-and-later will have these knobs tuned by default to prevent this issue from happening.

Workarounds

There are no known workarounds for this issue without upgrading. Embedders are recommended to upgrade and configure their embeddings as necessary to prevent possibly-malicious guests from triggering this issue.

Resources

Store::sethostcallfuel ResourceTable::setmaxcapacity WasiCtxBuilder::maxrandomsize Original PR showing resource exhaustion Issue about limiting max resource handles per-guest

Summary

Wasmtime contains a vulnerability where when transcoding a UTF-16 string to the latin1+utf16 component-model encoding it would incorrectly validate the byte length of the input string when performing a bounds check. Specifically the number of code units were checked instead of the byte length, which is twice the size of the code units.

This vulnerability can cause the host to read beyond the end of a WebAssembly's linear memory in an attempt to transcode nonexistent bytes. In Wasmtime's default configuration this will read unmapped memory on a guard page, terminating the process with a segfault. Wasmtime can be configured, however, without guard pages which would mean that host memory beyond the end of linear memory may be read and interpreted as UTF-16.

A host segfault is a denial-of-service vulnerability in Wasmtime, and possibly being able to read beyond the end of linear memory is additionally a vulnerability. Note that reading beyond the end of linear memory requires nonstandard configuration of Wasmtime, specifically with guard pages disabled.

Impact

This is an out-of-bounds memory access. Any user running untrusted wasm components that use cross-component string passing (with UTF-16 source and latin1+utf16 destination encodings) is affected.

- With guard pages: Denial of service. The host process crashes with SIGBUS/SIGSEGV. - Without guard pages: Potential information disclosure. The guest can read host memory beyond its linear memory allocation.

Patches

Wasmtime 24.0.7, 36.0.7, 42.0.2, and 43.0.1 have been issued to fix this bug. Users are recommended to update to these patched versions of Wasmtime. Workarounds

There is no workaround for this bug. Hosts are recommended to updated to a patched version of Wasmtime.

Impact

Wasmtime's implementation of the SIMD proposal for WebAssembly on x8664 contained two distinct bugs in the instruction lowerings implemented in Cranelift. The aarch64 implementation of the simd proposal is not affected. The bugs were presented in the i8x16.swizzle and select WebAssembly instructions. The select instruction is only affected when the inputs are of v128 type. The correspondingly affected Cranelift instructions were swizzle and select.

The swizzle instruction lowering in Cranelift erroneously overwrote the mask input register which could corrupt a constant value, for example. This means that future uses of the same constant may see a different value than the constant itself.

The select instruction lowering in Cranelift wasn't correctly implemented for vector types that are 128-bits wide. When the condition was 0 the wrong instruction was used to move the correct input to the output of the instruction meaning that only the low 32 bits were moved and the upper 96 bits of the result were left as whatever the register previously contained (instead of the input being moved from). The select instruction worked correctly if the condition was nonzero, however.

This bug in Wasmtime's implementation of these instructions on x8664 represents an incorrect implementation of the specified semantics of these instructions according to the WebAssembly specification. The impact of this is benign for hosts running WebAssembly but represents possible vulnerabilities within the execution of a guest program. For example a WebAssembly program could take unintended branches or materialize incorrect values internally which runs the risk of exposing the program itself to other related vulnerabilities which can occur from miscompilations.

Patches

We have released Wasmtime 0.38.1 and cranelift-codegen (and other associated cranelift crates) 0.85.1 which contain the corrected implementations of these two instructions in Cranelift.

Workarounds

If upgrading is not an option for you at this time, you can avoid the vulnerability by disabling the Wasm simd proposal

rust config.wasmsimd(false);

Additionally the bug is only present on x8664 hosts. Other aarch64 hosts are not affected. Note that s390x hosts don't yet implement the simd proposal and are not affected.

References

The WebAssembly simd proposal Original test case showing the erroneous behavior Fix for the swizzle instruction Fix for the select instruction

For more information

If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

Impact

As a Rust library the wasmtime crate clearly marks which functions are safe and which are unsafe, guaranteeing that if consumers never use unsafe then it should not be possible to have memory unsafety issues in their embeddings of Wasmtime. An issue was discovered in the safe API of Linker::func APIs. These APIs were previously not sound when one Engine was used to create the Linker and then a different Engine was used to create a Store and then the Linker was used to instantiate a module into that Store. Cross-Engine usage of functions is not supported in Wasmtime and this can result in type confusion of function pointers, resulting in being able to safely call a function with the wrong type.

Triggering this bug requires using at least two Engine values in an embedding and then additionally using two different values with a Linker (one at the creation time of the Linker and another when instantiating a module with the Linker).

It's expected that usage of more-than-one Engine in an embedding is relatively rare since an Engine is intended to be a globally shared resource, so the expectation is that the impact of this issue is relatively small.

The fix implemented is to change this behavior to panic!() in Rust instead of silently allowing it. Using different Engine instances with a Linker is a programmer bug that wasmtime catches at runtime.

Patches

This bug has been patched and users should upgrade to Wasmtime version 0.30.0.

Workarounds

If you cannot upgrade Wasmtime and are using more than one Engine in your embedding it's recommended to instead use only one Engine for the entire program if possible. An Engine is designed to be a globally shared resource that is suitable to have only one for the lifetime of an entire process. If using multiple Engines is required then code should be audited to ensure that Linker is only used with one Engine.

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If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

Impact

There was a use-after-free bug when passing externrefs from the host to guest Wasm content.

To trigger the bug, you have to explicitly pass multiple externrefs from the host to a Wasm instance at the same time, either by

passing multiple externrefs as arguments from host code to a Wasm function, or returning multiple externrefs to Wasm from a multi-value return function defined in the host. If you do not have host code that matches one of these shapes, then you are not impacted.

If Wasmtime's VMExternRefActivationsTable became filled to capacity after passing the first externref in, then passing in the second externref could trigger a garbage collection. However the first externref is not rooted until we pass control to Wasm, and therefore could be reclaimed by the collector if nothing else was holding a reference to it or otherwise keeping it alive. Then, when control was passed to Wasm after the garbage collection, Wasm could use the first externref, which at this point has already been freed.

We have reason to believe that the effective impact of this bug is relatively small because usage of externref is currently quite rare.

Patches

The bug has been fixed, and users should upgrade to Wasmtime 0.30.0.

Additionally, we have updated our primary externref fuzz target such that it better exercises these code paths and we can have greater confidence in their correctness going forward.

Workarounds

If you cannot upgrade Wasmtime yet, you can avoid the bug by disabling reference types support in Wasmtime by passing false to wasmtime::Config::wasmreferencetypes.

References

The reference types Wasm proposal, which introduces externref

For more information

If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository

Impact

There was an invalid free and out-of-bounds read and write bug when running Wasm that uses externrefs in Wasmtime.

To trigger this bug, Wasmtime needs to be running Wasm that uses externrefs, the host creates non-null externrefs, Wasmtime performs a garbage collection (GC), and there has to be a Wasm frame on the stack that is at a GC safepoint where

there are no live references at this safepoint, and there is a safepoint with live references earlier in this frame's function.

Under this scenario, Wasmtime would incorrectly use the GC stack map for the safepoint from earlier in the function instead of the empty safepoint. This would result in Wasmtime treating arbitrary stack slots as externrefs that needed to be rooted for GC. At the next GC, it would be determined that nothing was referencing these bogus externrefs (because nothing could ever reference them, because they are not really externrefs) and then Wasmtime would deallocate them and run <ExternRef as Drop>::drop on them. This results in a free of memory that is not necessarily on the heap (and shouldn't be freed at this moment even if it was), as well as potential out-of-bounds reads and writes.

Even though support for externrefs (via the reference types proposal) is enabled by default, unless you are creating non-null externrefs in your host code or explicitly triggering GCs, you cannot be affected by this bug.

We have reason to believe that the effective impact of this bug is relatively small because usage of externref is currently quite rare.

Patches

This bug has been patched and users should upgrade to Wasmtime version 0.30.0.

Additionally, we have updated our primary externref fuzz target such that it better exercises these code paths and we can have greater confidence in their correctness going forward.

Workarounds

If you cannot upgrade Wasmtime at this time, you can avoid this bug by disabling the reference types proposal by passing false to wasmtime::Config::wasmreferencetypes

References

The Wasm reference types proposal, which introduces externref

For more information

If you have any questions or comments about this advisory:

Reach out to us on the Bytecode Alliance Zulip chat Open an issue in the bytecodealliance/wasmtime repository