It was found that a malicious HVM guest administrator can cause DoS, specifically prevent use of physical CPU for significant, perhaps indefinite period. When a benign exception occurs while delivering another benign exception, it is architecturally specified that these would be delivered sequentially. There are, however, cases where this results in an infinite loop inside the CPU, which (in the virtualized case) can be broken only by intercepting delivery of the respective exception.
When a guest sets up a hardware breakpoint covering a data structure involved in delivering #DB (Debug Exception), upon completion of the delivery of the first exception another #DB will need to be delivered. The effects slightly differ depending on further guest characteristics:
Guests running in 32-bit mode would be expected to sooner or later encounter another fault due to the stack pointer decreasing during each iteration of the loop. The most likely case would be #PF (Page Fault) due to running into unmapped virtual space. However, an infinite loop cannot be excluded (e.g. when the guest is running with paging disabled).
Guests running in long mode, but not using the IST (Interrupt Stack Table) feature for the IDT entry corresponding to #DB would behave similarly to guests running in 32-bit mode, just that the larger virtual address space allows for a much longer loop. The loop can't, however, be infinite, as eventually the stack pointer would move into non-canonical address space, causing #SS (Stack Fault) instead.
Guests running in long mode and using the IST for the IDT entry corresponding to #DB would enter an infinite loop, as the stack pointer wouldn't change between #DB instances.
If a host watchdog (Xen or dom0) is in use, this can lead to a watchdog timeout and consequently a reboot of the host. If another, innocent, guest, is configured with a watchdog, this issue can lead to a reboot of such a guest.
A privileged user inside guest could use this flaw to crash the host kernel resulting in DoS.
For KVM virtualisation, it only affects the AMD processor support, as for Intel it already intercepts the #DB exception.
Upstream KVM patch: ------------------- -> http://permalink.gmane.org/gmane.linux.kernel/2082332
References: ----------- -> http://www.openwall.com/lists/oss-security/2015/11/10/1
The PV pagetable code in arch/x86/mm.c in Xen 4.7.x and earlier allows local 32-bit PV guest OS administrators to gain host OS privileges by leveraging fast-paths for updating pagetable entries.
Xen 4.6.x, 4.5.x, 4.4.x, 4.3.x, and earlier do not initialize x86 FPU stack and XMM registers when XSAVE/XRSTOR are not used to manage guest extended register state, which allows local guest domains to obtain sensitive information from other domains via unspecified vectors.
The PV superpage functionality in arch/x86/mm.c in Xen 3.4.0, 3.4.1, and 4.1.x through 4.6.x allows local PV guests to obtain sensitive information, cause a denial of service, gain privileges, or have unspecified other impact via a crafted page identifier (MFN) to the (1) MMUEXTMARKSUPER or (2) MMUEXTUNMARKSUPER sub-op in the HYPERVISORmmuextop hypercall or (3) unknown vectors related to page table updates.
The hvmmsrreadintercept function in arch/x86/hvm/hvm.c in Xen 4.1 through 4.4.x uses an improper MSR range for x2APIC emulation, which allows local HVM guests to cause a denial of service (host crash) or read data from the hypervisor or other guests via unspecified vectors.
Use-after-free vulnerability in Xen 4.2.x, 4.3.x, and 4.4.x allows remote domains to cause a denial of service (system crash) via a crafted hypercall during HVM guest teardown.
Xen 3.3.x through 4.5.x does not properly restrict access to PCI MSI mask bits, which allows local x86 HVM guest users to cause a denial of service (unexpected interrupt and host crash) via unspecified vectors.
The libxl toolstack library in Xen 4.1.x through 4.6.x does not properly release mappings of files used as kernels and initial ramdisks when managing multiple domains in the same process, which allows attackers to cause a denial of service (memory and disk consumption) by starting domains.
The x86 emulator in Xen 3.2.x through 4.5.x does not properly ignore segment overrides for instructions with register operands, which allows local guest users to obtain sensitive information, cause a denial of service (memory corruption), or possibly execute arbitrary code via unspecified vectors.
The modl2entry function in arch/x86/mm.c in Xen 3.4 through 4.6.x does not properly validate level 2 page table entries, which allows local PV guest administrators to gain privileges via a crafted superpage mapping.
The dommuupdate function in arch/x86/mm.c in Xen 3.2.x through 4.4.x does not properly manage page references, which allows remote domains to cause a denial of service by leveraging control over an HVM guest and a crafted MMUMACHPHYSUPDATE.
Xen 4.3.x, 4.4.x, and 4.5.x, when using toolstack disaggregation, allows remote domains with partial management control to cause a denial of service (host lock) via unspecified domctl operations.
Stack-based buffer overflow in the xl command line utility in Xen 4.1.x through 4.5.x allows local guest administrators to gain privileges via a long configuration argument.
The libxl device-handling in Xen 4.6.x and earlier allows local OS guest administrators to cause a denial of service (resource consumption or management facility confusion) or gain host OS privileges by manipulating information in guest controlled areas of xenstore.
The paginginvlpg function in include/asm-x86/paging.h in Xen 3.3.x through 4.6.x, when using shadow mode paging or nested virtualization is enabled, allows local HVM guest users to cause a denial of service (host crash) via a non-canonical guest address in an INVVPID instruction, which triggers a hypervisor bug check.
Race condition in HVMOPtrackdirtyvram in Xen 4.0.0 through 4.4.x does not ensure possession of the guarding lock for dirty video RAM tracking, which allows certain local guest domains to cause a denial of service via unspecified vectors.
The x86emulate function in arch/x86/x86emulate/x86emulate.c in Xen 4.4.x and earlier does not properly check supervisor mode permissions, which allows local HVM users to cause a denial of service (guest crash) or gain guest kernel mode privileges via vectors involving an (1) HLT, (2) LGDT, (3) LIDT, or (4) LMSW instruction.
Xen 4.2.x and 4.3.x, when nested virtualization is disabled, does not properly check the emulation paths for (1) VMLAUNCH and (2) VMRESUME, which allows local HVM guest users to cause a denial of service (host crash) via unspecified vectors related to "guest VMX instruction execution."
Memory leak in Xen 3.3 through 4.8.x allows guest OS users to cause a denial of service (ARM or x86 AMD host OS memory consumption) by continually rebooting, because certain cleanup is skipped if no pass-through device was ever assigned, aka XSA-207.
The dommuupdate function in arch/x86/mm.c in Xen 4.x through 4.4.x does not properly restrict updates to only PV page tables, which allows remote PV guests to cause a denial of service (NULL pointer dereference) by leveraging hardware emulation services for HVM guests using Hardware Assisted Paging (HAP).
The Ocaml xenstored implementation (oxenstored) in Xen 4.1.x, 4.2.x, and 4.3.x allows local guest domains to cause a denial of service (domain shutdown) via a large message reply.
The XENDOMCTLgetmemlist hypercall in Xen 3.4.x through 4.3.x (possibly 4.3.1) does not always obtain the pagealloclock and mmrwlock in the same order, which allows local guest administrators to cause a denial of service (host deadlock).
Xen 3.0.3 through 4.1.x (possibly 4.1.6.1), 4.2.x (possibly 4.2.3), and 4.3.x (possibly 4.3.1) does not properly prevent access to hypercalls, which allows local guest users to gain privileges via a crafted application running in ring 1 or 2.
It was found that a guest can DoS a host by triggering an infinite loop in microcode. If a guest in 32-bit mode enabled alignment exceptions, puts the exception handler in ring 3, and then triggers an alignment exception with an unaligned stack, then the microcode will enter an infinite loop. Because there's no instruction boundary the core never receives another interrupt (including SMIs). The host kernel panics pretty quickly due to the effects.
A privileged user inside guest could use this flaw to crash the host kernel resulting in DoS.
Upstream KVM patch: ------------------- -> http://permalink.gmane.org/gmane.linux.kernel/2082329
References: ----------- -> http://www.openwall.com/lists/oss-security/2015/11/10/1
QEMU, as used in Xen 3.3.x through 4.5.x, does not properly restrict access to PCI command registers, which might allow local HVM guest users to cause a denial of service (non-maskable interrupt and host crash) by disabling the (1) memory or (2) I/O decoding for a PCI Express device and then accessing the device, which triggers an Unsupported Request (UR) response.
The XENDOMCTLmemorymapping hypercall in Xen 3.2.x through 4.5.x, when using a PCI passthrough device, is not preemptible, which allows local x86 HVM domain users to cause a denial of service (host CPU consumption) via a crafted request to the device model (qemu-dm).
GNTTABOPswapgrantref in Xen 4.2 through 4.5 does not check the grant table operation version, which allows local guest domains to cause a denial of service (NULL pointer dereference) via a hypercall without a GNTTABOPsetuptable or GNTTABOPsetversion.
Xen 3.3.x through 4.5.x enables logging for PCI MSI-X pass-through error messages, which allows local x86 HVM guests to cause a denial of service (host disk consumption) via certain invalid operations.
The compatiret function in Xen 3.1 through 4.5 iterates the wrong way through a loop, which allows local 32-bit PV guest administrators to cause a denial of service (large loop and system hang) via a hypercalliret call with EFLAGS.VM set.
Xen 3.3.x through 4.5.x does not properly restrict write access to the host MSI message data field, which allows local x86 HVM guest administrators to cause a denial of service (host interrupt handling confusion) via vectors related to qemu and accessing spanning multiple fields.