A vulnerability was found in Linux Kernel (Operating System) (version unknown) and classified as problematic. This issue affects an unknown function of the component vm86 Syscall Handler. The manipulation as part of a FPU Exception leads to a denial of service vulnerability. Using CWE to declare the problem leads to CWE-404. The product does not release or incorrectly releases a resource before it is made available for re-use. Impacted is availability.

The bug was discovered 12/28/2013. The weakness was shared 12/28/2013 by halfdog as Vm86SyscallTaskSwitchKernelPanic as not defined advisory (Website). The advisory is shared at halfdog.net. The public release happened without involvement of Linux. An attack has to be approached locally. The successful exploitation requires a simple authentication. Technical details are unknown but a public exploit is available. The advisory points out:

The linux vm86 syscall allows to use the virtual-8086 mode from userspace, e.g. for emulating of old 8086 software as done with dosemu. When in virtual-8086-mode instructions work differently, e.g. 16 bit operand sizes, addressing as in real mode and interception of IO, interrupt calls and CPU exceptions. Usually those are handled by the software outside the virtual-8086 code to emulate the behavior of those instructions as if they were running on real hardware. As virtual-8086 mode might change eflags or FPU state, but some of these changes are problematic in protected mode, hence the kernel has to take care to permit not all of those and to revert them during task switching. It seems, that during task switching, unhandled FPU exceptions cause errors resulting in a kernel panic. Key player is the emms instruction, which will fault when e.g. cr0 has bits set due to unhandled errors.

A public exploit has been developed by halfdog in ANSI C and been published immediately after the advisory. The exploit is available at halfdog.net. It is declared as proof-of-concept. The code used by the exploit is:

socat TCP4-LISTEN:1234,reuseaddr=1,fork=1 EXEC:./Virtual86RandomCode,nofork=1

tee TestInput < /dev/urandom | socat - TCP4:x.x.x.x:1234 > ProcessedBlocks

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

License

• open-source

CPE 2.3info

• 🔍

CPE 2.2info

• 🔍

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