Re: Sanitize CPU-state when switching tasks (was sanitize CPU-state when switching from virtual-8086 mode to other task)

["H. Peter Anvin" <hpa@zytor.com>]

Adding Borislav.

Boris, do you happen to know of any erratum on AMD E-350 which may be
in play here?

	-hpa


On 01/07/2014 11:45 PM, halfdog wrote:
> Update to the issue:
> 
> * Although first observed with virtual-8086 mode, the bug is not 
> specific to virtual-8086 mode, it can be triggered with normal x86 
> userspace code also, even with better reproducibility.
> 
> * Ben Hutchings looked at the Debian bug report [1], he failed to 
> reproduce on his hardware, so it might be specific to some CPU
> models (currently my AMD E-350 is only machine known to be
> affected).
> 
> * When deactivating mmap_min_addr, the NULL-dereferences during 
> task-switch is exploitable, works both on native hardware and
> within VirtualBox. See [2] for POC to gain root privileges.
> 
> * It seems, that when changing the FPU control word with "fstcw"
> just before exit of the process, then another process could suffer
> when doing __do_switch, probably related to the xsave instruction
> and a x86 processor bug workaround, see "noxsave" switch [3]:
> [BUGS=X86] Disables x86 extended register state save and restore
> using xsave. The kernel will fallback to enabling legacy
> floating-point and sse state.
> 
> hd
> 
> [1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=733551 [2] 
> http://www.halfdog.net/Security/2013/Vm86SyscallTaskSwitchKernelPanic/
>
> 
[3] https://www.kernel.org/doc/Documentation/kernel-parameters.txt
> 
>