Re: [PATCH] kvm: x86: do not leak guest xcr0 into host interrupt handlers

[David Matlack]

On Fri, Apr 22, 2016 at 12:30 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> Hi Paolo and David,
> 2016-03-31 3:24 GMT+08:00 David Matlack <dmatlack@xxxxxxxxxx>:
>>
>> kernel_fpu_begin() saves the current fpu context. If this uses
>> XSAVE[OPT], it may leave the xsave area in an undesirable state.
>> According to the SDM, during XSAVE bit i of XSTATE_BV is not modified
>> if bit i is 0 in xcr0. So it's possible that XSTATE_BV[i] == 1 and
>> xcr0[i] == 0 following an XSAVE.
>
> How XSAVE save bit i since SDM mentioned that "XSAVE saves state
> component i if and only if RFBM[i] = 1. "?  RFBM[i] will be 0 if
> XSTATE_BV[i] == 1 && guest xcr0[i] == 0.

You are correct, RFBM[i] will be 0 and XSAVE does not save state
component i in this case. However, XSTATE_BV[i] is left untouched by
XSAVE (left as 1). On XRSTOR, the CPU checks if XSTATE_BV[i] == 1 &&
xcr0[i] == 0, and if so delivers a #GP.

If you are wondering how XSTATE_BV[i] could be 1 in the first place, I
suspect it is left over from a previous XSAVE (which sets XSTATE_BV[i]
to the value in XINUSE[i]).

>
> Regards,
> Wanpeng Li
>
>>
>> kernel_fpu_end() restores the fpu context. Now if any bit i in
>> XSTATE_BV == 1 while xcr0[i] == 0, XRSTOR generates a #GP. The
>> fault is trapped and SIGSEGV is delivered to the current process.