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

[Wanpeng Li]

2016-04-23 1:21 GMT+08:00 David Matlack <dmatlack@xxxxxxxxxx>:
> 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.

However, SDM also mentioned that "If RFBM[i] = 0, XRSTOR does not
update state component i." So we #GP on a don't need restore bit i if
XSTATE_BV[I] == 1 && xcr0[0] ==0. That's where I miss I think, thanks
for your explanation.

Regard,
Wanpeng Li

>
> 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]).