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

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Message-ID: <CANRm+Cz-Jrf3L0e6JbHGQwXAdubUuTzLYmSdctpa2X2NmAQ9Vg@mail.gmail.com>
Date:	Sun, 24 Apr 2016 06:37:22 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	David Matlack <dmatlack@...gle.com>
Cc:	kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>, stable@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: do not leak guest xcr0 into host interrupt handlers

2016-04-23 1:21 GMT+08:00 David Matlack <dmatlack@...gle.com>:
> On Fri, Apr 22, 2016 at 12:30 AM, Wanpeng Li <kernellwp@...il.com> wrote:
>> Hi Paolo and David,
>> 2016-03-31 3:24 GMT+08:00 David Matlack <dmatlack@...gle.com>:
>>>
>>> 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]).