Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0

[David Matlack]

On Mon, Mar 14, 2016 at 12:46 AM, Xiao Guangrong
<guangrong.xiao@xxxxxxxxxxxxxxx> wrote:
>
>
> On 03/12/2016 04:47 AM, David Matlack wrote:
>
>> I have not been able to trigger this bug on Linux 4.3, and suspect
>> it is due to this commit from Linux 4.2:
>>
>> 653f52c kvm,x86: load guest FPU context more eagerly
>>
>> With this commit, as long as the host is using eagerfpu, the guest's
>> fpu is always loaded just before the guest's xcr0 (vcpu->fpu_active
>> is always 1 in the following snippet):
>>
>> 6569         if (vcpu->fpu_active)
>> 6570                 kvm_load_guest_fpu(vcpu);
>> 6571         kvm_load_guest_xcr0(vcpu);
>>
>> When the guest's fpu is loaded, irq_fpu_usable() returns false.
>
>
> Er, i did not see that commit introduced this change.
>
>>
>> We've included our workaround for this bug, which applies to Linux 3.11.
>> It does not apply cleanly to HEAD since the fpu subsystem was refactored
>> in Linux 4.2. While the latest kernel does not look vulnerable, we may
>> want to apply a fix to the vulnerable stable kernels.
>
>
> Is the latest kvm safe if we use !eager fpu?

Yes I believe so. When !eagerfpu, interrupted_kernel_fpu_idle()
returns "!current->thread.fpu.fpregs_active && (read_cr0() &
X86_CR0_TS)". This should ensure the interrupt handler never does
XSAVE/XRSTOR with the guest's xcr0.

> Under this case,
> kvm_load_guest_fpu()
> is not called for every single VM-enter, that means kernel will use guest's
> xcr0 to
> save/restore XSAVE area.
>
> Maybe a simpler fix is just calling __kernel_fpu_begin() when the CPU
> switches
> to vCPU and reverts it when the vCPU is scheduled out or returns to
> userspace.
>