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

[Andy Lutomirski]

On Tue, Mar 15, 2016 at 8:43 PM, Xiao Guangrong
<guangrong.xiao@xxxxxxxxxxxxxxx> wrote:
>
>
> On 03/16/2016 03:01 AM, David Matlack wrote:
>>
>> 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.
>
>
>
> interrupted_kernel_fpu_idle() returns true if KVM-based hypervisor (e.g.
> QEMU)
> is not using fpu.ãThat can not stop handler using fpu.

Why is it safe to rely on interrupted_kernel_fpu_idle?  That function
is for interrupts, but is there any reason that KVM can't be preempted
(or explicitly schedule) with XCR0 having some funny value?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC