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

[Xiao Guangrong]

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? 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.