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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
