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. > -- 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
