Re: [PATCH 1/2] KVM: SVM: Fix NMI path when NMI happens in guest mode

Avi Kivity
2011-01-13T19:27:10Z

On 01/13/2011 05:51 PM, Roedel, Joerg wrote:
> On Thu, Jan 13, 2011 at 10:42:01AM -0500, Avi Kivity wrote:
> > On 01/13/2011 05:22 PM, Joerg Roedel wrote:
> > > The vmexit path on SVM needs to restore the KERNEL_GS_BASE
> > > MSR in order to savely execute the NMI handler. Otherwise a
> > > pending NMI can occur after the STGI instruction and crash
> > > the machine.
> > > This makes it impossible to run perf and kvm in parallel on
> > > an AMD machine in a stable way.
> > >
> > > Cc: stable@kernel.org
> > > Signed-off-by: Joerg Roedel<joerg.roedel@amd.com>
> > > ---
> > > arch/x86/kvm/svm.c | 1 +
> > > 1 files changed, 1 insertions(+), 0 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > > index 25bd1bc..8b9bc72 100644
> > > --- a/arch/x86/kvm/svm.c
> > > +++ b/arch/x86/kvm/svm.c
> > > @@ -3637,6 +3637,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> > >
> > > #ifdef CONFIG_X86_64
> > > wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> > > + wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gs);
> > > #else
> > > loadsegment(fs, svm->host.fs);
> > > #endif
> >
> > Why would an NMI crash if MSR_KERNEL_GS_BASE is bad?
> >
> > I see save_paranoid depends on MSR_GS_BASE (specifically its sign, which
> > is bad for the new instructions that allow userspace to write gsbase),
> > but not on MSR_KERNEL_GS_BASE.
>
> Thats a good question. I have not idea. I spent some time trying to
> figure this out (after I found out that wrong KERNEL_GS_BASE was the
> cause of the crashes) but had no luck.
>
> This also doesn't happen every time an NMI is delivered in svm_vcpu_run.
> Sometimes it runs perfectly in parallel for a few minutues before the
> machine triple-faults.
>
> I also had a look at entry_64.S. The save_paranoid could not be the
> cause because MSR_GS_BASE is already negative at this point. But the
> re-schedule condition check at the end of the NMI handler code could
> also not be the cause because the NMI happens while preemption (and
> interrupts) are disabled (a re-schedule should also trigger
> preempt-notifiers and restore KERNEL_GS_BASE).
>

I have it:

ENTRY(native_load_gs_index)
CFI_STARTPROC
pushfq_cfi
DISABLE_INTERRUPTS(CLBR_ANY & ~CLBR_RDI)
SWAPGS
gs_change:
movl %edi,%gs
2: mfence /* workaround */
SWAPGS
popfq_cfi
ret

If an nmi hits between the two SWAPGSs, it sees the guest's
MSR_KERNEL_GS_BASE as the host's MSR_GS_BASE.

An alternative to your fix would be to disable GIF around
load_gs_index() in kvm. I imagine it would be slower than your fix (not
a trivial tradeoff - wrmsr every lightweight exit, vs. clgi/stgi every
heavyweight exit).

Please update the changelog, and add a comment.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.