Re: [PATCH] KVM: SVM: Make sure GHCB is mapped before updating

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 07, 2021, Tom Lendacky wrote:
> On 4/7/21 3:08 PM, Sean Christopherson wrote:
> > On Wed, Apr 07, 2021, Tom Lendacky wrote:
> >> From: Tom Lendacky <thomas.lendacky@amd.com>
> >>
> >> The sev_vcpu_deliver_sipi_vector() routine will update the GHCB to inform
> >> the caller of the AP Reset Hold NAE event that a SIPI has been delivered.
> >> However, if a SIPI is performed without a corresponding AP Reset Hold,
> >> then the GHCB may not be mapped, which will result in a NULL pointer
> >> dereference.
> >>
> >> Check that the GHCB is mapped before attempting the update.
> > 
> > It's tempting to say the ghcb_set_*() helpers should guard against this, but
> > that would add a lot of pollution and the vast majority of uses are very clearly
> > in the vmgexit path.  svm_complete_emulated_msr() is the only other case that
> > is non-obvious; would it make sense to sanity check svm->ghcb there as well?
> 
> Hmm... I'm not sure if we can get here without having taken the VMGEXIT
> path to start, but it certainly couldn't hurt to add it.

Yeah, AFAICT it should be impossible to reach the callback without a valid ghcb,
it'd be purely be a sanity check.
 
> I can submit a v2 with that unless you want to submit it (with one small
> change below).

I'd say just throw it into v2.

> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 019ac836dcd0..abe9c765628f 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -2728,7 +2728,8 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  static int svm_complete_emulated_msr(struct kvm_vcpu *vcpu, int err)
> >  {
> >         struct vcpu_svm *svm = to_svm(vcpu);
> > -       if (!sev_es_guest(vcpu->kvm) || !err)
> > +
> > +       if (!err || !sev_es_guest(vcpu->kvm) || !WARN_ON_ONCE(svm->ghcb))
> 
> This should be WARN_ON_ONCE(!svm->ghcb), otherwise you'll get the right
> result, but get a stack trace immediately.

Doh, yep.