KVM: SVM: don't NMI singlestep over event injection

Singlestepping is enabled by setting the TF flag and care must be
taken to not let the guest see (and reuse at an inconvenient time)
the modified rflag value. One such case is event injection, as part
of which flags are pushed on the stack and restored later on iret.

This commit disables singlestepping when we're about to inject an
event and forces an immediate exit for us to re-evaluate the NMI
related state.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 6ac9bcd7f6f0d0b8738ccdb1d6e9574adfc03dda..597337987a167f38938f21ac7ad4b7c7c3a9faa1 100644 (file)
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -4823,6 +4823,22 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
        if (unlikely(svm->nested.exit_required))
                return;
 
+       /*
+        * Disable singlestep if we're injecting an interrupt/exception.
+        * We don't want our modified rflags to be pushed on the stack where
+        * we might not be able to easily reset them if we disabled NMI
+        * singlestep later.
+        */
+       if (svm->nmi_singlestep && svm->vmcb->control.event_inj) {
+               /*
+                * Event injection happens before external interrupts cause a
+                * vmexit and interrupts are disabled here, so smp_send_reschedule
+                * is enough to force an immediate vmexit.
+                */
+               disable_nmi_singlestep(svm);
+               smp_send_reschedule(vcpu->cpu);
+       }
+
        pre_svm_run(svm);
 
        sync_lapic_to_cr8(vcpu);