[PATCH 3/3] KVM: x86: Check for injected exceptions before queuing a debug exception

[Yosry Ahmed]

On KVM_SET_GUEST_DEBUG, if a #DB or #BP is injected with
KVM_GUESTDBG_INJECT_DB or KVM_GUESTDBG_INJECT_BP, KVM fails with -EBUSY
if there is an existing pending exception. This was introduced in
commit 4f926bf29186 ("KVM: x86: Polish exception injection via
KVM_SET_GUEST_DEBUG") to avoid a warning in kvm_queue_exception(),
presumably to avoid overriding a pending exception.

This added another (arguably nice) property, if there's a pending
exception, KVM_SET_GUEST_DEBUG cannot cause a #DF or triple fault.
However, if an exception is injected, KVM_SET_GUEST_DEBUG will cause
a #DF or triple fault in the guest, as kvm_multiple_exception() combines
them.

Check for both pending and injected exceptions for
KVM_GUESTDBG_INJECT_DB and KVM_GUESTDBG_INJECT_BP, to avoid accidentally
injecting a #DB or triple fault.

Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e39c5faf94230..0c8aacf1fa67f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12543,7 +12543,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 
 	if (dbg->control & (KVM_GUESTDBG_INJECT_DB | KVM_GUESTDBG_INJECT_BP)) {
 		r = -EBUSY;
-		if (kvm_is_exception_pending(vcpu))
+		if (kvm_is_exception_pending(vcpu) || vcpu->arch.exception.injected)
 			goto out;
 		if (dbg->control & KVM_GUESTDBG_INJECT_DB)
 			kvm_queue_exception(vcpu, DB_VECTOR);
-- 
2.53.0.473.g4a7958ca14-goog