KVM: x86: Check for injected exceptions before queuing a debug exception

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@kernel.org>
base-commit: a68a4bbc5b9ce5b722473399f05cb05217abaee8
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a03530795707797b73c701b0c952d46d27389e8d..658476815b6a6ccf2cd94e87cdea76629bec490a 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12529,7 +12529,7 @@ 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);