KVM: x86: Return to userspace with internal error on unexpected exit reason

Receiving an unexpected exit reason from hardware should be considered
as a severe bug in KVM. Therefore, instead of just injecting #UD to
guest and ignore it, exit to userspace on internal error so that
it could handle it properly (probably by terminating guest).

In addition, prefer to use vcpu_unimpl() instead of WARN_ONCE()
as handling unexpected exit reason should be a rare unexpected
event (that was expected to never happen) and we prefer to print
a message on it every time it occurs to guest.

Furthermore, dump VMCS/VMCB to dmesg to assist diagnosing such cases.
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

@@ -4981,9 +4981,14 @@ static int handle_exit(struct kvm_vcpu *vcpu)
 
 	if (exit_code >= ARRAY_SIZE(svm_exit_handlers)
 	    || !svm_exit_handlers[exit_code]) {
-		WARN_ONCE(1, "svm: unexpected exit reason 0x%x\n", exit_code);
-		kvm_queue_exception(vcpu, UD_VECTOR);
-		return 1;
+		vcpu_unimpl(vcpu, "svm: unexpected exit reason 0x%x\n", exit_code);
+		dump_vmcb(vcpu);
+		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		vcpu->run->internal.suberror =
+			KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON;
+		vcpu->run->internal.ndata = 1;
+		vcpu->run->internal.data[0] = exit_code;
+		return 0;
 	}
 
 	return svm_exit_handlers[exit_code](svm);

arch/x86/kvm/vmx/vmx.c

@@ -5870,8 +5870,13 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
 	else {
 		vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
 				exit_reason);
-		kvm_queue_exception(vcpu, UD_VECTOR);
-		return 1;
+		dump_vmcs();
+		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+		vcpu->run->internal.suberror =
+			KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON;
+		vcpu->run->internal.ndata = 1;
+		vcpu->run->internal.data[0] = exit_reason;
+		return 0;
 	}
 }
 

include/uapi/linux/kvm.h

@@ -243,6 +243,8 @@ struct kvm_hyperv_exit {
 #define KVM_INTERNAL_ERROR_SIMUL_EX	2
 /* Encounter unexpected vm-exit due to delivery event. */
 #define KVM_INTERNAL_ERROR_DELIVERY_EV	3
+/* Encounter unexpected vm-exit reason */
+#define KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON	4
 
 /* for KVM_RUN, returned by mmap(vcpu_fd, offset=0) */
 struct kvm_run {