KVM: emulate: warn on invalid or uninitialized exception numbers

These were reported when running Jailhouse on AMD processors.

Initialize ctxt->exception.vector with an invalid exception number,
and warn if it remained invalid even though the emulator got
an X86EMUL_PROPAGATE_FAULT return code.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -527,6 +527,7 @@ static unsigned long seg_base(struct x86_emulate_ctxt *ctxt, int seg)
 static int emulate_exception(struct x86_emulate_ctxt *ctxt, int vec,
 			     u32 error, bool valid)
 {
+	WARN_ON(vec > 0x1f);
 	ctxt->exception.vector = vec;
 	ctxt->exception.error_code = error;
 	ctxt->exception.error_code_valid = valid;
@@ -4827,8 +4828,10 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 	ctxt->eip = ctxt->_eip;
 
 done:
-	if (rc == X86EMUL_PROPAGATE_FAULT)
+	if (rc == X86EMUL_PROPAGATE_FAULT) {
+		WARN_ON(ctxt->exception.vector > 0x1f);
 		ctxt->have_exception = true;
+	}
 	if (rc == X86EMUL_INTERCEPTED)
 		return EMULATION_INTERCEPTED;
 

arch/x86/kvm/x86.c

@@ -5248,6 +5248,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 
 		ctxt->interruptibility = 0;
 		ctxt->have_exception = false;
+		ctxt->exception.vector = -1;
 		ctxt->perm_ok = false;
 
 		ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;