KVM: x86: Bug the VM if the emulator generates a bogus exception vector

Bug the VM if KVM's emulator attempts to inject a bogus exception vector.
The guest is likely doomed even if KVM continues on, and propagating a
bad vector to the rest of KVM runs the risk of breaking other assumptions
in KVM and thus triggering a more egregious bug.

All existing users of emulate_exception() have hardcoded vector numbers
(__load_segment_descriptor() uses a few different vectors, but they're
all hardcoded), and future users are likely to follow suit, i.e. the
change to emulate_exception() is a glorified nop.

As for the ctxt->exception.vector check in x86_emulate_insn(), the few
known times the WARN has been triggered in the past is when the field was
not set when synthesizing a fault, i.e. for all intents and purposes the
check protects against consumption of uninitialized data.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220526210817.3428868-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -624,7 +624,9 @@ 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);
+	if (KVM_EMULATOR_BUG_ON(vec > 0x1f, ctxt))
+		return X86EMUL_UNHANDLEABLE;
+
 	ctxt->exception.vector = vec;
 	ctxt->exception.error_code = error;
 	ctxt->exception.error_code_valid = valid;
@@ -5728,7 +5730,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 
 done:
 	if (rc == X86EMUL_PROPAGATE_FAULT) {
-		WARN_ON(ctxt->exception.vector > 0x1f);
+		if (KVM_EMULATOR_BUG_ON(ctxt->exception.vector > 0x1f, ctxt))
+			return EMULATION_FAILED;
 		ctxt->have_exception = true;
 	}
 	if (rc == X86EMUL_INTERCEPTED)