KVM: x86: Bug the VM on an out-of-bounds data read

Bug the VM and terminate emulation if an out-of-bounds read into the
emulator's data cache occurs.  Knowingly contuining on all but guarantees
that KVM will overwrite random kernel data, which is far, far worse than
killing the VM.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220526210817.3428868-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -1373,7 +1373,8 @@ static int read_emulated(struct x86_emulate_ctxt *ctxt,
 	if (mc->pos < mc->end)
 		goto read_cached;
 
-	WARN_ON((mc->end + size) >= sizeof(mc->data));
+	if (KVM_EMULATOR_BUG_ON((mc->end + size) >= sizeof(mc->data), ctxt))
+		return X86EMUL_UNHANDLEABLE;
 
 	rc = ctxt->ops->read_emulated(ctxt, addr, mc->data + mc->end, size,
 				      &ctxt->exception);