target/x86: Correctly handle invalid 0x0f 0xc7 0xxx insns

In the decode_group9() function, if we don't recognise the insn as
one that we should handle, we leave the 'entry' pointer unaltered.
Because the X86OpEntry struct has a union for the gen and decode
pointers, this means that the top level code will call decode.e.gen()
which tries to use the decode function pointer (still set to
decode_group9) as a gen function pointer.

This is undefined behaviour, but seems to be mostly harmless in
practice (we call decode_group9() again with bogus arguments and it
does nothing).  If you have CFI enabled then it will trip the CFI
check:

../target/i386/tcg/decode-new.c.inc:2862:9: runtime error: control flow integrity check for type 'void (struct DisasContext *, struct X86DecodedInsn *)' failed during indirect function call

Set *entry to UNKNOWN_OPCODE to provoke the #UD exception, as we do
in decode_group1A() and decode_group11() for similar situations.

Thanks to the bug reporter for the clear description and analysis of
the bug and the simple reproducer.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3172
Fixes: fcd16539eb ("target/i386: convert CMPXCHG8B/CMPXCHG16B to new decoder")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20251021173152.1695997-1-peter.maydell@linaro.org>

target/i386/tcg/decode-new.c.inc

@@ -335,6 +335,8 @@ static void decode_group9(DisasContext *s, CPUX86State *env, X86OpEntry *entry,
         *entry = group9_reg;
     } else if (op == 1) {
         *entry = REX_W(s) ? cmpxchg16b : cmpxchg8b;
+    } else {
+        *entry = UNKNOWN_OPCODE;
     }
 }