From: Ziqiao Kong <ziqiaokong@gmail.com>

target/i386: As specified by Intel Manual Vol2 3-180, cmp instructions
are not allowed to have lock prefix and a `UD` should be raised. Without
this patch, s1->T0 will be uninitialized and used in the case OP_CMPL.

Signed-off-by: Ziqiao Kong <ziqiaokong@gmail.com>
Message-ID: <20240215095015.570748-2-ziqiaokong@gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 99d0dcd7f102c07a510200d768cae65e5db25d23)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
index 8fd49ff474..83c2b40480 100644
--- a/target/i386/tcg/translate.c
+++ b/target/i386/tcg/translate.c
@@ -1480,12 +1480,13 @@ static bool check_iopl(DisasContext *s)
 /* if d == OR_TMP0, it means memory operand (address in A0) */
 static void gen_op(DisasContext *s1, int op, MemOp ot, int d)
 {
+    /* Invalid lock prefix when destination is not memory or OP_CMPL. */
+    if ((d != OR_TMP0 || op == OP_CMPL) && s1->prefix & PREFIX_LOCK) {
+        gen_illegal_opcode(s1);
+        return;
+    }
+
     if (d != OR_TMP0) {
-        if (s1->prefix & PREFIX_LOCK) {
-            /* Lock prefix when destination is not memory.  */
-            gen_illegal_opcode(s1);
-            return;
-        }
         gen_op_mov_v_reg(s1, ot, s1->T0, d);
     } else if (!(s1->prefix & PREFIX_LOCK)) {
         gen_op_ld_v(s1, ot, s1->T0, s1->A0);
-- 
2.39.2