tcg/i386: Check the size of instruction being translated

This fixes the bug: 'user-to-root privesc inside VM via bad translation
caching' reported by Jann Horn here:
https://bugs.chromium.org/p/project-zero/issues/detail?id=1122

Reviewed-by: Richard Henderson <rth@twiddle.net>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Message-Id: <20170323175851.14342-1-bobby.prani@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/translate.c

@@ -4418,6 +4418,13 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
     s->vex_l = 0;
     s->vex_v = 0;
  next_byte:
+    /* x86 has an upper limit of 15 bytes for an instruction. Since we
+     * do not want to decode and generate IR for an illegal
+     * instruction, the following check limits the instruction size to
+     * 25 bytes: 14 prefix + 1 opc + 6 (modrm+sib+ofs) + 4 imm */
+    if (s->pc - pc_start > 14) {
+        goto illegal_op;
+    }
     b = cpu_ldub_code(env, s->pc);
     s->pc++;
     /* Collect prefixes.  */