x86/kprobes: Retrieve correct opcode for group instruction

Since the opcodes start from 0xff are group5 instruction group which is
not 2 bytes opcode but the extended opcode determined by the MOD/RM byte.

The commit abd82e5 ("x86/kprobes: Do not decode opcode in resume_execution()")
used insn->opcode.bytes[1], but that is not correct. We have to refer
the insn->modrm.bytes[1] instead.

Fixes: abd82e5 ("x86/kprobes: Do not decode opcode in resume_execution()")
Signed-off-by: Masami Hiramatsu <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/161469872400.49483.18214724458034233166.stgit@devnote2

Author: mhiramat

arch/x86/kernel/kprobes/core.c

@@ -453,7 +453,11 @@ static void set_resume_flags(struct kprobe *p, struct insn *insn)
 		break;
 #endif
 	case 0xff:
-		opcode = insn->opcode.bytes[1];
+		/*
+		 * Since the 0xff is an extended group opcode, the instruction
+		 * is determined by the MOD/RM byte.
+		 */
+		opcode = insn->modrm.bytes[0];
 		if ((opcode & 0x30) == 0x10) {
 			/*
 			 * call absolute, indirect