[BUGFIX PATCH -tip 1/2] x86/kprobes: Retrieve correct opcode for group instruction

[Masami Hiramatsu]

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 abd82e533d88 ("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: abd82e533d88 ("x86/kprobes: Do not decode opcode in resume_execution()")
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
---
 arch/x86/kernel/kprobes/core.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index df776cdca327..08674e7a5d7b 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -448,7 +448,11 @@ 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