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 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@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/161469872400.49483.18214724458034233166.stgit@devnote2

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 60a540fcbd56b5de8aedbd71c4aec91e0632ae0c..9b31790a0b0ad2eef78c00d6ced14947a8e2799a 100644 (file)
--- a/arch/x86/kernel/kprobes/core.c
+++ b/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