linux-kernel - [tip: x86/core] x86/kprobes: Retrieve correct opcode for group instruction

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Message-ID: <161651250981.398.14785132555334741464.tip-bot2@tip-bot2>
Date:   Tue, 23 Mar 2021 15:15:09 -0000
From:   "tip-bot2 for Masami Hiramatsu" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/core] x86/kprobes: Retrieve correct opcode for group instruction

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     d60ad3d46f1d04a282c56159f1deb675c12733fd
Gitweb:        https://git.kernel.org/tip/d60ad3d46f1d04a282c56159f1deb675c12733fd
Author:        Masami Hiramatsu <mhiramat@...nel.org>
AuthorDate:    Wed, 03 Mar 2021 00:25:24 +09:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 23 Mar 2021 16:07:55 +01:00

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@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/161469872400.49483.18214724458034233166.stgit@devnote2
---
 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 60a540f..9b31790 100644
--- 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