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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Masami Hiramatsu <mhiramat@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.12 368/677] x86/kprobes: Retrieve correct opcode for group instruction
Date:   Wed, 12 May 2021 16:46:54 +0200
Message-Id: <20210512144849.552383168@linuxfoundation.org>
In-Reply-To: <20210512144837.204217980@linuxfoundation.org>
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From: Masami Hiramatsu <mhiramat@kernel.org>

[ Upstream commit d60ad3d46f1d04a282c56159f1deb675c12733fd ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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
-- 
2.30.2