linux-kernel - [PATCH] x86/kprobes: Fix 1 byte conditional jump target

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Message-Id: <20230204210807.3930-1-namit@vmware.com>
Date:   Sat,  4 Feb 2023 21:08:07 +0000
From:   Nadav Amit <nadav.amit@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Nadav Amit <namit@...are.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] x86/kprobes: Fix 1 byte conditional jump target

From: Nadav Amit <namit@...are.com>

Commit 3bc753c06dd0 ("kbuild: treat char as always unsigned") broke
kprobes.  Setting a probe-point on 1 byte conditional jump can cause the
kernel to crash, as the branch target is not sign extended.

Fix by using s8 instead of char and use immediate.value instead of
immediate.bytes for consistency.

Cc: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Nadav Amit <namit@...are.com>
---
 arch/x86/kernel/kprobes/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index b36f3c367cb2..6a56d56b3817 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -625,7 +625,7 @@ static int prepare_emulation(struct kprobe *p, struct insn *insn)
 		/* 1 byte conditional jump */
 		p->ainsn.emulate_op = kprobe_emulate_jcc;
 		p->ainsn.jcc.type = opcode & 0xf;
-		p->ainsn.rel32 = *(char *)insn->immediate.bytes;
+		p->ainsn.rel32 = *(s8 *)&insn->immediate.value;
 		break;
 	case 0x0f:
 		opcode = insn->opcode.bytes[1];
-- 
2.34.1