linux-kernel - [PATCH v2] x86/kprobes: Fix 1 byte conditional jump target

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Date:   Wed,  8 Feb 2023 07:17:08 +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>,
        Dave Hansen <dave.hansen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH v2] 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 and instead
zero-extended.

In fact, there is no need for any casting of immediate.value since sign
extension is already done during its decoding in insn_get_immediate().

Fix by removing the casting of the 1 byte conditional jump target.
Future patches can also remove the casting (and sign extension) in other
cases in which immediate.value is being used.

Fixes: 3bc753c06dd0 ("kbuild: treat char as always unsigned")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Suggested-by: Dave Hansen <dave.hansen@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Nadav Amit <namit@...are.com>

---

v1->v2: Removing the casting completely [Dave]
---
 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..695873c0f50b 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 = insn->immediate.value;
 		break;
 	case 0x0f:
 		opcode = insn->opcode.bytes[1];
-- 
2.34.1