[tip: x86/urgent] x86/kprobes: Fix 1 byte conditional jump target

[tip-bot2 for Nadav Amit]

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

Commit-ID:     bd467ddd80ab3ecdd846ac148274603e1214ae73
Gitweb:        https://git.kernel.org/tip/bd467ddd80ab3ecdd846ac148274603e1214ae73
Author:        Nadav Amit <namit@xxxxxxxxxx>
AuthorDate:    Wed, 08 Feb 2023 07:17:08 
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Wed, 08 Feb 2023 08:26:21 -08:00

x86/kprobes: Fix 1 byte conditional jump target

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 when the (signed) relative jump offset gets treated as
unsigned.

Fix by replacing the unsigned 'immediate.bytes' (plus a cast) with the
signed 'immediate.value' when assigning to the relative jump offset.

[ dhansen: clarified changelog ]

Fixes: 3bc753c06dd0 ("kbuild: treat char as always unsigned")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20230208071708.4048-1-namit%40vmware.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 b36f3c3..695873c 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];