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) <[email protected]>
Suggested-by: Dave Hansen <[email protected]>
Signed-off-by: Nadav Amit <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/all/20230208071708.4048-1-namit%40vmware.com

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index b36f3c367cb24c5e8fcec80271098d1ead7e727e..695873c0f50b5bdb2363c3e58dfc5191908164fa 100644 (file)
--- 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];