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SubjectRe: [PATCH v2] x86/kprobes: Fix 1 byte conditional jump target
On Wed,  8 Feb 2023 07:17:08 +0000
Nadav Amit <nadav.amit@gmail.com> wrote:

> From: Nadav Amit <namit@vmware.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@kernel.org>
> Suggested-by: Dave Hansen <dave.hansen@intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Nadav Amit <namit@vmware.com>

Thanks, looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>


>
> ---
>
> 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
>

Nadav
--
Masami Hiramatsu (Google) <mhiramat@kernel.org>

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