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

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Date:   Thu, 9 Feb 2023 23:24:12 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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: Re: [PATCH v2] x86/kprobes: Fix 1 byte conditional jump target

On Wed,  8 Feb 2023 07:17:08 +0000
Nadav Amit <nadav.amit@...il.com> wrote:

> 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>

Thanks, looks good to me.

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