linux-kernel - Re: [PATCH bpf-next v3] bpf: Clamp trace length in __bpf_get

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Message-ID: <43aa4ed3-d9c0-4f60-b850-d345cb85fe41@arnaud-lcm.com>
Date: Thu, 13 Nov 2025 13:26:10 +0000
From: "Lecomte, Arnaud" <contact@...aud-lcm.com>
To: Brahmajit Das <listout@...tout.xyz>
Cc: syzbot+d1b7fa1092def3628bd7@...kaller.appspotmail.com, andrii@...nel.org,
 ast@...nel.org, bpf@...r.kernel.org, daniel@...earbox.net,
 eddyz87@...il.com, haoluo@...gle.com, john.fastabend@...il.com,
 jolsa@...nel.org, kpsingh@...nel.org, linux-kernel@...r.kernel.org,
 martin.lau@...ux.dev, netdev@...r.kernel.org, sdf@...ichev.me,
 song@...nel.org, syzkaller-bugs@...glegroups.com, yonghong.song@...ux.dev
Subject: Re: [PATCH bpf-next v3] bpf: Clamp trace length in __bpf_get_stack to
 fix OOB write


On 13/11/2025 12:49, Brahmajit Das wrote:
> On 12.11.2025 08:40, 'Lecomte, Arnaud' via syzkaller-bugs wrote:
>> I am a not sure this is the right solution and I am scared that by
>> forcing this clamping, we are hiding something else.
>> If we have a look at the code below:
>> ```
>>
>> |
>>
>> 	if (trace_in) {
>> 		trace = trace_in;
>> 		trace->nr = min_t(u32, trace->nr, max_depth);
>> 	} else if (kernel && task) {
>> 		trace = get_callchain_entry_for_task(task, max_depth);
>> 	} else {
>> 		trace = get_perf_callchain(regs, kernel, user, max_depth,
>> 					crosstask, false, 0);
>> 	} ``` trace should be (if I remember correctly) clamped there. If not, it
>> might hide something else. I would like to have a look at the return for
>> each if case through gdb. |
> Hi Arnaud,
> So I've been debugging this the reproducer always takes the else branch
> so trace holds whatever get_perf_callchain returns; in this situation.
>
> I mostly found it to be a value around 4.
>
> In some case the value would exceed to something 27 or 44, just after
> the code block
>
> 	if (unlikely(!trace) || trace->nr < skip) {
> 		if (may_fault)
> 			rcu_read_unlock();
> 		goto err_fault;
> 	}
>
> So I'm assuming there's some race condition that might be going on
> somewhere.
Which value ? trace->nr ?
> I'm still debugging bug I'm open to ideas and definitely I could be
> wrong here, please feel free to correct/point out.

I should be able to have a look tomorrow evening as I am currently a bit 
overloaded
with my work.

Thanks,
Arnaud