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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <kjjn3mvfp2gf5iyeyukthgluayrkefonfmqbugrsreeeqfwde5@rxrzxrsobt54>
Date: Thu, 13 Nov 2025 18:19:00 +0530
From: Brahmajit Das <listout@...tout.xyz>
To: "Lecomte, Arnaud" <contact@...aud-lcm.com>
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 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.
I'm still debugging bug I'm open to ideas and definitely I could be
wrong here, please feel free to correct/point out.

-- 
Regards,
listout