netdev - Re: [PATCH net] net/sched: sch_qfq: Fix NULL deref when deactivating

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Message-ID: <CAPpSM+T51DDkcSehkc-3r3FbcYQzXkTq4LGx8RRfD2fACwM8pg@mail.gmail.com>
Date: Mon, 8 Dec 2025 15:17:57 -0700
From: Xiang Mei <xmei5@....edu>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: security@...nel.org, netdev@...r.kernel.org, jhs@...atatu.com, 
	jiri@...nulli.us
Subject: Re: [PATCH net] net/sched: sch_qfq: Fix NULL deref when deactivating

Sorry for not explaining that. This PoC could be complex to use TC to
trigger. I was thinking about the same thing: transforming this C
program to `tc` commands so we can have a tc-tests case, but this bug
is related to a race condition.

We have to use racing to create the state mentioned in the commit
message: "Two qfq_class objects may point to the same
leaf_qdisc"(Racing between tc_new_tfilter and qdisc_delete). I failed
to find a clean way to use `tc` to trigger this race after several
hours of trial, so I gave up on that. For non-race condition bugs,
I'll try to provide self-tests.

Thanks,
Xiang

On Sun, Dec 7, 2025 at 3:45 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Thu, Dec 04, 2025 at 07:11:12PM -0700, Xiang Mei wrote:
> > The PoC and intended crash are attached for your reference:
> >
> > PoC:
> > ```c
>
> I hate to ask every time, but is it possible to turn this C reproducer
> into a selftest? To save your time, use AI?
>
> Maybe I should add a warning in checkpatch.pl to catch net_sched fixes
> without selftests. :)
>
> Thanks,
> Cong