linux-kernel - Re: [REGRESSION] 6.14.3 panic - kernel NULL pointer dereference in htb

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Message-ID: <aAf/K7F9TmCJIT+N@pop-os.localdomain>
Date: Tue, 22 Apr 2025 13:42:19 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: Holger Hoffstätte <holger@...lied-asynchrony.com>
Cc: "Alan J. Wylie" <alan@...ie.me.uk>, Jamal Hadi Salim <jhs@...atatu.com>,
	regressions@...ts.linux.dev, Jiri Pirko <jiri@...nulli.us>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Octavian Purdila <tavip@...gle.com>,
	Toke Høiland-Jørgensen <toke@...hat.com>,
	stable@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [REGRESSION] 6.14.3 panic - kernel NULL pointer dereference in
 htb_dequeue

On Tue, Apr 22, 2025 at 07:20:24PM +0200, Holger Hoffstätte wrote:
> (cc: Greg KH)
> 
> On 2025-04-22 18:51, Alan J. Wylie wrote:
> > On Mon, 21 Apr 2025 21:09:27 +0100
> > "Alan J. Wylie" <alan@...ie.me.uk> wrote:
> > 
> > > On Mon, 21 Apr 2025 21:47:44 +0200
> > > Holger Hoffstätte <holger@...lied-asynchrony.com> wrote:
> > > 
> > > > > I'm afraid that didn't help. Same panic.
> > > > 
> > > > Bummer :-(
> > > > 
> > > > Might be something else missing then - so for now the only other
> > > > thing I'd suggest is to revert the removal of the qlen check in
> > > > fq_codel.
> > > 
> > > Like this?
> > > 
> > > $ git diff  sch_fq_codel.c
> > > diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
> > > index 6c9029f71e88..4fdf317b82ec 100644
> > > --- a/net/sched/sch_fq_codel.c
> > > +++ b/net/sched/sch_fq_codel.c
> > > @@ -316,7 +316,7 @@ static struct sk_buff *fq_codel_dequeue(struct
> > > Qdisc *sch) qdisc_bstats_update(sch, skb);
> > >          flow->deficit -= qdisc_pkt_len(skb);
> > > -       if (q->cstats.drop_count) {
> > > +       if (q->cstats.drop_count && sch->q.qlen) {
> > >                  qdisc_tree_reduce_backlog(sch, q->cstats.drop_count,
> > >                                            q->cstats.drop_len);
> > >                  q->cstats.drop_count = 0;
> > > $
> > > 
> > 
> > It's been about 21 hours and no crash yet. I had an excellent day down
> > a cave, so there's not been as much Internet traffic as usual, but
> > there's a good chance the above patch as at least worked around, if not
> > fixed the issue.
> 
> Thought so .. \o/
> 
> I guess now the question is what to do about it. IIUC the fix series [1]
> addressed some kind of UAF problem, but obviously was not applied
> correctly or is missing follow-ups. It's also a bit mysterious why
> adding the HTB patch didn't work.
> 
> Maybe Cong Wang can advise what to do here?

I guess my patch caused some regression, I am still decoding the crashes
reported here.

Meanwhile, if you could provide a reliable (and ideally minimum)
reproducer, it would help me a lot to debug.

Thanks!