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

["Holger Hoffstätte" <holger@applied-asynchrony.com>]

(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@wylie.me.uk> wrote:
> 
>> On Mon, 21 Apr 2025 21:47:44 +0200
>> Holger Hoffstätte <holger@applied-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?

So unless someone else has any ideas: Greg, please revert:

6.14.y/a57fe60ef4cf96bfbb6b58397ec28bdb5a5c6b31
("codel: remove sch->q.qlen check before qdisc_tree_reduce_backlog()")

and probably from 6.12 as well.

cheers
Holger

[1] https://lore.kernel.org/all/20250403211033.166059-1-xiyou.wangcong@gmail.com/