netdev - Re: [Patch net v2 00/11] net_sched: make ->qlen

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Message-ID: <584c74f6-a5c1-4842-ac5b-9f3639b5b5c4@redhat.com>
Date: Tue, 8 Apr 2025 11:01:26 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc: jhs@...atatu.com, jiri@...nulli.us, victor@...atatu.com
Subject: Re: [Patch net v2 00/11] net_sched: make ->qlen_notify() idempotent

On 4/3/25 11:10 PM, Cong Wang wrote:
> Gerrard reported a vulnerability exists in fq_codel where manipulating
> the MTU can cause codel_dequeue() to drop all packets. The parent qdisc's
> sch->q.qlen is only updated via ->qlen_notify() if the fq_codel queue
> remains non-empty after the drops. This discrepancy in qlen between
> fq_codel and its parent can lead to a use-after-free condition.
> 
> Let's fix this by making all existing ->qlen_notify() idempotent so that
> the sch->q.qlen check will be no longer necessary.
> 
> Patch 1~5 make all existing ->qlen_notify() idempotent to prepare for
> patch 6 which removes the sch->q.qlen check. They are followed by 5
> selftests for each type of Qdisc's we touch here.
> 
> All existing and new Qdisc selftests pass after this patchset.
> 
> Fixes: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM")
> Fixes: 76e3cc126bb2 ("codel: Controlled Delay AQM")

FTR, I think it would be better to include the fixes tag in the relevant
commit message, as such I propagated the above tags in patch 6.

/P