Re: [Patch net 1/2] sch_hfsc: Fix qlen accounting bug when using peek in hfsc_enqueue()

[Simon Horman <horms@kernel.org>]

On Sun, May 18, 2025 at 03:20:37PM -0700, Cong Wang wrote:
> When enqueuing the first packet to an HFSC class, hfsc_enqueue() calls the
> child qdisc's peek() operation before incrementing sch->q.qlen and
> sch->qstats.backlog. If the child qdisc uses qdisc_peek_dequeued(), this may
> trigger an immediate dequeue and potential packet drop. In such cases,
> qdisc_tree_reduce_backlog() is called, but the HFSC qdisc's qlen and backlog
> have not yet been updated, leading to inconsistent queue accounting. This
> can leave an empty HFSC class in the active list, causing further
> consequences like use-after-free.
> 
> This patch fixes the bug by moving the increment of sch->q.qlen and
> sch->qstats.backlog before the call to the child qdisc's peek() operation.
> This ensures that queue length and backlog are always accurate when packet
> drops or dequeues are triggered during the peek.
> 
> Fixes: 12d0ad3be9c3 ("net/sched/sch_hfsc.c: handle corner cases where head may change invalidating calculated deadline")
> Reported-by: Mingi Cho <mincho@theori.io>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>

Reviewed-by: Simon Horman <horms@kernel.org>