Subject: Re: [PATCH net v3 4/7] net/sched: netem: restructure dequeue to avoid re-entrancy with child qdisc

From: Jakub Kicinski <[email-masked]>
Date: Mon, 6 Apr 2026 08:41:33 -0700

On Sat, 4 Apr 2026 10:49:46 +0100 Simon Horman wrote:
> On Thu, Apr 02, 2026 at 01:19:32PM -0700, Stephen Hemminger wrote:
> > netem_dequeue() enqueues packets into its child qdisc while being
> > called from the parent's dequeue path. This causes two problems:
> > 
> > - HFSC tracks class active/inactive state on qlen transitions.
> >   A child enqueue during dequeue causes double-insertion into
> >   the eltree (CVE-2025-37890, CVE-2025-38001).
> > 
> > - Non-work-conserving children like TBF may refuse to dequeue
> >   packets just enqueued, causing netem to return NULL despite
> >   having backlog. Parents like DRR then incorrectly deactivate
> >   the class.
> > 
> > Split the dequeue into helpers:
> > 
> >   netem_pull_tfifo()    - remove head packet from tfifo
> >   netem_slot_account()  - update slot pacing counters
> >   netem_dequeue_child() - batch-transfer ready packets to the
> >                           child, then dequeue from the child
> >   netem_dequeue_direct()- dequeue from tfifo when no child
> > 
> > When a child qdisc is present, all time-ready packets are moved
> > into the child before calling its dequeue. This separates the
> > enqueue and dequeue phases so the parent sees consistent qlen
> > transitions.
> > 
> > Fixes: 50612537e9ab ("netem: fix classful handling")
> > Signed-off-by: Stephen Hemminger <[email-masked]>
> > ---
> >  net/sched/sch_netem.c | 201 +++++++++++++++++++++++++++---------------
> >  1 file changed, 128 insertions(+), 73 deletions(-)  
> 
> Hi Stephen,
> 
> As a fix this is a large and complex patch.
> Could it be split up somehow to aid review?

+1, FWIW it's perfectly fine to have refactoring patch in a net series
(without a Fixes tag) if it makes the fix a lot easier to review.