Subject: Re: [PATCH] net/sched: sch_dualpi2: fix NULL pointer dereference in dualpi2_change()

From: Simon Horman <[email-masked]>
Date: Tue, 14 Apr 2026 19:36:01 +0100

On Tue, Apr 14, 2026 at 07:31:32PM +0100, Simon Horman wrote:
> On Mon, Apr 13, 2026 at 03:57:40PM +0800, Kito Xu (veritas501) wrote:
> > dualpi2_change() uses a trim loop to enforce the new queue limit after a
> > configuration change. The loop calls qdisc_dequeue_internal(sch, true)
> > which only dequeues from the C-queue (sch->q) and the requeue list
> > (sch->gso_skb). It does not dequeue from the L-queue (q->l_queue).
> > 
> > However, the loop continuation condition checks qdisc_qlen(sch), which
> > reflects the total packet count across both queues because
> > dualpi2_enqueue_skb() manually increments sch->q.qlen for L-queue
> > packets (line 418). Similarly, q->memory_used accounts for memory from
> > both queues.
> > 
> > When all packets reside in the L-queue and the C-queue is empty, the
> > loop condition remains true but qdisc_dequeue_internal() returns NULL.
> > The subsequent skb->truesize dereference causes a NULL pointer oops.
> > 
> > An unprivileged user can trigger this from a user namespace:
> > 
> >   1. unshare(CLONE_NEWUSER | CLONE_NEWNET)
> >   2. Create a dummy device and attach dualpi2 qdisc
> >   3. Send ECT(1)-marked packets to fill the L-queue
> >   4. Reduce the qdisc limit via RTM_NEWQDISC
> 
> ...
> 
> > Fix this by adding a NULL check after qdisc_dequeue_internal(). When
> > the C-queue is exhausted but L-queue packets keep qdisc_qlen(sch) above
> > the limit, the loop breaks safely. Remaining excess L-queue packets will
> > be drained by the normal dequeue path.
> > 
> > Fixes: 320d031ad6e4 ("sched: Struct definition and parsing of dualpi2 qdisc")
> > Signed-off-by: Kito Xu (veritas501) <[email-masked]>
> 
> Reviewed-by: Simon Horman <[email-masked]>

Sorry, I now see that a more comprehensive fix for this code path
is available from the original author of the code.

- [PATCH v1 net 1/1] net/sched: sch_dualpi2: fix limit/memlimit enforcement when dequeueing L-queue
  https://lore.kernel.org/all/[email-masked]/