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

[Simon Horman <horms@kernel.org>]

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) <hxzene@gmail.com>

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