net_sched: fix skb memory leak in deferred qdisc drops

When the network stack cleans up the deferred list via qdisc_run_end(),
it operates on the root qdisc. If the root qdisc do not implement the
TCQ_F_DEQUEUE_DROPS flag the packets queue to free are never freed and
gets stranded on the child's local to_free list.

Fix this by making qdisc_dequeue_drop() aware of the root qdisc. It
fetches the root qdisc and check for the TCQ_F_DEQUEUE_DROPS flag. If
the flag is present, the packet is appended directly to the root's
to_free list. Otherwise, drop it directly as it was done before the
optimization was implemented.

Fixes: a6efc273ab82 ("net_sched: use qdisc_dequeue_drop() in cake, codel, fq_codel")
Reported-by: Damilola Bello <damilola@aterlo.com>
Closes: https://lore.kernel.org/netdev/CAPgFtOLaedBMU0f_BxV2bXftTJSmJr018Q5uozOo5vVo6b9tjw@mail.gmail.com/
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260408100044.4530-1-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index c3d657359a3d2d9d5eef72d3792b0d258b572ae1..5fc0b1ebaf25c0175d8fd225db45d4af52e26b96 100644 (file)
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -1170,12 +1170,22 @@ static inline void tcf_kfree_skb_list(struct sk_buff *skb)
 static inline void qdisc_dequeue_drop(struct Qdisc *q, struct sk_buff *skb,
                                      enum skb_drop_reason reason)
 {
+       struct Qdisc *root;
+
        DEBUG_NET_WARN_ON_ONCE(!(q->flags & TCQ_F_DEQUEUE_DROPS));
        DEBUG_NET_WARN_ON_ONCE(q->flags & TCQ_F_NOLOCK);
 
-       tcf_set_drop_reason(skb, reason);
-       skb->next = q->to_free;
-       q->to_free = skb;
+       rcu_read_lock();
+       root = qdisc_root_sleeping(q);
+
+       if (root->flags & TCQ_F_DEQUEUE_DROPS) {
+               tcf_set_drop_reason(skb, reason);
+               skb->next = root->to_free;
+               root->to_free = skb;
+       } else {
+               kfree_skb_reason(skb, reason);
+       }
+       rcu_read_unlock();
 }
 
 /* Instead of calling kfree_skb() while root qdisc lock is held,