[PATCH net v2] net_sched: fix skb memory leak in deferred qdisc drops

[PATCH net v2] net_sched: fix skb memory leak in deferred qdisc drops

[Fernando Fernandez Mancera]

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>
---
v2: added RCU read lock protection around root pointer and used root
qdisc to append the packets to free
---
 include/net/sch_generic.h | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index c3d657359a3d..5fc0b1ebaf25 100644
--- 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,
-- 
2.53.0

Re: [PATCH net v2] net_sched: fix skb memory leak in deferred qdisc drops

[Eric Dumazet]

On Wed, Apr 8, 2026 at 3:01 AM Fernando Fernandez Mancera
<fmancera@suse.de> wrote:
>
> 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>

Thanks !

Re: [PATCH net v2] net_sched: fix skb memory leak in deferred qdisc drops

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed,  8 Apr 2026 12:00:44 +0200 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [net,v2] net_sched: fix skb memory leak in deferred qdisc drops
    https://git.kernel.org/netdev/net/c/a6bd339dbb35

You are awesome, thank you!
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