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

[PATCH net] 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>
---
 include/net/sch_generic.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index c3d657359a3d..61ba54e909f2 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -1170,12 +1170,18 @@ 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 = qdisc_root_sleeping(q);
+
 	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;
+	if (root->flags & TCQ_F_DEQUEUE_DROPS) {
+		tcf_set_drop_reason(skb, reason);
+		skb->next = q->to_free;
+		q->to_free = skb;
+	} else {
+		kfree_skb_reason(skb, reason);
+	}
 }
 
 /* Instead of calling kfree_skb() while root qdisc lock is held,
-- 
2.53.0

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

[Fernando Fernandez Mancera]

On 4/7/26 7:02 PM, Fernando Fernandez Mancera 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>
> ---
>   include/net/sch_generic.h | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index c3d657359a3d..61ba54e909f2 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -1170,12 +1170,18 @@ 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 = qdisc_root_sleeping(q);
> +
>   	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;
> +	if (root->flags & TCQ_F_DEQUEUE_DROPS) {
> +		tcf_set_drop_reason(skb, reason);
> +		skb->next = q->to_free;
> +		q->to_free = skb;
> +	} else {
> +		kfree_skb_reason(skb, reason);
> +	}
>   }
>   

*sigh* this is missing the following diff - tested it but forgot to 
amend it. Anyway, Eric let me know what do you think, if you like this 
solution I can send the v2 appending the skb to the root's to_free list 
properly.

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 61ba54e909f2..0e4166319c02 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -1177,8 +1177,8 @@ static inline void qdisc_dequeue_drop(struct Qdisc 
*q, struct sk_buff *skb,

         if (root->flags & TCQ_F_DEQUEUE_DROPS) {
                 tcf_set_drop_reason(skb, reason);
-               skb->next = q->to_free;
-               q->to_free = skb;
+               skb->next = root->to_free;
+               root->to_free = skb;
         } else {
                 kfree_skb_reason(skb, reason);
         }

[syzbot ci] Re: net_sched: fix skb memory leak in deferred qdisc drops

[syzbot ci]

syzbot ci has tested the following series

[v1] net_sched: fix skb memory leak in deferred qdisc drops
https://lore.kernel.org/all/20260407170214.4157-2-fmancera@suse.de
* [PATCH net] net_sched: fix skb memory leak in deferred qdisc drops

and found the following issue:
WARNING: suspicious RCU usage in cake_dequeue

Full report is available here:
https://ci.syzbot.org/series/90acbb62-3e93-4bb7-af7a-51d555f5956b

***

WARNING: suspicious RCU usage in cake_dequeue

tree:      net
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net.git
base:      a9b8b18364fffce4c451e6f6fd218fa4ab646705
arch:      amd64
compiler:  Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
config:    https://ci.syzbot.org/builds/a5e0fad0-c9c8-4102-b2af-b4d1c7947ee3/config
syz repro: https://ci.syzbot.org/findings/b1dd239a-f59f-414f-8bde-578258899e94/syz_repro

=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
./include/net/sch_generic.h:581 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
2 locks held by syz.0.51/6016:
 #0: ffffffff8e75e640 (rcu_read_lock_bh){....}-{1:3}, at: local_bh_disable include/linux/bottom_half.h:20 [inline]
 #0: ffffffff8e75e640 (rcu_read_lock_bh){....}-{1:3}, at: rcu_read_lock_bh include/linux/rcupdate.h:903 [inline]
 #0: ffffffff8e75e640 (rcu_read_lock_bh){....}-{1:3}, at: __dev_queue_xmit+0x277/0x3890 net/core/dev.c:4778
 #1: ffff8881ad9e8170 (&sch->root_lock_key#31){+...}-{3:3}, at: spin_lock include/linux/spinlock.h:341 [inline]
 #1: ffff8881ad9e8170 (&sch->root_lock_key#31){+...}-{3:3}, at: __dev_xmit_skb net/core/dev.c:4241 [inline]
 #1: ffff8881ad9e8170 (&sch->root_lock_key#31){+...}-{3:3}, at: __dev_queue_xmit+0xbf2/0x3890 net/core/dev.c:4819

stack backtrace:
CPU: 1 UID: 0 PID: 6016 Comm: syz.0.51 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 lockdep_rcu_suspicious+0x13f/0x1d0 kernel/locking/lockdep.c:6876
 qdisc_root_sleeping include/net/sch_generic.h:581 [inline]
 qdisc_dequeue_drop include/net/sch_generic.h:1173 [inline]
 cake_dequeue+0x23a7/0x4e50 net/sched/sch_cake.c:2240
 dequeue_skb net/sched/sch_generic.c:297 [inline]
 qdisc_restart net/sched/sch_generic.c:402 [inline]
 __qdisc_run+0x26c/0x15d0 net/sched/sch_generic.c:420
 qdisc_run include/net/pkt_sched.h:120 [inline]
 __dev_xmit_skb net/core/dev.c:4286 [inline]
 __dev_queue_xmit+0x1bad/0x3890 net/core/dev.c:4819
 packet_snd net/packet/af_packet.c:3076 [inline]
 packet_sendmsg+0x3eb6/0x50f0 net/packet/af_packet.c:3108
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg net/socket.c:742 [inline]
 __sys_sendto+0x672/0x710 net/socket.c:2206
 __do_sys_sendto net/socket.c:2213 [inline]
 __se_sys_sendto net/socket.c:2209 [inline]
 __x64_sys_sendto+0xde/0x100 net/socket.c:2209
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f077b39c819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f077c28c028 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f077b615fa0 RCX: 00007f077b39c819
RDX: 000000000000fc13 RSI: 0000200000000800 RDI: 0000000000000003
RBP: 00007f077b432c91 R08: 0000000000000000 R09: fffffffffffffef0
R10: 0000000000000880 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f077b616038 R14: 00007f077b615fa0 R15: 00007fffceb20038
 </TASK>


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: syzbot@syzkaller.appspotmail.com

---
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