This is a note to let you know that I've just added the patch titled sch_api: Don't skip qdisc attach on ingress to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: sch_api-don-t-skip-qdisc-attach-on-ingress.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From de2d807b294d3d2ce5e59043ae2634016765d076 Mon Sep 17 00:00:00 2001 From: Maxim Mikityanskiy <maximmi@xxxxxxxxxx> Date: Wed, 12 Jan 2022 12:28:05 +0200 Subject: sch_api: Don't skip qdisc attach on ingress From: Maxim Mikityanskiy <maximmi@xxxxxxxxxx> commit de2d807b294d3d2ce5e59043ae2634016765d076 upstream. The attach callback of struct Qdisc_ops is used by only a few qdiscs: mq, mqprio and htb. qdisc_graft() contains the following logic (pseudocode): if (!qdisc->ops->attach) { if (ingress) do ingress stuff; else do egress stuff; } if (!ingress) { ... if (qdisc->ops->attach) qdisc->ops->attach(qdisc); } else { ... } As we see, the attach callback is not called if the qdisc is being attached to ingress (TC_H_INGRESS). That wasn't a problem for mq and mqprio, since they contain a check that they are attached to TC_H_ROOT, and they can't be attached to TC_H_INGRESS anyway. However, the commit cited below added the attach callback to htb. It is needed for the hardware offload, but in the non-offload mode it simulates the "do egress stuff" part of the pseudocode above. The problem is that when htb is attached to ingress, neither "do ingress stuff" nor attach() is called. It results in an inconsistency, and the following message is printed to dmesg: unregister_netdevice: waiting for lo to become free. Usage count = 2 This commit addresses the issue by running "do ingress stuff" in the ingress flow even in the attach callback is present, which is fine, because attach isn't going to be called afterwards. The bug was found by syzbot and reported by Eric. Fixes: d03b195b5aa0 ("sch_htb: Hierarchical QoS hardware offload") Signed-off-by: Maxim Mikityanskiy <maximmi@xxxxxxxxxx> Reported-by: Eric Dumazet <edumazet@xxxxxxxxxx> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- net/sched/sch_api.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/net/sched/sch_api.c +++ b/net/sched/sch_api.c @@ -1062,7 +1062,7 @@ static int qdisc_graft(struct net_device qdisc_offload_graft_root(dev, new, old, extack); - if (new && new->ops->attach) + if (new && new->ops->attach && !ingress) goto skip; for (i = 0; i < num_q; i++) { Patches currently in stable-queue which might be from maximmi@xxxxxxxxxx are queue-5.15/bpf-fix-the-test_task_vma-selftest-to-support-output.patch queue-5.15/sch_api-don-t-skip-qdisc-attach-on-ingress.patch