netdev - [PATCH] sch_api: Don't skip qdisc attach on ingress

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Message-ID: <20220112102805.488510-1-maximmi@nvidia.com>
Date:   Wed, 12 Jan 2022 12:28:05 +0200
From:   Maxim Mikityanskiy <maximmi@...dia.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
CC:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Tariq Toukan <tariqt@...dia.com>, <netdev@...r.kernel.org>,
        Maxim Mikityanskiy <maximmi@...dia.com>
Subject: [PATCH] sch_api: Don't skip qdisc attach on ingress

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@...dia.com>
Reported-by: Eric Dumazet <edumazet@...gle.com>
---
 net/sched/sch_api.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index c9c6f49f9c28..2cb496c84878 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -1062,7 +1062,7 @@ static int qdisc_graft(struct net_device *dev, struct Qdisc *parent,

 		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++) {
-- 
2.25.1