[PATCH 5.4] net_sched: prevent NULL dereference if default qdisc setup failed

[Pratyush Yadav <ptyadav@amazon.de>]

If qdisc_create_dflt() fails, it returns NULL. With CONFIG_NET_SCHED
enabled, the check qdisc != &noop_qdisc passes and qdisc will be passed
to qdisc_hash_add(), which dereferences it.

This assignment was present in the upstream commit
5891cd5ec46c2 ("net_sched: add __rcu annotation to netdev->qdisc") but
was missed in the backport 22d95b5449249 ("net_sched: add __rcu
annotation to netdev->qdisc"), perhaps due to merge conflicts.
dev->qdisc is &noop_qdisc by default and if qdisc_create_dflt() fails,
this assignment will make sure qdisc == &noop_qdisc and no NULL
dereference will take place.

This bug was discovered and resolved using Coverity Static Analysis
Security Testing (SAST) by Synopsys, Inc.

Fixes: 22d95b5449249 ("net_sched: add __rcu annotation to netdev->qdisc")
Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
---

As usual, this was found by our static code analysis bot. I have
compile-tested this patch and ran a simple boot test. Did not do any
testing specifically to hit this bug.

 net/sched/sch_generic.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 1f055c21be4cf..4250f3cf30e72 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -1116,6 +1116,7 @@ static void attach_default_qdiscs(struct net_device *dev)
 			qdisc->ops->attach(qdisc);
 		}
 	}
+	qdisc = rtnl_dereference(dev->qdisc);

 #ifdef CONFIG_NET_SCHED
 	if (qdisc != &noop_qdisc)
--
2.39.2