net/sched: fix race between deactivation and dequeue for NOLOCK qdisc

The test implemented by some_qdisc_is_busy() is somewhat loosy for
NOLOCK qdisc, as we may hit the following scenario:

CPU1						CPU2
// in net_tx_action()
clear_bit(__QDISC_STATE_SCHED...);
						// in some_qdisc_is_busy()
						val = (qdisc_is_running(q) ||
						       test_bit(__QDISC_STATE_SCHED,
								&q->state));
						// here val is 0 but...
qdisc_run(q)
// ... CPU1 is going to run the qdisc next

As a conseguence qdisc_run() in net_tx_action() can race with qdisc_reset()
in dev_qdisc_reset(). Such race is not possible for !NOLOCK qdisc as
both the above bit operations are under the root qdisc lock().

After commit 021a17ed796b ("pfifo_fast: drop unneeded additional lock on dequeue")
the race can cause use after free and/or null ptr dereference, but the root
cause is likely older.

This patch addresses the issue explicitly checking for deactivation under
the seqlock for NOLOCK qdisc, so that the qdisc_run() in the critical
scenario becomes a no-op.

Note that the enqueue() op can still execute concurrently with dev_qdisc_reset(),
but that is safe due to the skb_array() locking, and we can't avoid that
for NOLOCK qdiscs.

Fixes: 021a17ed796b ("pfifo_fast: drop unneeded additional lock on dequeue")
Reported-by: Li Shuang <shuali@redhat.com>
Reported-and-tested-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 1 deletions

diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
index a16fbe9a2a67..aa99c73c3fbd 100644
--- a/include/net/pkt_sched.h
+++ b/include/net/pkt_sched.h
@@ -118,7 +118,12 @@ void __qdisc_run(struct Qdisc *q);
 static inline void qdisc_run(struct Qdisc *q)
 {
 	if (qdisc_run_begin(q)) {
-		__qdisc_run(q);
+		/* NOLOCK qdisc must check 'state' under the qdisc seqlock
+		 * to avoid racing with dev_qdisc_reset()
+		 */
+		if (!(q->flags & TCQ_F_NOLOCK) ||
+		    likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
+			__qdisc_run(q);
 		qdisc_run_end(q);
 	}
 }