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 021a17ed79 ("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: 021a17ed79 ("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>
d518d2ed86