[NET_SCHED]: Fix fallout from dev->qdisc RCU change

The move of qdisc destruction to a rcu callback broke locking in the
entire qdisc layer by invalidating previously valid assumptions about
the context in which changes to the qdisc tree occur.

The two assumptions were:

- since changes only happen in process context, read_lock doesn't need
  bottem half protection. Now invalid since destruction of inner qdiscs,
  classifiers, actions and estimators happens in the RCU callback unless
  they're manually deleted, resulting in dead-locks when read_lock in
  process context is interrupted by write_lock_bh in bottem half context.

- since changes only happen under the RTNL, no additional locking is
  necessary for data not used during packet processing (f.e. u32_list).
  Again, since destruction now happens in the RCU callback, this assumption
  is not valid anymore, causing races while using this data, which can
  result in corruption or use-after-free.

Instead of "fixing" this by disabling bottem halfs everywhere and adding
new locks/refcounting, this patch makes these assumptions valid again by
moving destruction back to process context. Since only the dev->qdisc
pointer is protected by RCU, but ->enqueue and the qdisc tree are still
protected by dev->qdisc_lock, destruction of the tree can be performed
immediately and only the final free needs to happen in the rcu callback
to make sure dev_queue_xmit doesn't access already freed memory.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 6 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 14de297d024d..4d891beab138 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1480,14 +1480,16 @@ gso:
 	if (q->enqueue) {
 		/* Grab device queue */
 		spin_lock(&dev->queue_lock);
+		q = dev->qdisc;
+		if (q->enqueue) {
+			rc = q->enqueue(skb, q);
+			qdisc_run(dev);
+			spin_unlock(&dev->queue_lock);
 
-		rc = q->enqueue(skb, q);
-
-		qdisc_run(dev);
-
+			rc = rc == NET_XMIT_BYPASS ? NET_XMIT_SUCCESS : rc;
+			goto out;
+		}
 		spin_unlock(&dev->queue_lock);
-		rc = rc == NET_XMIT_BYPASS ? NET_XMIT_SUCCESS : rc;
-		goto out;
 	}
 
 	/* The device has no queue. Common case for software devices: