[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, 2 insertions, 2 deletions

diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index 7e14f14058e9..37a184021647 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -401,7 +401,7 @@ static int tc_dump_tfilter(struct sk_buff *skb, struct netlink_callback *cb)
         if ((dev = dev_get_by_index(tcm->tcm_ifindex)) == NULL)
                 return skb->len;
 
-        read_lock_bh(&qdisc_tree_lock);
+        read_lock(&qdisc_tree_lock);
         if (!tcm->tcm_parent)
                 q = dev->qdisc_sleeping;
         else
@@ -458,7 +458,7 @@ errout:
         if (cl)
                 cops->put(q, cl);
 out:
-        read_unlock_bh(&qdisc_tree_lock);
+        read_unlock(&qdisc_tree_lock);
         dev_put(dev);
         return skb->len;
 }