[PATCH,v2 net] net: sched: validate that class is found in qdisc_tree_decrease_qlen

[Alex Gartrell <agartrell@fb.com>]

We have an application that invokes tc to delete the root every time the
config changes. As a result we stress the cleanup code and were seeing the
following panic:

  crash> bt
  PID: 630839  TASK: ffff8823c990d280  CPU: 14  COMMAND: "tc"
   [... snip ...]
   #8 [ffff8820ceec17a0] page_fault at ffffffff8160a8c2
      [exception RIP: htb_qlen_notify+24]
      RIP: ffffffffa0841718  RSP: ffff8820ceec1858  RFLAGS: 00010282
      RAX: 0000000000000000  RBX: 0000000000000000  RCX: ffff88241747b400
      RDX: ffff88241747b408  RSI: 0000000000000000  RDI: ffff8811fb27d000
      RBP: ffff8820ceec1868   R8: ffff88120cdeff24   R9: ffff88120cdeff30
      R10: 0000000000000bd4  R11: ffffffffa0840919  R12: ffffffffa0843340
      R13: 0000000000000000  R14: 0000000000000001  R15: ffff8808dae5c2e8
      ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
   #9 [...] qdisc_tree_decrease_qlen at ffffffff81565375
  #10 [...] fq_codel_dequeue at ffffffffa084e0a0 [sch_fq_codel]
  #11 [...] fq_codel_reset at ffffffffa084e2f8 [sch_fq_codel]
  #12 [...] qdisc_destroy at ffffffff81560d2d
  #13 [...] htb_destroy_class at ffffffffa08408f8 [sch_htb]
  #14 [...] htb_put at ffffffffa084095c [sch_htb]
  #15 [...] tc_ctl_tclass at ffffffff815645a3
  #16 [...] rtnetlink_rcv_msg at ffffffff81552cb0
  [... snip ...]

To my understanding, the following situation is taking place.

  tc_ctl_tclass
   -> htb_delete
     -> class is deleted from clhash
   -> htb_put
     -> qdisc_destroy
       -> fq_codel_reset
         -> fq_codel_dequeue
           -> qdidsc_tree_decrease_qlen
             -> cl = htb_get # returns NULL, removed in htb_delete
               -> htb_qlen_notify(sch, NULL) # BOOM

This patch checks cl for 0 before invoking qlen_notify and put.  The notify
is not necessary in this case, as the parent has already been deactivated
anyway.

Signed-off-by: Alex Gartrell <agartrell@fb.com>
---
 net/sched/sch_api.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index f06aa01..46be5e5 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -762,8 +762,15 @@ void qdisc_tree_decrease_qlen(struct Qdisc *sch, unsigned int n)
 		cops = sch->ops->cl_ops;
 		if (cops->qlen_notify) {
 			cl = cops->get(sch, parentid);
-			cops->qlen_notify(sch, cl);
-			cops->put(sch, cl);
+			/* This will be 0 in the event that cl is being
+			 * destroyed, in which case we should not attempt
+			 * to invoke qlen_notify upon it (it must be
+			 * cleaned up otherwise anyway.
+			 */
+			if (cl != 0) {
+				cops->qlen_notify(sch, cl);
+				cops->put(sch, cl);
+			}
 		}
 		sch->q.qlen -= n;
 		__qdisc_qstats_drop(sch, drops);
-- 
Alex Gartrell <agartrell@fb.com>