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Subject[PATCH] net: sched: fix handling of singleton qdiscs with qdisc_hash
From: Jiri Kosina <>

qdisc_match_from_root() is now iterating over per-netdevice qdisc
hashtable instead of going through a linked-list of qdiscs (independently
on the actual underlying netdev), which used to be the case before the
switch to hashtable for qdiscs.

For singleton qdiscs, there is no underlying netdev associated though, and
therefore dumping a singleton qdisc will panic, as qdisc_dev(root) will
always be NULL.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000410
IP: [<ffffffff8167efac>] qdisc_match_from_root+0x2c/0x70
PGD 1aceba067 PUD 1aceb7067 PMD 0
Oops: 0000 [#1] PREEMPT SMP
[ ... ]
task: ffff8801ec996e00 task.stack: ffff8801ec934000
RIP: 0010:[<ffffffff8167efac>] [<ffffffff8167efac>] qdisc_match_from_root+0x2c/0x70
RSP: 0018:ffff8801ec937ab0 EFLAGS: 00010203
RAX: 0000000000000408 RBX: ffff88025e612000 RCX: ffffffffffffffd8
RDX: 0000000000000000 RSI: 00000000ffff0000 RDI: ffffffff81cf8100
RBP: ffff8801ec937ab0 R08: 000000000001c160 R09: ffff8802668032c0
R10: ffffffff81cf8100 R11: 0000000000000030 R12: 00000000ffff0000
R13: ffff88025e612000 R14: ffffffff81cf3140 R15: 0000000000000000
FS: 00007f24b9af6740(0000) GS:ffff88026f280000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000410 CR3: 00000001aceec000 CR4: 00000000001406e0
ffff8801ec937ad0 ffffffff81681210 ffff88025dd51a00 00000000fffffff1
ffff8801ec937b88 ffffffff81681e4e ffffffff81c42bc0 ffff880262431500
ffffffff81cf3140 ffff88025dd51a10 ffff88025dd51a24 00000000ec937b38
Call Trace:
[<ffffffff81681210>] qdisc_lookup+0x40/0x50
[<ffffffff81681e4e>] tc_modify_qdisc+0x21e/0x550
[<ffffffff8166ae25>] rtnetlink_rcv_msg+0x95/0x220
[<ffffffff81209602>] ? __kmalloc_track_caller+0x172/0x230
[<ffffffff8166ad90>] ? rtnl_newlink+0x870/0x870
[<ffffffff816897b7>] netlink_rcv_skb+0xa7/0xc0
[<ffffffff816657c8>] rtnetlink_rcv+0x28/0x30
[<ffffffff8168919b>] netlink_unicast+0x15b/0x210
[<ffffffff81689569>] netlink_sendmsg+0x319/0x390
[<ffffffff816379f8>] sock_sendmsg+0x38/0x50
[<ffffffff81638296>] ___sys_sendmsg+0x256/0x260
[<ffffffff811b1275>] ? __pagevec_lru_add_fn+0x135/0x280
[<ffffffff811b1a90>] ? pagevec_lru_move_fn+0xd0/0xf0
[<ffffffff811b1140>] ? trace_event_raw_event_mm_lru_insertion+0x180/0x180
[<ffffffff811b1b85>] ? __lru_cache_add+0x75/0xb0
[<ffffffff817708a6>] ? _raw_spin_unlock+0x16/0x40
[<ffffffff811d8dff>] ? handle_mm_fault+0x39f/0x1160
[<ffffffff81638b15>] __sys_sendmsg+0x45/0x80
[<ffffffff81638b62>] SyS_sendmsg+0x12/0x20
[<ffffffff810038e7>] do_syscall_64+0x57/0xb0

Fix this by special-casing singleton qdiscs (those that don't have underlying
netdevice) and introduce immediate handling of those rather than trying to
go over an underlying netdevice. We're in the same situation in
tc_dump_qdisc_root() and tc_dump_tclass_root() as well.

Ultimately, this will have to be slightly reworked so that we are actually
able to show singleton qdiscs (noop) in the dump properly, which is our
ultimate goal. But we're not currently doing that anyway, so no regression
there, and better do this in a gradual manner.

Fixes: 59cc1f61f ("net: sched: convert qdisc linked list to hashtable")
Reported-by: Daniel Borkmann <>
Tested-by: Daniel Borkmann <>
Signed-off-by: Jiri Kosina <>

Daniel, I've covered two more cases of the same in tc_dump_qdisc_root()
and tc_dump_tclass_root(), but preserved your Reported-by / Tested-by: for
the one you did wrt. qdisc_match_from_root(). Please shout out loud if you
object. Thanks.

net/sched/sch_api.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index c093d32..83413e7 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -262,6 +262,9 @@ static struct Qdisc *qdisc_match_from_root(struct Qdisc *root, u32 handle)
struct Qdisc *q;

+ if (!qdisc_dev(root))
+ return (root->handle == handle ? root : NULL);
if (!(root->flags & TCQ_F_BUILTIN) &&
root->handle == handle)
return root;
@@ -1456,6 +1459,10 @@ static int tc_dump_qdisc_root(struct Qdisc *root, struct sk_buff *skb,
goto done;
+ if (!qdisc_dev(root))
+ goto done;
hash_for_each(qdisc_dev(root)->qdisc_hash, b, q, hash) {
if (q_idx < s_q_idx) {
@@ -1781,6 +1788,9 @@ static int tc_dump_tclass_root(struct Qdisc *root, struct sk_buff *skb,
if (tc_dump_tclass_qdisc(root, skb, tcm, cb, t_p, s_t) < 0)
return -1;

+ if (!qdisc_dev(root))
+ return 0;
hash_for_each(qdisc_dev(root)->qdisc_hash, b, q, hash) {
if (tc_dump_tclass_qdisc(q, skb, tcm, cb, t_p, s_t) < 0)
return -1;
 \ /
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