netdev - Re: size overflow in function qdisc_tree_decrease_qlen net/sched/sch

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Date:	Tue, 01 Dec 2015 12:06:11 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <cwang@...pensource.com>
Cc:	pageexec@...email.hu, Daniele Fucini <dfucini@...il.com>,
	netdev <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	David Miller <davem@...emloft.net>, spender@...ecurity.net,
	re.emese@...il.com
Subject: Re: size overflow in function qdisc_tree_decrease_qlen
 net/sched/sch_api.c

On Tue, 2015-12-01 at 11:17 -0800, Cong Wang wrote:
> On Tue, Dec 1, 2015 at 11:09 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Tue, 2015-12-01 at 10:43 -0800, Cong Wang wrote:
> >
> >> This smells hacky... Another way to fix this is to hold the qdisc tree
> >> lock in mq_dump(), since it is not a hot path (comparing with
> >> enqueue/dequeue)?
> >
> > Really ? Which qdisc tree lock will protect you exactly ???
> >
> > Whole point of MQ is that each TX queue has its own lock.
> >
> > So multiple cpus can call qdisc_tree_decrease_qlen() at the same time,
> > holding their own lock.
> >
> > Clearly modifying mq 'data' is wrong.
> 
> Ah, yeah, but mq _seems_ also the only one who modifies sch->q.qlen
> in ->dump(), which is the root cause of this bug. I am wondering if it should
> just compute the qlen and return it without modifying sch->q.qlen.

Sure, but then we still would get PAX underflows warnings ...

Also need to take care of sch->qstats.drops += count;

Also that would require a change of ->dump() api, since tc_fill_qdisc()
does :

if (q->ops->dump && q->ops->dump(q, skb) < 0)
    goto nla_put_failure;
qlen = q->q.qlen;

Not sure it is worth the pain, changing signature of all ->dump()
handlers...


What about adding TCQ_F_NOPARENT and then :

Note : Seems to be more invasive patch for net tree (need to properly
set TCQ_F_NOPARENT)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index f43c8f33f09e..20c462b48404 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -753,6 +753,9 @@ void qdisc_tree_decrease_qlen(struct Qdisc *sch, unsigned int n)
        while ((parentid = sch->parent)) {
                if (TC_H_MAJ(parentid) == TC_H_MAJ(TC_H_INGRESS))
                        return;
+               /* This qdisc has no 'parent', we are at the top of the tree */
+               if (sch->flags & TCQ_F_NOPARENT)
+                       return;
 
                sch = qdisc_lookup(qdisc_dev(sch), TC_H_MAJ(parentid));
                if (sch == NULL) {
 



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