Re: size overflow in function qdisc_tree_decrease_qlen net/sched/sch_api.c

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2015-12-01 at 12:06 -0800, Eric Dumazet wrote:
> 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@gmail.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)


Hmm... it looks like we have a much more serious bug :

qdisc_lookup() calls qdisc_match_from_root(dev->qdisc, handle) without
proper lock being held, so we might actually crash the host,
if qdisc_tree_decrease_qlen() happens at the time qdiscs are changed. 

qdisc_tree_decrease_qlen() needs serious care :(

Damned.