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Message-ID: <SJ0PR18MB4009D2CF6AA2DFF25732A4BFB2B79@SJ0PR18MB4009.namprd18.prod.outlook.com>
Date: Wed, 13 Oct 2021 09:43:48 +0000
From: "Volodymyr Mytnyk [C]" <vmytnyk@...vell.com>
To: Vlad Buslov <vladbu@...dia.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
Serhiy Boiko <serhiy.boiko@...ision.eu>,
Jamal Hadi Salim <jhs@...atatu.com>,
Cong Wang <xiyou.wangcong@...il.com>,
Jiri Pirko <jiri@...nulli.us>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Vlad Buslov <vladbu@...lanox.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] sched: fix infinite loop when creating tc filter
Hi Vlad,
Thanks for your review comments and good explanation of the problem you observe. I will
take a look at this and will back to you.
Regards,
Volodymyr
> Hi Volodymyr,
>
> On Sun 10 Oct 2021 at 09:55, Volodymyr Mytnyk <volodymyr.mytnyk@...ision.eu> wrote:
> > From: Volodymyr Mytnyk <vmytnyk@...vell.com>
> >
> > After running a specific set of commands tc will become unresponsive:
> >
> > $ ip link add dev DEV type veth
> > $ tc qdisc add dev DEV clsact
> > $ tc chain add dev DEV chain 0 ingress
> > $ tc filter del dev DEV ingress
> > $ tc filter add dev DEV ingress flower action pass
> >
> > When executing chain flush, the "chain->flushing" field is set
> > to true, which prevents insertion of new classifier instances.
> > It is unset in one place under two conditions:
> >
> > `refcnt - chain->action_refcnt == 0` and `!by_act`.
> >
> > Ignoring the by_act and action_refcnt arguments the `flushing procedure`
> > will be over when refcnt is 0.
> >
> > But if the chain is explicitly created (e.g. `tc chain add .. chain 0 ..`)
> > refcnt is set to 1 without any classifier instances. Thus the condition
> > is never met and the chain->flushing field is never cleared.
> > And because the default chain is `flushing` new classifiers cannot
> > be added. tc_new_tfilter is stuck in a loop trying to find a chain
> > where chain->flushing is false.
> >
> > By moving `chain->flushing = false` from __tcf_chain_put to the end
> > of tcf_chain_flush will avoid the condition and the field will always
> > be reset after the flush procedure.
> >
> > Fixes: 91052fa1c657 ("net: sched: protect chain->explicitly_created with block->lock")
>
> Thanks for working on this!
>
> >
> > Co-developed-by: Serhiy Boiko <serhiy.boiko@...ision.eu>
> > Signed-off-by: Serhiy Boiko <serhiy.boiko@...ision.eu>
> > Signed-off-by: Volodymyr Mytnyk <vmytnyk@...vell.com>
> > ---
> > net/sched/cls_api.c | 5 +++--
> > 1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> > index d73b5c5514a9..327594cce554 100644
> > --- a/net/sched/cls_api.c
> > +++ b/net/sched/cls_api.c
> > @@ -563,8 +563,6 @@ static void __tcf_chain_put(struct tcf_chain *chain, bool by_act,
> > if (refcnt - chain->action_refcnt == 0 && !by_act) {
> > tc_chain_notify_delete(tmplt_ops, tmplt_priv, chain->index,
> > block, NULL, 0, 0, false);
> > - /* Last reference to chain, no need to lock. */
> > - chain->flushing = false;
> > }
> >
> > if (refcnt == 0)
> > @@ -615,6 +613,9 @@ static void tcf_chain_flush(struct tcf_chain *chain, bool rtnl_held)
> > tcf_proto_put(tp, rtnl_held, NULL);
> > tp = tp_next;
> > }
> > +
> > + /* Last reference to chain, no need to lock. */
>
> But after moving the code block here you can no longer guarantee that
> this is the last reference, right?
>
> > + chain->flushing = false;
>
> Resetting the flag here is probably correct for actual flush use-case
> (e.g. RTM_DELTFILTER message with prio==0), but can cause undesired
> side-effects for other users of tcf_chain_flush(). Consider following
> interaction between new filter creation and explicit chain deletion that
> also uses tcf_chanin_flush():
>
> RTM_DELCHAIN RTM_NEWTFILTER
> + +
> | |
> | +----------v-----------+
> | | |
> | | __tcf_block_find |
> | | |
> | +----------+-----------+
> | |
> | |
> | +----------v------------+
> | | |
> | | tcf_chain_get |
> | | |
> | +----------+------------+
> | |
> +--------v--------+ |
> | | |
> | tcf_chain_flush | |
> | | |
> +--------+--------+ |
> | |
> | +----------v------------+
> | | |
> | | tcf_chain_tp_find |
> | | |
> | +----------+------------+
> | |
> | |tp==NULL
> | |chain->flushing==false
> | |
> | +---------------v----------------+
> | | |
> | | tp_created = 1 |
> | | tcf_chain_tp_insert_unique |
> | | |
> | +---------------+----------------+
> | |
> | |
> +---------------v-----------------+ |
> | | |
> |tcf_chain_put_explicitly_created | |
> | | |
> +---------------+-----------------+ |
> | |
> v v
>
> In this example tc_new_tfilter() holds chain reference during flush. If
> flush finishes concurrently before the check for chain->flushing, the
> chain reference counter will not reach 0 (because new filter creation
> code will not back off and release the reference). In the described
> example tc_chain_notify_delete() will not be called which will confuse
> any userland code that expects to receive delete chain notification
> after sending RTM_DELCHAIN message.
>
> With these considerations I can propose following approach to fix the
> issue:
>
> 1. Extend tcf_chain_flush() with additional boolean argument and only
> call it with 'true' value from tc_del_tfilter(). (or create helper
> function that calls tcf_chain_flush() and then resets chain->flushing
> flag)
>
> 2. Reset the 'flushing' flag when new argument is true.
>
> 3. Wrap the 'flushing' flag reset code in filter_chain_lock critical
> section.
>
> > }
> >
>
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