Re: [PATCH net] sched: fix infinite loop when creating tc filter

["Volodymyr Mytnyk [C]" <vmytnyk@marvell.com>]

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@plvision.eu> wrote:
> > From: Volodymyr Mytnyk <vmytnyk@marvell.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@plvision.eu>
> > Signed-off-by: Serhiy Boiko <serhiy.boiko@plvision.eu>
> > Signed-off-by: Volodymyr Mytnyk <vmytnyk@marvell.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.
> 
> >  }
> >  
>