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Message-ID: <ygnhbl3vbrto.fsf@nvidia.com>
Date: Mon, 11 Oct 2021 16:42:59 +0300
From: Vlad Buslov <vladbu@...dia.com>
To: Volodymyr Mytnyk <volodymyr.mytnyk@...ision.eu>
CC: <netdev@...r.kernel.org>, Volodymyr Mytnyk <vmytnyk@...vell.com>,
"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>
Subject: Re: [PATCH net] sched: fix infinite loop when creating tc filter
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.
> }
>
> static int tcf_block_setup(struct tcf_block *block,
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