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 [off-list ref] wrote:

quoted

From: Volodymyr Mytnyk <redacted>

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!

quoted

Co-developed-by: Serhiy Boiko <redacted>
Signed-off-by: Serhiy Boiko <redacted>
Signed-off-by: Volodymyr Mytnyk <redacted>
---
 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?

quoted

+     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.

quoted

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