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Date:   Wed, 13 Oct 2021 23:09:59 +0200
From:   Paul Menzel <>
To:     Jamal Hadi Salim <>,
        Cong Wang <>,
        Jiri Pirko <>,
        "David S. Miller" <>,
        Jakub Kicinski <>
Cc:     Serhiy Boiko <>,
        Paul Menzel <>,,
Subject: [PATCH] net: sched: fix infinite loop when trying to create tcf rule

From: Serhiy Boiko <>

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.

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.

Signed-off-by: Serhiy Boiko <>
[Upstreamed from]
Signed-off-by: Paul Menzel <>
 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 2ef8f5a6205a..405f955bef1e 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. */
+	chain->flushing = false;
 static int tcf_block_setup(struct tcf_block *block,

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