net/sched: act_api: use RCU with deferred freeing for action lifecycle

jira VULN-XXXX
cve CVE-2026-53264
commit-author Jamal Hadi Salim <jhs@mojatatu.com>
commit 5057e1a

When NEWTFILTER and DELFILTER are run concurrently it is possible to create a
race with an associated action.

Let's illustrate with CPU0 running NEWTFILTER and CPU1 running DELFILTER:

 0: mutex_lock() <-- holds the idr lock
 0: rcu_read_lock()
 0: p = idr_find(idr, index) <-- action p is valid (RCU protects IDR)
 0: mutex_unlock() <-- releases the idr lock
 1: refcount_dec_and_mutex_lock() <-- refcnt 1->0, mutex held
 1: idr_remove(idr, index) <-- Action removed from IDR
 1: mutex_unlock() <-- mutex released allowing us to delete the action
 1: tcf_action_cleanup(p); kfree(p) <-- Kfrees p immediately, no deferral
 0: refcount_inc_not_zero(&p->tcfa_refcnt) <-- ouch, UAF p points to freed memory

This patch fixes the race condition between NEWTFILTER and DELFILTER by
adding struct rcu_head to tc_action used in the deferral and introducing a
call_rcu() in the delete path to defer the final kfree().

Note: this is a revert of commit d7fb60b ("net_sched: get rid of tcfa_rcu")
but also modernization/simplification to directly use kfree_rcu().

Let's illustrate the new restored code path:

 0: rcu_read_lock()
 1: refcount_dec_and_mutex_lock() <-- refcnt 1->0, mutex held
 1: idr_remove(idr, index)
 1: mutex_unlock()
 1: call_rcu(&p->tcfa_rcu, tcf_action_rcu_free) <-- defer kfree after grace period
 0: p = idr_find(idr, index)
 0: refcount_inc_not_zero(&p->tcfa_refcnt) <-- fails, refcnt already 0
 1: rcu_read_unlock() <-- release so freeing can run after grace period

After CPU1 calls idr_remove(), the object is no longer reachable through the IDR.
CPU0's subsequent idr_find() will return NULL, and even if it still held a
stale pointer, the immediate kfree() is now deferred until after the RCU grace
period, so no UAF can occur.

Fixes: d7fb60b ("net_sched: get rid of tcfa_rcu")
	Suggested-by: Jakub Kicinski <kuba@kernel.org>
	Reported-by: Kyle Zeng <kylebot@openai.com>
	Tested-by: Victor Nogueira <victor@mojatatu.com>
	Tested-by: syzbot@syzkaller.appspotmail.com
	Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
	Tested-by: Kyle Zeng <kylebot@openai.com>
	Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
	Reviewed-by: Eric Dumazet <edumazet@google.com>
	Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20260531160812.68020-1-jhs@mojatatu.com
	Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit 5057e1a)
	Signed-off-by: Brett Mastbergen <bmastbergen@ciq.com>

Author: bmastbergen

include/net/act_api.h

@@ -41,6 +41,7 @@ struct tc_action {
 	struct tc_cookie	__rcu *act_cookie;
 	struct tcf_chain	__rcu *goto_chain;
 	u32			tcfa_flags;
+	struct rcu_head         tcfa_rcu;
 	u8			hw_stats;
 	u8			used_hw_stats;
 	bool			used_hw_stats_valid;

net/sched/act_api.c

@@ -115,11 +115,6 @@ struct tcf_chain *tcf_action_set_ctrlact(struct tc_action *a, int action,
 }
 EXPORT_SYMBOL(tcf_action_set_ctrlact);
 
-/* XXX: For standalone actions, we don't need a RCU grace period either, because
- * actions are always connected to filters and filters are already destroyed in
- * RCU callbacks, so after a RCU grace period actions are already disconnected
- * from filters. Readers later can not find us.
- */
 static void free_tcf(struct tc_action *p)
 {
 	struct tcf_chain *chain = rcu_dereference_protected(p->goto_chain, 1);
@@ -132,7 +127,7 @@ static void free_tcf(struct tc_action *p)
 	if (chain)
 		tcf_chain_put_by_act(chain);
 
-	kfree(p);
+	kfree_rcu(p, tcfa_rcu);
 }
 
 static void tcf_action_cleanup(struct tc_action *p)