CVE-2026-53264

About vulnerability

In the Linux kernel, the following vulnerability has been resolved:

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

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 d7fb60b9cafb (“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.

Affected product:

AlmaLinux 9.2 ESU , CentOS 7 ELS , CentOS 8.4 ELS , CentOS 8.5 ELS , CentOS Stream 8 ELS , TuxCare 9.6 ESU

Affected packages:

kernel @ 4.18.0 (+5 more)

Live-patch status from KernelCare for each operating system.