CVE-2026-53264

Description

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.

Predictions

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

References

• https://git.kernel.org/stable/c/98b2e40879abf0245be5a5b7af69e0f6ff524ac3
• https://git.kernel.org/stable/c/18af5d2ef0c4f65787fd1280c8b23286b9f2a835
• https://git.kernel.org/stable/c/1f1b98fea6b9ea30507d0f2fbff6750292d097e2
• https://git.kernel.org/stable/c/8b136f18ac4b2ace5aaad3305b3f8a5d8165a009
• https://git.kernel.org/stable/c/5dd51e09020c65aa53cf128e5e3517cd53b3c113
• https://git.kernel.org/stable/c/b60e9391142e983fab2be53497aa8f71fdd09cd5
• https://git.kernel.org/stable/c/91d105d2cbe002f9c7b43a6183adedc37e1da1f7
• https://git.kernel.org/stable/c/5057e1aca011e51ef51498c940ef96f3d3e8a305
• https://security-tracker.debian.org/tracker/CVE-2026-53264