net/sched: act_ct: Only release RCU read lock after ct_ft

Summary

In the Linux kernel, the following vulnerability has been resolved: net/sched: act_ct: Only release RCU read lock after ct_ft When looking up a flow table in act_ct in tcf_ct_flow_table_get(), rhashtable_lookup_fast() internally opens and closes an RCU read critical section before returning ct_ft. The tcf_ct_flow_table_cleanup_work() can complete before refcount_inc_not_zero() is invoked on the returned ct_ft resulting in a UAF on the already freed ct_ft object. This vulnerability can lead to privilege escalation. Analysis from zdi-disclosures@trendmicro.com: When initializing act_ct, tcf_ct_init() is called, which internally triggers tcf_ct_flow_table_get(). static int tcf_ct_flow_table_get(struct net *net, struct tcf_ct_params *params) { struct zones_ht_key key = { .net = net, .zone = params->zone }; struct tcf_ct_flow_table *ct_ft; int err = -ENOMEM; mutex_lock(&zones_mutex); ct_ft = rhashtable_lookup_fast(&zones_ht, &key, zones_params); // [1] if (ct_ft && refcount_inc_not_zero(&ct_ft->ref)) // [2] goto out_unlock; ... } static __always_inline void *rhashtable_lookup_fast( struct rhashtable *ht, const void *key, const struct rhashtable_params params) { void *obj; rcu_read_lock(); obj = rhashtable_lookup(ht, key, params); rcu_read_unlock(); return obj; } At [1], rhashtable_lookup_fast() looks up and returns the corresponding ct_ft from zones_ht . The lookup is performed within an RCU read critical section through rcu_read_lock() / rcu_read_unlock(), which prevents the object from being freed. However, at the point of function return, rcu_read_unlock() has already been called, and there is nothing preventing ct_ft from being freed before reaching refcount_inc_not_zero(&ct_ft->ref) at [2]. This interval becomes the race window, during which ct_ft can be freed. Free Process: tcf_ct_flow_table_put() is executed through the path tcf_ct_cleanup() call_rcu() tcf_ct_params_free_rcu() tcf_ct_params_free() tcf_ct_flow_table_put(). static void tcf_ct_flow_table_put(struct tcf_ct_flow_table *ct_ft) { if (refcount_dec_and_test(&ct_ft->ref)) { rhashtable_remove_fast(&zones_ht, &ct_ft->node, zones_params); INIT_RCU_WORK(&ct_ft->rwork, tcf_ct_flow_table_cleanup_work); // [3] queue_rcu_work(act_ct_wq, &ct_ft->rwork); } } At [3], tcf_ct_flow_table_cleanup_work() is scheduled as RCU work static void tcf_ct_flow_table_cleanup_work(struct work_struct *work) { struct tcf_ct_flow_table *ct_ft; struct flow_block *block; ct_ft = container_of(to_rcu_work(work), struct tcf_ct_flow_table, rwork); nf_flow_table_free(&ct_ft->nf_ft); block = &ct_ft->nf_ft.flow_block; down_write(&ct_ft->nf_ft.flow_block_lock); WARN_ON(!list_empty(&block->cb_list)); up_write(&ct_ft->nf_ft.flow_block_lock); kfree(ct_ft); // [4] module_put(THIS_MODULE); } tcf_ct_flow_table_cleanup_work() frees ct_ft at [4]. When this function executes between [1] and [2], UAF occurs. This race condition has a very short race window, making it generally difficult to trigger. Therefore, to trigger the vulnerability an msleep(100) was inserted after[1]

Timeline

1. Jun 9, 2026, 12:11 PM UTCCVE published
2. Oct 16, 2026, 3:21 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Oct 19, 2026, 7:36 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 138470a9b2cc2e26e6018300394afc3858a54e6a · fixed in ece578ca61e572df96cfc80456357ebfae0b4b9e; From 138470a9b2cc2e26e6018300394afc3858a54e6a · fixed in a2e0c045c87aa252eb61412e67dd91f2c2b19f81; From 138470a9b2cc2e26e6018300394afc3858a54e6a · fixed in 67c9ecc9f2575273ed1323e312881fc98ac83d6d; From 138470a9b2cc2e26e6018300394afc3858a54e6a · fixed in f23424a0ddadb494d4bd57056a7ca703312d3a7b; From 138470a9b2cc2e26e6018300394afc3858a54e6a · fixed in 17dfb67cb399b660105d9a8c6100851c0d0cdc70; From 138470a9b2cc2e26e6018300394afc3858a54e6a · fixed in 4c727c6967a41b37efe0f26332ca9ec5b74785a3; From 138470a9b2cc2e26e6018300394afc3858a54e6a · fixed in 3e20e1b3058e0b94638e7b931c138e840e266724; From 138470a9b2cc2e26e6018300394afc3858a54e6a · fixed in f462dca0c8415bf0058d0ffa476354c4476d0f09

• Linux Linux

  5.7

• KernelLinux

  From 5.7.0 · fixed in 5.10.258; From 5.11.0 · fixed in 5.15.209; From 5.16.0 · fixed in 6.1.175; From 6.2.0 · fixed in 6.6.141; From 6.7.0 · fixed in 6.12.91; From 6.13.0 · fixed in 6.18.33; From 6.19.0 · fixed in 7.0.10

References 8