CVE-2026-46319

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 [email protected]:

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]