CVE-2026-46319: net/sched: act_ct: Only release RCU read lock after ct_ft

CVE-2026-46319: net/sched: act_ct: Only release RCU read lock after ct_ft

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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]

The Linux kernel CVE team has assigned CVE-2026-46319 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.7 with commit 138470a9b2cc2e26e6018300394afc3858a54e6a and fixed in 5.10.258 with commit ece578ca61e572df96cfc80456357ebfae0b4b9e
	Issue introduced in 5.7 with commit 138470a9b2cc2e26e6018300394afc3858a54e6a and fixed in 5.15.209 with commit a2e0c045c87aa252eb61412e67dd91f2c2b19f81
	Issue introduced in 5.7 with commit 138470a9b2cc2e26e6018300394afc3858a54e6a and fixed in 6.1.175 with commit 67c9ecc9f2575273ed1323e312881fc98ac83d6d
	Issue introduced in 5.7 with commit 138470a9b2cc2e26e6018300394afc3858a54e6a and fixed in 6.6.141 with commit f23424a0ddadb494d4bd57056a7ca703312d3a7b
	Issue introduced in 5.7 with commit 138470a9b2cc2e26e6018300394afc3858a54e6a and fixed in 6.12.91 with commit 17dfb67cb399b660105d9a8c6100851c0d0cdc70
	Issue introduced in 5.7 with commit 138470a9b2cc2e26e6018300394afc3858a54e6a and fixed in 6.18.33 with commit 4c727c6967a41b37efe0f26332ca9ec5b74785a3
	Issue introduced in 5.7 with commit 138470a9b2cc2e26e6018300394afc3858a54e6a and fixed in 7.0.10 with commit 3e20e1b3058e0b94638e7b931c138e840e266724
	Issue introduced in 5.7 with commit 138470a9b2cc2e26e6018300394afc3858a54e6a and fixed in 7.1-rc1 with commit f462dca0c8415bf0058d0ffa476354c4476d0f09

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-46319
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/sched/act_ct.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/ece578ca61e572df96cfc80456357ebfae0b4b9e
	https://git.kernel.org/stable/c/a2e0c045c87aa252eb61412e67dd91f2c2b19f81
	https://git.kernel.org/stable/c/67c9ecc9f2575273ed1323e312881fc98ac83d6d
	https://git.kernel.org/stable/c/f23424a0ddadb494d4bd57056a7ca703312d3a7b
	https://git.kernel.org/stable/c/17dfb67cb399b660105d9a8c6100851c0d0cdc70
	https://git.kernel.org/stable/c/4c727c6967a41b37efe0f26332ca9ec5b74785a3
	https://git.kernel.org/stable/c/3e20e1b3058e0b94638e7b931c138e840e266724
	https://git.kernel.org/stable/c/f462dca0c8415bf0058d0ffa476354c4476d0f09