CVE-2026-74700: net/sched: cls_api: Always acquire rtnl_lock when destroying locked classifiers

From: Greg Kroah-Hartman <[email protected]>

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

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

net/sched: cls_api: Always acquire rtnl_lock when destroying locked classifiers

Another challenge with unlocked filters.
There is a short window in tc_new_tfilter where a tcf_proto can be found
and briefly referenced by a totally unrelated, unlocked classifier's request
and cause a race.

Feng created a poc which created this race with two threads, one creating a
u32 filter and other a flower filter in the same chain/prio:

1. Both threads enter tc_new_tfilter, both find the chain empty, both
   drop filter_chain_lock
2. u32 finishes tcf_proto_create("u32") first, calls
   tcf_chain_tp_insert_unique() -> inserts u32_tp into the chain
3. flower finishes tcf_proto_create("flower") later, calls
   tcf_chain_tp_insert_unique() -> tcf_chain_tp_find() now sees u32_tp
   already there, takes a reference on it, destroys flower's own tp_new
   and returns u32_tp to the caller.

Flower then hits the kind mismatch check (because it requested for kind
"flower" but tp->ops->kind is "u32") and goes through the errout path
which calls tcf_proto_put() on u32_tp. If the u32 thread has already
gone through its own errout (its change() call failed on the PoC's empty
options) and dropped its create and insert refs, flower's put is the
last one and drops u32_tp's refcnt to zero.

At this point tp->ops->destroy() runs in a context that never took
rtnl_lock. When that happens, it might cause a UAF like the following
(illustrated by the PoC):

[  +0.000710] BUG: KASAN: slab-use-after-free in u32_init (net/sched/cls_u32.c:393)
[  +0.000281] Read of size 8 at addr ffff888120022f00 by task poc_feng_xue/524

  Call Trace:
   u32_init (net/sched/cls_u32.c:393)
   tc_new_tfilter (net/sched/cls_api.c:2378)

  Allocated by task 526:
   u32_init (net/sched/cls_u32.c:378)
   tc_new_tfilter (net/sched/cls_api.c:2378)

  Freed by task 522:
   kfree
   u32_destroy (net/sched/cls_u32.c:662)
   tcf_proto_destroy (net/sched/cls_api.c:446)
   tcf_proto_put (net/sched/cls_api.c:459)
   tc_new_tfilter (net/sched/cls_api.c:2459)

Fix this by having tcf_proto_destroy() take rtnl_lock around
tp->ops->destroy() for locked classifiers whenever rtnl is not held.

To explain why I used a temp variable "not_lockless" I'd like to point to a
semi-related note on rtnl_held vs TCF_PROTO_OPS_DOIT_UNLOCKED (adding here
for future cleanup if deemed necessary):
The rtnl_held parameter and the TCF_PROTO_OPS_DOIT_UNLOCKED flag are
redundant sources of truth for whether rtnl_lock is held. Among the nine
classifier destroy(..rtnl_held..) callbacks, only flower consults the
rtnl_held parameter which it propagates to tc_setup_cb_destroy()
and tc_setup_cb_call(). The other eight (u32, flow, bpf, cgroup, route, basic,
fw, mall) ignore it entirely;-> those that call tc_setup_cb_destroy()
(u32, bpf, mall) hardcode true always instead of forwarding the parameter.

A future cleanup should remove the rtnl_held parameter from the destroy callback
signature entirely and have callers rely solely on their knowledge whether
they are running in an unlocked context.

The Linux kernel CVE team has assigned CVE-2026-74700 to this issue.


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

	Issue introduced in 5.1 with commit 12db03b65c2b90752e4c37666977fd4a1b5f5824 and fixed in 6.6.152 with commit 34e77d8e3570f9df3952496ddb402833695662fd
	Issue introduced in 5.1 with commit 12db03b65c2b90752e4c37666977fd4a1b5f5824 and fixed in 6.12.104 with commit b648c8a56531aeabd1c14f6b5cf1891e269b3756
	Issue introduced in 5.1 with commit 12db03b65c2b90752e4c37666977fd4a1b5f5824 and fixed in 6.18.45 with commit d6222af7274f08e7a1848131dc30319993d8f377
	Issue introduced in 5.1 with commit 12db03b65c2b90752e4c37666977fd4a1b5f5824 and fixed in 7.1.9 with commit a81f9c44d87fb59d99fce72e29e02cab3a49a1c3
	Issue introduced in 5.1 with commit 12db03b65c2b90752e4c37666977fd4a1b5f5824 and fixed in 7.2 with commit a347304b2ca1a5377d5bd2d8a72e4b4f12afe648

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-2026-74700
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/cls_api.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/34e77d8e3570f9df3952496ddb402833695662fd
	https://git.kernel.org/stable/c/b648c8a56531aeabd1c14f6b5cf1891e269b3756
	https://git.kernel.org/stable/c/d6222af7274f08e7a1848131dc30319993d8f377
	https://git.kernel.org/stable/c/a81f9c44d87fb59d99fce72e29e02cab3a49a1c3
	https://git.kernel.org/stable/c/a347304b2ca1a5377d5bd2d8a72e4b4f12afe648