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

Affected: Linux

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…

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