GHSA-696W-35HF-M34Q
Vulnerability from github – Published: 2026-08-22 18:30 – Updated: 2026-08-25 06:31In 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:
- Both threads enter tc_new_tfilter, both find the chain empty, both drop filter_chain_lock
- u32 finishes tcf_proto_create("u32") first, calls tcf_chain_tp_insert_unique() -> inserts u32_tp into the chain
- 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.
{
"affected": [],
"aliases": [
"CVE-2026-74700"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-08-22T16:16:44Z",
"severity": "HIGH"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: cls_api: Always acquire rtnl_lock when destroying locked classifiers\n\nAnother challenge with unlocked filters.\nThere is a short window in tc_new_tfilter where a tcf_proto can be found\nand briefly referenced by a totally unrelated, unlocked classifier\u0027s request\nand cause a race.\n\nFeng created a poc which created this race with two threads, one creating a\nu32 filter and other a flower filter in the same chain/prio:\n\n1. Both threads enter tc_new_tfilter, both find the chain empty, both\n drop filter_chain_lock\n2. u32 finishes tcf_proto_create(\"u32\") first, calls\n tcf_chain_tp_insert_unique() -\u003e inserts u32_tp into the chain\n3. flower finishes tcf_proto_create(\"flower\") later, calls\n tcf_chain_tp_insert_unique() -\u003e tcf_chain_tp_find() now sees u32_tp\n already there, takes a reference on it, destroys flower\u0027s own tp_new\n and returns u32_tp to the caller.\n\nFlower then hits the kind mismatch check (because it requested for kind\n\"flower\" but tp-\u003eops-\u003ekind is \"u32\") and goes through the errout path\nwhich calls tcf_proto_put() on u32_tp. If the u32 thread has already\ngone through its own errout (its change() call failed on the PoC\u0027s empty\noptions) and dropped its create and insert refs, flower\u0027s put is the\nlast one and drops u32_tp\u0027s refcnt to zero.\n\nAt this point tp-\u003eops-\u003edestroy() runs in a context that never took\nrtnl_lock. When that happens, it might cause a UAF like the following\n(illustrated by the PoC):\n\n[ +0.000710] BUG: KASAN: slab-use-after-free in u32_init (net/sched/cls_u32.c:393)\n[ +0.000281] Read of size 8 at addr ffff888120022f00 by task poc_feng_xue/524\n\n Call Trace:\n u32_init (net/sched/cls_u32.c:393)\n tc_new_tfilter (net/sched/cls_api.c:2378)\n\n Allocated by task 526:\n u32_init (net/sched/cls_u32.c:378)\n tc_new_tfilter (net/sched/cls_api.c:2378)\n\n Freed by task 522:\n kfree\n u32_destroy (net/sched/cls_u32.c:662)\n tcf_proto_destroy (net/sched/cls_api.c:446)\n tcf_proto_put (net/sched/cls_api.c:459)\n tc_new_tfilter (net/sched/cls_api.c:2459)\n\nFix this by having tcf_proto_destroy() take rtnl_lock around\ntp-\u003eops-\u003edestroy() for locked classifiers whenever rtnl is not held.\n\nTo explain why I used a temp variable \"not_lockless\" I\u0027d like to point to a\nsemi-related note on rtnl_held vs TCF_PROTO_OPS_DOIT_UNLOCKED (adding here\nfor future cleanup if deemed necessary):\nThe rtnl_held parameter and the TCF_PROTO_OPS_DOIT_UNLOCKED flag are\nredundant sources of truth for whether rtnl_lock is held. Among the nine\nclassifier destroy(..rtnl_held..) callbacks, only flower consults the\nrtnl_held parameter which it propagates to tc_setup_cb_destroy()\nand tc_setup_cb_call(). The other eight (u32, flow, bpf, cgroup, route, basic,\nfw, mall) ignore it entirely;-\u003e those that call tc_setup_cb_destroy()\n(u32, bpf, mall) hardcode true always instead of forwarding the parameter.\n\nA future cleanup should remove the rtnl_held parameter from the destroy callback\nsignature entirely and have callers rely solely on their knowledge whether\nthey are running in an unlocked context.",
"id": "GHSA-696w-35hf-m34q",
"modified": "2026-08-25T06:31:28Z",
"published": "2026-08-22T18:30:29Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-74700"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/34e77d8e3570f9df3952496ddb402833695662fd"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/a347304b2ca1a5377d5bd2d8a72e4b4f12afe648"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/a81f9c44d87fb59d99fce72e29e02cab3a49a1c3"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/b648c8a56531aeabd1c14f6b5cf1891e269b3756"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/d6222af7274f08e7a1848131dc30319993d8f377"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Confirmed: The vulnerability has been validated from an analyst's perspective.
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- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.