FKIE_CVE-2026-74700
Vulnerability from fkie_nvd - Published: 2026-08-22 16:16 - Updated: 2026-10-14 13:10
Severity
Summary
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.
References
Impacted products
| Vendor | Product | Version |
|---|
{
"affected": [
{
"affectedData": [
{
"defaultStatus": "unaffected",
"product": "Linux",
"programFiles": [
"net/sched/cls_api.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"lessThan": "34e77d8e3570f9df3952496ddb402833695662fd",
"status": "affected",
"version": "12db03b65c2b90752e4c37666977fd4a1b5f5824",
"versionType": "git"
},
{
"lessThan": "b648c8a56531aeabd1c14f6b5cf1891e269b3756",
"status": "affected",
"version": "12db03b65c2b90752e4c37666977fd4a1b5f5824",
"versionType": "git"
},
{
"lessThan": "d6222af7274f08e7a1848131dc30319993d8f377",
"status": "affected",
"version": "12db03b65c2b90752e4c37666977fd4a1b5f5824",
"versionType": "git"
},
{
"lessThan": "a81f9c44d87fb59d99fce72e29e02cab3a49a1c3",
"status": "affected",
"version": "12db03b65c2b90752e4c37666977fd4a1b5f5824",
"versionType": "git"
},
{
"lessThan": "a347304b2ca1a5377d5bd2d8a72e4b4f12afe648",
"status": "affected",
"version": "12db03b65c2b90752e4c37666977fd4a1b5f5824",
"versionType": "git"
}
]
},
{
"defaultStatus": "affected",
"product": "Linux",
"programFiles": [
"net/sched/cls_api.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"status": "affected",
"version": "5.1"
},
{
"lessThan": "5.1",
"status": "unaffected",
"version": "0",
"versionType": "semver"
},
{
"lessThanOrEqual": "6.6.*",
"status": "unaffected",
"version": "6.6.152",
"versionType": "semver"
},
{
"lessThanOrEqual": "6.12.*",
"status": "unaffected",
"version": "6.12.104",
"versionType": "semver"
},
{
"lessThanOrEqual": "6.18.*",
"status": "unaffected",
"version": "6.18.45",
"versionType": "semver"
},
{
"lessThanOrEqual": "7.1.*",
"status": "unaffected",
"version": "7.1.9",
"versionType": "semver"
},
{
"lessThanOrEqual": "*",
"status": "unaffected",
"version": "7.2",
"versionType": "original_commit_for_fix"
}
]
}
],
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}
],
"cveTags": [],
"descriptions": [
{
"lang": "en",
"value": "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."
},
{
"lang": "es",
"value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nnet/sched: cls_api: Adquirir siempre rtnl_lock al destruir clasificadores bloqueados\n\nOtro desaf\u00edo con filtros desbloqueados.\nExiste una ventana corta en tc_new_tfilter donde un tcf_proto puede ser encontrado y brevemente referenciado por la solicitud de un clasificador totalmente no relacionado y desbloqueado, y causar una condici\u00f3n de carrera.\n\nFeng cre\u00f3 una PoC que gener\u00f3 esta condici\u00f3n de carrera con dos hilos, uno creando un filtro u32 y el otro un filtro flower en la misma cadena/prioridad:\n\n1. Ambos hilos entran en tc_new_tfilter, ambos encuentran la cadena vac\u00eda, ambos sueltan filter_chain_lock\n2. u32 termina tcf_proto_create(\u0027u32\u0027) primero, llama a tcf_chain_tp_insert_unique() -\u003e inserta u32_tp en la cadena\n3. flower termina tcf_proto_create(\u0027flower\u0027) m\u00e1s tarde, llama a tcf_chain_tp_insert_unique() -\u003e tcf_chain_tp_find() ahora ve u32_tp ya all\u00ed, toma una referencia sobre \u00e9l, destruye el propio tp_new de flower y devuelve u32_tp al llamador.\n\nFlower entonces encuentra la verificaci\u00f3n de tipo no coincidente (porque solicit\u00f3 el tipo \u0027flower\u0027 pero tp -\u003e ops -\u003e kind es \u0027u32\u0027) y pasa por la ruta de error que llama a tcf_proto_put() en u32_tp. Si el hilo u32 ya ha pasado por su propio error (su llamada a change() fall\u00f3 en las opciones vac\u00edas de la PoC) y ha soltado sus referencias de creaci\u00f3n e inserci\u00f3n, el put de flower es el \u00faltimo y reduce el refcnt de u32_tp a cero.\n\nEn este punto, tp -\u003e ops -\u003e destroy() se ejecuta en un contexto que nunca tom\u00f3 rtnl_lock. Cuando eso sucede, podr\u00eda causar un UAF como el siguiente (ilustrado por la PoC):\n\n[ +0.000710] BUG: KASAN: uso despu\u00e9s de liberaci\u00f3n de slab en u32_init (net/sched/cls_u32.c:393)\n[ +0.000281] Lectura de tama\u00f1o 8 en la direcci\u00f3n ffff888120022f00 por la tarea poc_feng_xue/524\n\n Rastro de Llamada:\n u32_init (net/sched/cls_u32.c:393)\n tc_new_tfilter (net/sched/cls_api.c:2378)\n\n Asignado por la tarea 526:\n u32_init (net/sched/cls_u32.c:378)\n tc_new_tfilter (net/sched/cls_api.c:2378)\n\n Liberado por la tarea 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\nSolucione esto haciendo que tcf_proto_destroy() tome rtnl_lock alrededor de tp -\u003e ops -\u003e destroy() para clasificadores bloqueados siempre que rtnl no est\u00e9 retenido.\n\nPara explicar por qu\u00e9 us\u00e9 una variable temporal \u0027not_lockless\u0027 me gustar\u00eda se\u00f1alar una nota semi-relacionada sobre rtnl_held vs TCF_PROTO_OPS_DOIT_UNLOCKED (a\u00f1adiendo aqu\u00ed para una futura limpieza si se considera necesario):\nEl par\u00e1metro rtnl_held y la bandera TCF_PROTO_OPS_DOIT_UNLOCKED son fuentes de verdad redundantes sobre si rtnl_lock est\u00e1 retenido. Entre las nueve devoluciones de llamada destroy(..rtnl_held..) del clasificador, solo flower consulta el par\u00e1metro rtnl_held, el cual propaga a tc_setup_cb_destroy() y tc_setup_cb_call(). Los otros ocho (u32, flow, bpf, cgroup, route, basic, fw, mall) lo ignoran por completo; -\u003e aquellos que llaman a tc_setup_cb_destroy() (u32, bpf, mall) codifican \u0027true\u0027 siempre en lugar de reenviar el par\u00e1metro.\n\nUna futura limpieza deber\u00eda eliminar el par\u00e1metro rtnl_held de la firma de la devoluci\u00f3n de llamada destroy por completo y hacer que los llamadores conf\u00eden \u00fanicamente en su conocimiento sobre si se est\u00e1n ejecutando en un contexto desbloqueado."
}
],
"id": "CVE-2026-74700",
"lastModified": "2026-10-14T13:10:00.187",
"metrics": {
"cvssMetricV31": [
{
"cvssData": {
"attackComplexity": "LOW",
"attackVector": "LOCAL",
"availabilityImpact": "HIGH",
"baseScore": 7.8,
"baseSeverity": "HIGH",
"confidentialityImpact": "HIGH",
"integrityImpact": "HIGH",
"privilegesRequired": "LOW",
"scope": "UNCHANGED",
"userInteraction": "NONE",
"vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"version": "3.1"
},
"exploitabilityScore": 1.8,
"impactScore": 5.9,
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"type": "Secondary"
}
]
},
"published": "2026-08-22T16:16:44.610",
"references": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/34e77d8e3570f9df3952496ddb402833695662fd"
},
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/a347304b2ca1a5377d5bd2d8a72e4b4f12afe648"
},
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/a81f9c44d87fb59d99fce72e29e02cab3a49a1c3"
},
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/b648c8a56531aeabd1c14f6b5cf1891e269b3756"
},
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/d6222af7274f08e7a1848131dc30319993d8f377"
}
],
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"vulnStatus": "Received"
}
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Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
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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.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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