CVE-2026-74700 (GCVE-0-2026-74700)
Vulnerability from cvelistv5
Published
2026-08-22 15:33
Modified
2026-08-25 05:41
Severity ?
VLAI Severity ?
EPSS score ?
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
{
"containers": {
"cna": {
"affected": [
{
"defaultStatus": "unaffected",
"product": "Linux",
"programFiles": [
"net/sched/cls_api.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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{
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},
{
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{
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"versionType": "git"
}
]
},
{
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"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",
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},
{
"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"
}
]
}
],
"cpeApplicability": [
{
"nodes": [
{
"cpeMatch": [
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "6.6.152",
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"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
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{
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{
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"negate": false,
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],
"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."
}
],
"metrics": [
{
"cvssV3_1": {
"baseScore": 7.8,
"baseSeverity": "HIGH",
"vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"version": "3.1"
},
"scenarios": [
{
"lang": "en",
"value": "AV:L - The bug is reached only via RTM_NEWTFILTER/rtnetlink from a local process (tc filter add); tc_new_tfilter() is a TC configuration path, not packet receive or any remote network service, and per kernel CNA guidance tc/netlink qdisc/classifier operations are Local.\nAC:L - A PoC triggers this with two attacker-controlled threads concurrently creating u32 and flower filters on the same chain/priority; the attacker creates and wins the tcf_chain_tp_insert_unique race without uncontrollable victim timing or memory layout.\nPR:L - rtnetlink enforces CAP_NET_ADMIN via netlink_net_capable() before tc_new_tfilter(); this capability is routinely obtainable in an unprivileged-created user+network namespace (unshare -Urn) and is commonly delegated to containers with NET_ADMIN.\nUI:N - Exploitation requires only the attacker\u0027s own concurrent tc/netlink operations to race u32 and flower filter creation; no victim action such as opening a file or mounting a filesystem is needed.\nS:U - The UAF corrupts kernel TC classifier state (tc_u_common hash/hlist) during cls_api teardown on the same host kernel; this is standard kernel memory corruption, not a VM escape or cross-security-authority boundary.\nC:H - Concurrent u32_destroy() without rtnl_lock frees tp_c while another thread\u0027s u32_init() reads tp_c-\u003ehlist (cls_u32.c:393); KASAN-confirmed slab use-after-free that per kernel guidance enables arbitrary kernel memory disclosure.\nI:H - The race frees and reuses tc_u_common/tcf_proto structures without rtnl synchronization, corrupting shared classifier hash lists; UAF on these heap objects enables heap spraying and arbitrary kernel write or code execution primitives.\nA:H - The PoC triggers KASAN slab-use-after-free in u32_init(); UAF on core net/sched classifier teardown can cause kernel oops/panic/hang, denying system availability until reboot."
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-08-25T05:41:42.546Z",
"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"shortName": "Linux"
},
"references": [
{
"url": "https://git.kernel.org/stable/c/34e77d8e3570f9df3952496ddb402833695662fd"
},
{
"url": "https://git.kernel.org/stable/c/b648c8a56531aeabd1c14f6b5cf1891e269b3756"
},
{
"url": "https://git.kernel.org/stable/c/d6222af7274f08e7a1848131dc30319993d8f377"
},
{
"url": "https://git.kernel.org/stable/c/a81f9c44d87fb59d99fce72e29e02cab3a49a1c3"
},
{
"url": "https://git.kernel.org/stable/c/a347304b2ca1a5377d5bd2d8a72e4b4f12afe648"
}
],
"title": "net/sched: cls_api: Always acquire rtnl_lock when destroying locked classifiers",
"x_generator": {
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}
}
},
"cveMetadata": {
"assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"assignerShortName": "Linux",
"cveId": "CVE-2026-74700",
"datePublished": "2026-08-22T15:33:00.933Z",
"dateReserved": "2026-08-15T05:44:03.927Z",
"dateUpdated": "2026-08-25T05:41:42.546Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
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Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.
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