CVE-2026-74583 (GCVE-0-2026-74583)
Vulnerability from cvelistv5
Published
2026-08-21 16:31
Modified
2026-08-25 05:40
Severity ?
VLAI Severity ?
EPSS score ?
Summary
In the Linux kernel, the following vulnerability has been resolved:
net/sched: cls_route: fix fastmap use-after-free on filter
The route4 classifier maintains a 16-slot fastmap cache that stores raw
struct route4_filter pointers indexed by (id, iif). The reader
(route4_classify) populates this cache via route4_set_fastmap() for every
classified packet that hits a filter. The writer (route4_delete,
route4_change) clears the cache via route4_reset_fastmap() before
RCU-deferred kfree of the filter.
This creates a UAF race:
1. Reader walks the RCU-protected bucket chain, finds filter f
2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work()
3. Reader calls route4_set_fastmap() and writes f into the cache
*after* the writer's reset, caching a pointer about to be freed
4. After the RCU grace period, kfree(f) executes
5. Next classified packet on the same (id, iif) tuple hits the stale
fastmap entry and reads f->res from freed memory
Reproduced with an mdelay(100) accelerator in route4_set_fastmap() and a
concurrent add/delete stress test (provided by both zdi and Santosh).
Both triggered KASAN slab-use-after-free reports in the route4 fastmap
paths.
Fix:
Introduce a per-filter boolean dying flag to suppress stale fastmap
republishing by in-flight readers.
References
Impacted products
| Vendor | Product | Version | ||
|---|---|---|---|---|
| Linux | Linux |
Version: 1109c00547fc66df45b9ff923544be4c1e1bec13 Version: 1109c00547fc66df45b9ff923544be4c1e1bec13 Version: 1109c00547fc66df45b9ff923544be4c1e1bec13 Version: 1109c00547fc66df45b9ff923544be4c1e1bec13 Version: 1109c00547fc66df45b9ff923544be4c1e1bec13 Version: 1109c00547fc66df45b9ff923544be4c1e1bec13 Version: 1109c00547fc66df45b9ff923544be4c1e1bec13 Version: 1109c00547fc66df45b9ff923544be4c1e1bec13 |
||
{
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"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: cls_route: fix fastmap use-after-free on filter\n\nThe route4 classifier maintains a 16-slot fastmap cache that stores raw\nstruct route4_filter pointers indexed by (id, iif). The reader\n(route4_classify) populates this cache via route4_set_fastmap() for every\nclassified packet that hits a filter. The writer (route4_delete,\nroute4_change) clears the cache via route4_reset_fastmap() before\nRCU-deferred kfree of the filter.\n\nThis creates a UAF race:\n 1. Reader walks the RCU-protected bucket chain, finds filter f\n 2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work()\n 3. Reader calls route4_set_fastmap() and writes f into the cache\n *after* the writer\u0027s reset, caching a pointer about to be freed\n 4. After the RCU grace period, kfree(f) executes\n 5. Next classified packet on the same (id, iif) tuple hits the stale\n fastmap entry and reads f-\u003eres from freed memory\n\nReproduced with an mdelay(100) accelerator in route4_set_fastmap() and a\nconcurrent add/delete stress test (provided by both zdi and Santosh).\nBoth triggered KASAN slab-use-after-free reports in the route4 fastmap\npaths.\n\nFix:\nIntroduce a per-filter boolean dying flag to suppress stale fastmap\nrepublishing by in-flight readers."
}
],
"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 - Exploitation requires local rtnetlink RTM_NEWTFILTER/RTM_DELTFILTER (tc filter add/change/del route) through sendmsg\u2192rtnetlink_rcv_msg\u2192tc_new_tfilter/tc_del_tfilter\u2192route4_change/route4_delete; remote packets alone cannot trigger the filter-deletion side of the race.\nAC:L - The attacker controls both race sides\u2014concurrent route filter add/change/delete via netlink plus traffic through route4_classify on clsact/ingress/egress qdiscs; ZDI and independent reporter stress tests reproduced KASAN slab-use-after-free without uncontrollable prerequisites.\nPR:L - Non-GET rtnetlink handlers require netlink_net_capable(CAP_NET_ADMIN), evaluated against the target network namespace user_ns; an unprivileged local user obtains CAP_NET_ADMIN via user+network namespaces (unshare -Urn) and configures clsact with route filters.\nUI:N - No victim interaction is needed; the attacker configures the route classifier, drives concurrent filter churn, and generates matching traffic from their own processes.\nS:U - Impact is kernel memory corruption within the same kernel security boundary (local privilege escalation/DoS); it does not cross VM, hypervisor, or IOMMU boundaries.\nC:H - Slab use-after-free: a stale fastmap entry retains a freed route4_filter pointer and route4_classify dereferences f-\u003eres from freed kmalloc memory; KASAN confirmed slab-use-after-free, enabling arbitrary kernel read via heap reclaim/spray.\nI:H - UAF of the kmalloc route4_filter object allows attacker-controlled heap spraying to reclaim the freed slot and obtain write/control-flow hijacking primitives; memory corruption is exploitable beyond a simple crash.\nA:H - UAF on the fastmap hot path causes kernel oops/panic under concurrent filter churn; KASAN slab-use-after-free was confirmed during add/delete stress testing, providing reliable denial of service even without full exploitation."
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-08-25T05:40:10.077Z",
"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"shortName": "Linux"
},
"references": [
{
"url": "https://git.kernel.org/stable/c/7897198b26445b4009a057bda1986b94a99e5d5f"
},
{
"url": "https://git.kernel.org/stable/c/820f083c294ad6d319c02a7d43294f2ed2565139"
},
{
"url": "https://git.kernel.org/stable/c/5ec9001be6d0eb527251125632ec8fe88278897f"
},
{
"url": "https://git.kernel.org/stable/c/b969984b2bdc85d721ce4047cd270cd37ec705a2"
},
{
"url": "https://git.kernel.org/stable/c/a17f636c9330eac879822ce29f998e5abd1b72c1"
},
{
"url": "https://git.kernel.org/stable/c/0e7a8cf8895b06d07c7311f028eba16ad742b9bc"
},
{
"url": "https://git.kernel.org/stable/c/ae9aff87025219005a2d16b4fe83d6f24643e50d"
},
{
"url": "https://git.kernel.org/stable/c/47d7f7051253bdc02b1d245d87e38f16d31a74df"
}
],
"title": "net/sched: cls_route: fix fastmap use-after-free on filter",
"x_generator": {
"engine": "bippy-1.2.0"
}
}
},
"cveMetadata": {
"assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"assignerShortName": "Linux",
"cveId": "CVE-2026-74583",
"datePublished": "2026-08-21T16:31:55.886Z",
"dateReserved": "2026-08-15T05:44:03.918Z",
"dateUpdated": "2026-08-25T05:40:10.077Z",
"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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