GHSA-4HV8-GC9J-4WJF
Vulnerability from github – Published: 2026-08-22 18:30 – Updated: 2026-08-25 06:31In the Linux kernel, the following vulnerability has been resolved:
ipv4: fix use-after-free in fib_nhc_update_mtu()
fib_nhc_update_mtu() walks the nexthop exception table under RTNL, but RTNL does not serialize this walk with PMTU exception updates. The walk uses rcu_dereference_protected() with a constant true condition without holding fnhe_lock.
The following interleaving can therefore occur:
CPU 0 CPU 1 fib_nhc_update_mtu() update_or_create_fnhe() load fnhe spin_lock_bh(&fnhe_lock) fnhe_remove_oldest() unlink fnhe kfree_rcu(fnhe, rcu) access fnhe after grace period
KASAN reported:
BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410 Read of size 8 at addr ffff888107d49000 by task poc/90 Call Trace: fib_nhc_update_mtu+0x3df/0x410 fib_sync_mtu+0x7a/0xd0 fib_netdev_event+0x229/0x3f0 netif_set_mtu_ext+0x33a/0x570 dev_set_mtu+0x88/0x120
The same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a pair and other writers serialize them with fnhe_lock. RCU alone prevents reclamation, but would still allow concurrent writers to leave a mixed pair.
Walk the table under RCU and acquire fnhe_lock only while updating each exception. RCU keeps the current entry alive while the short critical section serializes its paired PMTU fields. This avoids holding the global lock while scanning all 2048 buckets for every nexthop.
{
"affected": [],
"aliases": [
"CVE-2026-74656"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-08-22T16:16:39Z",
"severity": "HIGH"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nipv4: fix use-after-free in fib_nhc_update_mtu()\n\nfib_nhc_update_mtu() walks the nexthop exception table under RTNL, but\nRTNL does not serialize this walk with PMTU exception updates. The walk\nuses rcu_dereference_protected() with a constant true condition without\nholding fnhe_lock.\n\nThe following interleaving can therefore occur:\n\n CPU 0 CPU 1\n fib_nhc_update_mtu() update_or_create_fnhe()\n load fnhe spin_lock_bh(\u0026fnhe_lock)\n fnhe_remove_oldest()\n unlink fnhe\n kfree_rcu(fnhe, rcu)\n \u003cquiescent state\u003e\n access fnhe after grace period\n\nKASAN reported:\n\n BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410\n Read of size 8 at addr ffff888107d49000 by task poc/90\n Call Trace:\n fib_nhc_update_mtu+0x3df/0x410\n fib_sync_mtu+0x7a/0xd0\n fib_netdev_event+0x229/0x3f0\n netif_set_mtu_ext+0x33a/0x570\n dev_set_mtu+0x88/0x120\n\nThe same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a\npair and other writers serialize them with fnhe_lock. RCU alone prevents\nreclamation, but would still allow concurrent writers to leave a mixed\npair.\n\nWalk the table under RCU and acquire fnhe_lock only while updating each\nexception. RCU keeps the current entry alive while the short critical\nsection serializes its paired PMTU fields. This avoids holding the global\nlock while scanning all 2048 buckets for every nexthop.",
"id": "GHSA-4hv8-gc9j-4wjf",
"modified": "2026-08-25T06:31:25Z",
"published": "2026-08-22T18:30:28Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-74656"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/5a28a4b22dde92f9d293b94236314b8d6181dc4a"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/63996ffc594d128ccec8fc0983f91effd2d3adc4"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/bc5bde9ce3cc36502839dfe98e068f7303a50982"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/dfe388da13aa784851e5ebbea90afbb099075761"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/e00f7d2b5f2540a3415a229c982af7a25ff6362e"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/e1e602d6b22d5cb1641c4459c487eb18bf569e0a"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ed503eaad62f20cdd5122d7c3078a648a99c8f16"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/fd39e711866498ae94fcf9acf6f422a4f045b681"
}
],
"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 confirmed: The user expressed doubt about the validity of the vulnerability.
- 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.