GHSA-J9W6-P85X-R6R9
Vulnerability from github – Published: 2026-10-23 15:31 – Updated: 2026-10-30 15:34In the Linux kernel, the following vulnerability has been resolved:
ipv4: stop PMTU walk when nexthop group shrinks
Commit 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if multipath enabled") made __ip_rt_update_pmtu() update every path.
For nexthop objects, fib_info_num_path() and fib_info_nhc() independently load nh->nh_grp. RCU protects each group's lifetime but does not make the loads observe the same group. If replacement shrinks the group after the loop accepts an index, fib_info_nhc() returns NULL and update_or_create_fnhe() dereferences it.
A deterministic probe only widened the existing window between the real operations. On v7.2, an RTM_NEWNEXTHOP replacement published a one-member group after the PMTU reader accepted index 1 from a two-member group, producing:
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] RIP: update_or_create_fnhe+0x45/0x15b0
Stop when the indexed path is absent. Groups are dense, so no later index exists in that snapshot. The same real-writer window completed 114,423 PMTU iterations without an oops with this check. A reproducer is available on request.
Replacement requires CAP_NET_ADMIN in the network namespace. Where unprivileged user namespaces are permitted, a local user can obtain it in a new user and network namespace.
{
"affected": [],
"aliases": [
"CVE-2026-99074"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-10-23T13:17:31Z",
"severity": null
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nipv4: stop PMTU walk when nexthop group shrinks\n\nCommit 7d3f3b4367f3 (\"net: ipv4: Cache pmtu for all packet paths if\nmultipath enabled\") made __ip_rt_update_pmtu() update every path.\n\nFor nexthop objects, fib_info_num_path() and fib_info_nhc()\nindependently load nh-\u003enh_grp. RCU protects each group\u0027s lifetime but\ndoes not make the loads observe the same group. If replacement shrinks\nthe group after the loop accepts an index, fib_info_nhc() returns NULL\nand update_or_create_fnhe() dereferences it.\n\nA deterministic probe only widened the existing window between the real\noperations. On v7.2, an RTM_NEWNEXTHOP replacement published a\none-member group after the PMTU reader accepted index 1 from a two-member\ngroup, producing:\n\n KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]\n RIP: update_or_create_fnhe+0x45/0x15b0\n\nStop when the indexed path is absent. Groups are dense, so no later\nindex exists in that snapshot. The same real-writer window completed\n114,423 PMTU iterations without an oops with this check. A reproducer\nis available on request.\n\nReplacement requires CAP_NET_ADMIN in the network namespace. Where\nunprivileged user namespaces are permitted, a local user can obtain it\nin a new user and network namespace.",
"id": "GHSA-j9w6-p85x-r6r9",
"modified": "2026-10-30T15:34:17Z",
"published": "2026-10-23T15:31:37Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99074"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/6619e01855e65ef35723685a557e3eb106f4ecf5"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/82d83eec893d8ae84f280aec44c60d9fdecc62b5"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/a411df69524f91fe3adb8f9544d33e701fc9732c"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/c60d17d6dd61adc761ac6f65a008fefbc54cbf87"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/c6d670ac656badfd0e15355e07a13a6bdf3b69fd"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/f27483f9975b5065ff67054cc7eb8edac6083488"
}
],
"schema_version": "1.4.0",
"severity": []
}
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