CVE-2025-21971 (GCVE-0-2025-21971)
Vulnerability from cvelistv5
Published
2025-04-01 15:47
Modified
2026-08-05 11:55
Severity ?
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: net_sched: Prevent creation of classes with TC_H_ROOT The function qdisc_tree_reduce_backlog() uses TC_H_ROOT as a termination condition when traversing up the qdisc tree to update parent backlog counters. However, if a class is created with classid TC_H_ROOT, the traversal terminates prematurely at this class instead of reaching the actual root qdisc, causing parent statistics to be incorrectly maintained. In case of DRR, this could lead to a crash as reported by Mingi Cho. Prevent the creation of any Qdisc class with classid TC_H_ROOT (0xFFFFFFFF) across all qdisc types, as suggested by Jamal.
References
Impacted products
Vendor Product Version
Linux Linux Version: 066a3b5b2346febf9a655b444567b7138e3bb939
Version: 066a3b5b2346febf9a655b444567b7138e3bb939
Version: 066a3b5b2346febf9a655b444567b7138e3bb939
Version: 066a3b5b2346febf9a655b444567b7138e3bb939
Version: 066a3b5b2346febf9a655b444567b7138e3bb939
Version: 066a3b5b2346febf9a655b444567b7138e3bb939
Version: 066a3b5b2346febf9a655b444567b7138e3bb939
Version: 066a3b5b2346febf9a655b444567b7138e3bb939
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet_sched: Prevent creation of classes with TC_H_ROOT\n\nThe function qdisc_tree_reduce_backlog() uses TC_H_ROOT as a termination\ncondition when traversing up the qdisc tree to update parent backlog\ncounters. However, if a class is created with classid TC_H_ROOT, the\ntraversal terminates prematurely at this class instead of reaching the\nactual root qdisc, causing parent statistics to be incorrectly maintained.\nIn case of DRR, this could lead to a crash as reported by Mingi Cho.\n\nPrevent the creation of any Qdisc class with classid TC_H_ROOT\n(0xFFFFFFFF) across all qdisc types, as suggested by Jamal."
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              "value": "AV:L - The vulnerability is reached through RTM_NEWTCLASS rtnetlink messages (tc class add) plus locally generated traffic on a local netdev, requiring local system access. Per kernel scoring guidance, tc/netlink qdisc bugs are Local.\nAC:L - Exploitation is a deterministic fixed sequence \u2014 create a drr/hfsc/qfq/ets qdisc with handle ffff:, add a class with classid ffff:ffff, direct a packet to it, drop the filter, then delete the class \u2014 with no race window or attacker-uncontrollable precondition. Heap grooming to occupy the freed drr_class slab is fully under attacker control.\nPR:L - tc class creation requires only CAP_NET_ADMIN in the network namespace\u0027s owning user namespace (netlink_net_capable in rtnetlink_rcv_msg), which any unprivileged user obtains via unshare -Urn, where they can also create a dummy/veth device and generate the traffic needed. No real root in the init namespace is needed.\nUI:N - The attacker performs every step themselves \u2014 netlink configuration and packet transmission \u2014 with no action required from any other user or administrator.\nS:U - The corruption stays within the kernel\u0027s own security authority; there is no crossing of a VM, IOMMU, or sandbox boundary.\nC:H - The stale active-list entry produces a use-after-free on a kzalloc\u0027d drr_class object; the attacker can reclaim that slab and have the kernel read pointers and data out of attacker-adjacent memory, and the resulting primitives can be leveraged for arbitrary kernel memory disclosure.\nI:H - The UAF yields both an unlink write (list_del on freed cl-\u003ealist) and an indirect call through cl-\u003eqdisc-\u003eops-\u003epeek read from reclaimed heap memory, giving arbitrary write and control-flow hijack potential, i.e. full kernel integrity loss.\nA:H - Even without full exploitation, the dangling list entry causes a use-after-free oops in softirq context on the next dequeue, and the broken accounting permanently stalls the qdisc (drr_dequeue always returns NULL), halting transmission on the device \u2014 a repeatable kernel crash/DoS."
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        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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      "title": "net_sched: Prevent creation of classes with TC_H_ROOT",
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    "dateReserved": "2024-12-29T08:45:45.797Z",
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