CVE-2026-80630
massLinux kernel fq_codel qdisc qlen-accounting flaw causes wild memory access and crash
CVE-2026-80630 is a memory-safety flaw in the Linux kernel's fair queue controlled delay (fq_codel) traffic-control queueing discipline (net/sched/sch_fq_codel). When fq_codel drops packets during a 'peek' operation, it calls qdisc_tree_reduce_backlog before restoring its queue length, so if qlen momentarily reaches zero the parent qdisc's qlen_notify callback deactivates the parent's class even though fq_codel still holds one packet. This is triggered with fq_codel stacked under a classful parent qdisc such as qfq, for example when tc-driven qdisc reset/graft operations run while packets are queued and drops occur; the observed result is a general protection fault with wild memory access in qfq_deactivate_agg, i.e., a kernel crash (denial of service). The provided CVSS 3.1 score is 9.8 (critical, AV:N/AC:L/PR:N/UI:N), though the reported crash path involves local traffic-control qdisc management on systems using the affected qdisc stack. Any Linux deployment using fq_codel as a child of a classful qdisc is potentially affected, while systems not using this scheduler configuration are not impacted. No exploitation is currently known: the flaw is not in CISA KEV, no public proof-of-concept is known, and EPSS estimates only a 0.5% probability of exploitation in the next 30 days.
What to do: Track and apply the upstream fix 'net/sched: sch_fq_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen' when it reaches the stable kernel branches you run, and move to a patched kernel release as soon as it is published. Until then, audit 'tc qdisc show' output for fq_codel instances attached under classful qdiscs (e.g., qfq) and avoid live tc qdisc replace/change/graft operations on such stacks, since qdisc reconfiguration is the documented crash trigger. No mitigation other than a patched kernel or avoiding the fq_codel-under-classful-parent configuration is known.
| Linux kernel (net/sched/sch_fq_codel queueing discipline) | Affected/fixed version ranges not specified in the source data; bug reproduced on a 7.1.0-rc5 development kernel and resolved by the upstream sch_fq_codel patch |
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
In the Linux kernel, the following vulnerability has been resolved: net/sched: sch_fq_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen Whenever fq_codel drops packets during peek, it calls qdisc_tree_reduce_backlog. An issue arises because it calls qdisc_tree_reduce_backlog before it reincrements the qlen. If qlen drops to zero, but peek returns an skb, the parent's qlen_notify callback will be executed even though fq_codel still has 1 packet on the queue and, thus, will mistakenly deactivate the parent's class causing issues like a recent report [1] and a wild memory access in qfq: [ 29.371146][ T360] Oops: general protection fault, probably for non-canonical address 0xfbd59c0000000024: 0000 [#1] SMP KASAN NOPTI [ 29.371666][ T360] KASAN: maybe wild-memory-access in range [0xdead000000000120-0xdead000000000127] [ 29.371987][ T360] CPU: 6 UID: 0 PID: 360 Comm: tc Not tainted 7.1.0-rc5-00285-gc530e5b2dbc6-dirty #82 PREEMPT(full) [ 29.372384][ T360] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 [ 29.372620][ T360] RIP: 0010:qfq_deactivate_agg (include/linux/list.h:1029 (discriminator 2) include/linux/list.h:1043 (discriminator 2) net/sched/sch_qfq.c:1369 (discriminator 2) net/sched/sch_qfq.c:1395 (discriminator 2)) sch_qfq [ 29.373544][ T360] RSP: 0018:ffff888102417370 EFLAGS: 00010216 [ 29.373800][ T360] RAX: 0000000000000000 RBX: ffff88811224d568 RCX: dffffc0000000000 [ 29.374079][ T360] RDX: 1ffff11021fe1543 RSI: ffff88810ff0aa00 RDI: dffffc0000000000 [ 29.374368][ T360] RBP: ffff88811224c280 R08: dead000000000122 R09: 1bd5a00000000024 [ 29.374649][ T360] R10: fffffbfff7940329 R11: fffffbfff7940329 R12: 0000000000000000 [ 29.374926][ T360] R13: dead000000000100 R14: ffff88811224d580 R15: ffff88811224d578 [ 29.375207][ T360] FS: 00007f5b794e5780(0000) GS:ffff88815d1e9000(0000) knlGS:0000000000000000 [ 29.375545][ T360] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 29.375823][ T360] CR2: 000055ffb091f000 CR3: 000000010a305000 CR4: 0000000000750ef0 [ 29.376103][ T360] PKRU: 55555554 [ 29.376258][ T360] Call Trace: [ 29.376401][ T360] ... [ 29.376885][ T360] qfq_reset_qdisc (net/sched/sch_qfq.c:357 net/sched/sch_qfq.c:1487) sch_qfq [ 29.377074][ T360] qdisc_reset (net/sched/sch_generic.c:1057) [ 29.377414][ T360] __qdisc_destroy (net/sched/sch_generic.c:1096) [ 29.377600][ T360] qdisc_graft (net/sched/sch_api.c:1062 net/sched/sch_api.c:1053 net/sched/sch_api.c:1159) [ 29.378593][ T360] tc_get_qdisc (net/sched/sch_api.c:1528 net/sched/sch_api.c:1556) Fix this by only calling qdisc_tree_reduce_backlog in peek after the qlen is restored. [1] http://lore.kernel.org/netdev/CAN2cbVe79oj0O9==m4+4x3v+O+qzRagA=2=wkrp9i9=CqYvyZA@mail.gmail.com/
- Vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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