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CVE-2026-74267

net/sched: sch_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen

9.8 EPSS 0.7% AV:N

Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem net/sched. Debian urgency: not yet assigned.

What to do, per Debian release

Debian's own status for CVE-2026-74267 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source

Release	Status	Version	What this means
Debian 12bookworm	fixed	6.1.180-1	Update the kernel and reboot. Fixed in 6.1.180-1. A running kernel keeps the old code until the machine restarts.
Debian 13trixie	fixed	6.12.100-1	Update the kernel and reboot. Fixed in 6.12.100-1. A running kernel keeps the old code until the machine restarts.
Debian 14forky	fixed	7.1.5-1	Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts.
Debian unstablesid	fixed	7.1.5-1	Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts.

The vulnerability

What the bug is

Whenever 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 codel still has 1 packet on the queue and, thus, will mistakenly deactivate the parent's class causing issues like a wild memory access when qfq has codel as a child: [ 36.339843][ T370] Oops: general protection fault, probably for non-canonical address 0xfbd59c0000000024: 0000 [#1] SMP KASAN NOPTI [ 36.340408][ T370] KASAN: maybe wild-memory-access in range [0xdead000000000120-0xdead000000000127] [ 36.340737][ T370] CPU: 2 UID: 0 PID: 370 Comm: tc Not tainted 7.1.0-rc5-00287-g66e13b626592 #87 PREEMPT(full) [ 36.341113][ T370] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 [ 36.341357][ T370] 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 [ 36.342221][ T370] RSP: 0018:ffff8881100ef370 EFLAGS: 00010216 [ 36.342422][ T370] RAX: 0000000000000000 RBX: ffff8881058a9568 RCX: dffffc0000000000 [ 36.342664][ T370] RDX: 1ffff11021064dc3 RSI: ffff888108326e00 RDI: dffffc0000000000 [ 36.342905][ T370] RBP: ffff8881058a8280 R08: dead000000000122 R09: 1bd5a00000000024 [ 36.343140][ T370] R10: fffffbfff2940329 R11: fffffbfff2940329 R12: 0000000000000000 [ 36.343383][ T370] R13: dead000000000100 R14: ffff8881058a9580 R15: ffff8881058a9578 [ 36.343631][ T370] FS: 00007fc04b0ca780(0000) GS:ffff888184fef000(0000) knlGS:0000000000000000 [ 36.343911][ T370] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 36.344116][ T370] CR2: 0000557c02c02000 CR3: 000000010e0ba000 CR4: 0000000000750ef0 [ 36.344359][ T370] PKRU: 55555554 [ 36.344481][ T370] Call Trace: ... [ 36.345054][ T370] qfq_reset_qdisc (net/sched/sch_qfq.c:357 net/sched/sch_qfq.c:1487) sch_qfq [ 36.345222][ T370] qdisc_reset (net/sched/sch_generic.c:1057) [ 36.345503][ T370] __qdisc_destroy (net/sched/sch_generic.c:1096) [ 36.345677][ T370] qdisc_graft (net/sched/sch_api.c:1062 net/sched/sch_api.c:1053 net/sched/sch_api.c:1159) [ 36.346335][ T370] 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.

Where it lives

The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.

net/sched/sch_codel.c

What this is: TC subsystemMAINTAINERS section matched on net/sched/ · netdev@vger.kernel.org · Maintained
Kernel module: sch_codel

Check it on the machine

grep -w CONFIG_NET_SCH_CODEL /boot/config-$(uname -r)

Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.

lsmod | grep -w sch_codel

Prints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.

Show whether Debian builds this, per release

Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.

bookworm, trixie, forky, sidm

m: a loadable module on every flavour - check lsmod, can be blacklisted

Build switch: CONFIG_NET_SCH_CODEL

From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages

Debian tracker CVE record NVD kernel CNA record

Signals

Published: 2026-08-15 (2 months ago)source
CVSS: 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it
Attack vector: network (AV:N)source
EPSS: 0.71%, higher than 52.8% of all CVEssource
CISA KEV: not listedthe KEV catalogue
Debian urgency: not yet assignedsource
Subsystem: net/sched

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
5.10.241	5.10.261	bb9cfd874ee8
5.15.190	5.15.212	7a05af7f5856
6.1.135	6.1.178	eba49fd85995
6.6.88	6.6.145	e8c6dbadf139
6.12.24	6.12.97	e4615aa6bb78
6.15	6.18.40	755108bb7a50
6.15	7.1.5	91e0a793a723
6.15	7.2	52f1da34c9f4
5.4.297	0	0
6.13.12	0	0
6.14.3	0	0

Why the CNA scored it this way

Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source

AV:NAttack vector, network: Remote senders reach the bug when packets traverse a QFQ (or other qlen_notify parent) with a codel child: qfq_enqueue() and qfq_dequeue() call the child peek handler during __dev_queue_xmit scheduling, and CoDel can drop there before qlen is restored.
AC:LAttack complexity, low: The attacker controls traffic rate, packet size, and CoDel drop behavior to force drops during peek while qlen reads zero; this is a deterministic logic bug, not a race on uncontrollable kernel state or a rare compile-time configuration.
PR:NPrivileges required, none: On systems where an operator has already deployed a parent qdisc with a codel child, any remote party that can send packets into the shaped class can trigger the faulty peek/drop path with no account or capabilities on the victim host.
UI:NUser interaction, none: Exploitation requires only attacker-generated traffic (and, for full crash during teardown, attacker-initiated tc reconfiguration); no independent action by another user or administrator is needed at trigger time.
S:UScope, unchanged: The flaw corrupts in-kernel QFQ/CoDel scheduler structures and memory within the same kernel security domain; it does not cross VM, container, or IOMMU authority boundaries.
C:HConfidentiality impact, high: False qlen_notify prematurely deactivates QFQ aggregates while packets remain; the fix commit shows KASAN wild-memory-access on poisoned aggregate list nodes in qfq_deactivate_agg(), giving attacker-influenced reads from freed/corrupted scheduler memory.
I:HIntegrity impact, high: Corrupted aggregate/class linkage leads to invalid hlist operations and inconsistent scheduler state that can be steered with heap layout control, yielding memory corruption primitives usable for arbitrary kernel writes or control-flow hijack.
A:HAvailability impact, high: The published oops is a general protection fault from corrupted QFQ state during qdisc teardown, and the same corruption during live packet scheduling can reliably kernel-panic or hang affected hosts.