CVE-2026-90053

net/sched: sch_htb: limit htb_classify inner-class filter hops

Published Sep 17, 2026 CVSS n/a EPSS < 1% KEV · Not listed

Description

In the Linux kernel, the following vulnerability has been resolved:

net/sched: sch_htb: limit htb_classify inner-class filter hops

htb_classify() follows each filter-selected inner class by switching
to cl->filter_list, but never bounds the number of hops. A filter on
an inner class can point back to itself or to another inner class that
points back, creating an infinite loop in the packet classification
path with the qdisc lock held and BH disabled — a soft lockup / panic
from a single packet.

Bound the traversal with a hop counter and drop the packet with a
rate-limited warning once the bound is exceeded. The counter is
incremented at the point the inner filter chain is picked up, after the
TC_ACT_* switch has consumed the classifier verdict, so a terminal
TC_ACT_QUEUED/STOLEN/TRAP on the last permitted chain still sets *qerr
to __NET_XMIT_STOLEN and the packet is not charged as a drop by this
qdisc or its parent.

The bound is TC_HTB_MAXDEPTH, taken from HTB's own parameters rather than
from the qdisc hierarchy depth limit. Class levels run from 0 to
TC_HTB_MAXDEPTH - 1, so a traversal that strictly descends in level can
take at most TC_HTB_MAXDEPTH hops. That descent is what a sane
configuration does, but it is assumed here rather than enforced:
htb_find() resolves a classid against every class in the qdisc, so a
filter may equally select a sibling or an ancestor. The normal
root -> inner -> leaf path takes a single hop, so the bound does not
affect legitimate classification.

htb_classify() can now return NULL irrespective of CONFIG_NET_CLS_ACT,
whereas previously every NULL return sat inside that ifdef. The NULL
handler in htb_enqueue() therefore cannot stay conditional either, so
drop the ifdef around it. This matches hfsc_enqueue(), which has always
handled a NULL class unconditionally. Without it, a kernel built
without actions would dereference a NULL class instead of dropping.

Conditions to recreate the bug:
- CONFIG_NET_SCHED, CONFIG_NET_SCH_HTB, CONFIG_NET_CLS_U32,
CONFIG_LOCKUP_DETECTOR.
- Create an HTB qdisc on a device (e.g. lo), add an inner class
1:1 with a leaf child 1:10, install a root u32 filter selecting
1:1, and an inner-class u32 filter on 1:1 also selecting 1:1.
- Send one packet (ping). On the unfixed kernel the classify loop
spins with the qdisc lock held; with softlockup_panic=1 it panics.
- Reachable from unprivileged user via unshare -Urn (CAP_NET_ADMIN).

Affected Products

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Linux

unaffected

Version	Status	Constraints
`1da177e4c3f41524e886b7f1b8a0c1fc7321cac2`	affected	< 83bb11805b620c498d6417e0e409f26887981489
`1da177e4c3f41524e886b7f1b8a0c1fc7321cac2`	affected	< 90a149ab1f8344b7f713e9a838f69fde9e8d2a51
`1da177e4c3f41524e886b7f1b8a0c1fc7321cac2`	affected	< 010d05df52cec053bdc67d82c5b61cc4996f3fde
`1da177e4c3f41524e886b7f1b8a0c1fc7321cac2`	affected	< 8c9611f7ba3ea5594e41bac952363aa0966d3280
`1da177e4c3f41524e886b7f1b8a0c1fc7321cac2`	affected	< e54be9aa503a0297b63aec530fbf760350effd45
`1da177e4c3f41524e886b7f1b8a0c1fc7321cac2`	affected	< 872317e84fd019ceffba125e62d31586e3336d41
`1da177e4c3f41524e886b7f1b8a0c1fc7321cac2`	affected	< 729c4896ab829169f95915d65edd530325910b37

Linux

affected

Version	Status	Constraints
`2.6.12`	affected	—
`0`	unaffected	< 2.6.12
`5.15.221`	unaffected	≤ 5.15.*
`6.1.188`	unaffected	≤ 6.1.*
`6.6.157`	unaffected	≤ 6.6.*
`6.12.110`	unaffected	≤ 6.12.*
`6.18.52`	unaffected	≤ 6.18.*
`7.2.6`	unaffected	≤ 7.2.*
`7.3`	unaffected	≤ *

No data.

Vendor Product Versions

Linux

Linux Kernel

Version	Status	Scheme	Platform
`[1da177e4c3f41524e886b7f1b8a0c1fc7321cac2,83bb11805b620c498d6417e0e409f26887981489)`	affected	code_commit	—
`[1da177e4c3f41524e886b7f1b8a0c1fc7321cac2,90a149ab1f8344b7f713e9a838f69fde9e8d2a51)`	affected	code_commit	—
`[1da177e4c3f41524e886b7f1b8a0c1fc7321cac2,010d05df52cec053bdc67d82c5b61cc4996f3fde)`	affected	code_commit	—
`[1da177e4c3f41524e886b7f1b8a0c1fc7321cac2,8c9611f7ba3ea5594e41bac952363aa0966d3280)`	affected	code_commit	—
`[1da177e4c3f41524e886b7f1b8a0c1fc7321cac2,e54be9aa503a0297b63aec530fbf760350effd45)`	affected	code_commit	—
`[1da177e4c3f41524e886b7f1b8a0c1fc7321cac2,872317e84fd019ceffba125e62d31586e3336d41)`	affected	code_commit	—
`[1da177e4c3f41524e886b7f1b8a0c1fc7321cac2,729c4896ab829169f95915d65edd530325910b37)`	affected	code_commit	—

Linux

Linux Kernel

Version	Status	Scheme	Platform
`2.6.12`	affected	semver	—
`[0,2.6.12)`	unaffected	semver	—
`[5.15.221,5.16.0)`	unaffected	semver	—
`[6.1.188,6.2.0)`	unaffected	semver	—
`[6.6.157,6.7.0)`	unaffected	semver	—
`[6.12.110,6.13.0)`	unaffected	semver	—
`[6.18.52,6.19.0)`	unaffected	semver	—
`[7.2.6,7.3.0)`	unaffected	semver	—
`[7.3-rc1,*]`	unaffected	semver	—

Remediation

No solution or workaround provided in the CVE record

Tracking

Advisories

Source	ID	Title
Debian DLA	DLA-4817-1	linux-6.12 security update
Debian DLA	DLA-4841-1	linux security update
Debian DSA	DSA-6528-1	linux security update
Ubuntu USN	USN-8961-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9019-1	Linux kernel (Azure) vulnerabilities
Ubuntu USN	USN-9020-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9022-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9023-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9024-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9025-1	Linux kernel vulnerabilities
Ubuntu USN	USN-9026-1	Linux kernel (Azure) vulnerabilities
Ubuntu USN	USN-9029-1	Linux kernel (Raspberry Pi) vulnerabilities
Ubuntu USN	USN-9039-1	Linux kernel (Azure CVM) vulnerabilities
Ubuntu USN	USN-9040-1	Linux kernel (Low Latency NVIDIA) vulnerabilities
Ubuntu USN	USN-9066-1	Linux kernel (HWE) vulnerabilities
Ubuntu USN	USN-9091-1	Linux kernel (NVIDIA Tegra) vulnerabilities

Metrics

No CVSS v4.0

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Exploit prediction score: < 1% (0.211% probability within 30 days)

Key SSVC decision points have not yet been added.

References

Link	Providers
https://git.kernel.org/stable/c/010d05df52cec053bdc67d82c5b61cc4996f3fde
https://git.kernel.org/stable/c/729c4896ab829169f95915d65edd530325910b37
https://git.kernel.org/stable/c/83bb11805b620c498d6417e0e409f26887981489
https://git.kernel.org/stable/c/872317e84fd019ceffba125e62d31586e3336d41
https://git.kernel.org/stable/c/8c9611f7ba3ea5594e41bac952363aa0966d3280
https://git.kernel.org/stable/c/90a149ab1f8344b7f713e9a838f69fde9e8d2a51
https://git.kernel.org/stable/c/e54be9aa503a0297b63aec530fbf760350effd45

History

Sep 20, 2026 4:30 AM Weaknesses updated View changes

Type	Values Removed	Values Added
Weaknesses		CWE-666 CWE-674

Sep 17, 2026 4:30 PM Description updated. Title updated. First appearance in the feed. Affected versions updated. Affected vendors updated. Added references View changes

Type	Values Removed	Values Added
Description		In the Linux kernel, the following vulnerability has been resolved: net/sched: sch_htb: limit htb_classify inner-class filter hops htb_classify() follows each filter-selected inner class by switching to cl->filter_list, but never bounds the number of hops. A filter on an inner class can point back to itself or to another inner class that points back, creating an infinite loop in the packet classification path with the qdisc lock held and BH disabled — a soft lockup / panic from a single packet. Bound the traversal with a hop counter and drop the packet with a rate-limited warning once the bound is exceeded. The counter is incremented at the point the inner filter chain is picked up, after the TC_ACT_* switch has consumed the classifier verdict, so a terminal TC_ACT_QUEUED/STOLEN/TRAP on the last permitted chain still sets *qerr to __NET_XMIT_STOLEN and the packet is not charged as a drop by this qdisc or its parent. The bound is TC_HTB_MAXDEPTH, taken from HTB's own parameters rather than from the qdisc hierarchy depth limit. Class levels run from 0 to TC_HTB_MAXDEPTH - 1, so a traversal that strictly descends in level can take at most TC_HTB_MAXDEPTH hops. That descent is what a sane configuration does, but it is assumed here rather than enforced: htb_find() resolves a classid against every class in the qdisc, so a filter may equally select a sibling or an ancestor. The normal root -> inner -> leaf path takes a single hop, so the bound does not affect legitimate classification. htb_classify() can now return NULL irrespective of CONFIG_NET_CLS_ACT, whereas previously every NULL return sat inside that ifdef. The NULL handler in htb_enqueue() therefore cannot stay conditional either, so drop the ifdef around it. This matches hfsc_enqueue(), which has always handled a NULL class unconditionally. Without it, a kernel built without actions would dereference a NULL class instead of dropping. Conditions to recreate the bug: - CONFIG_NET_SCHED, CONFIG_NET_SCH_HTB, CONFIG_NET_CLS_U32, CONFIG_LOCKUP_DETECTOR. - Create an HTB qdisc on a device (e.g. lo), add an inner class 1:1 with a leaf child 1:10, install a root u32 filter selecting 1:1, and an inner-class u32 filter on 1:1 also selecting 1:1. - Send one packet (ping). On the unfixed kernel the classify loop spins with the qdisc lock held; with softlockup_panic=1 it panics. - Reachable from unprivileged user via unshare -Urn (CAP_NET_ADMIN).
Title		net/sched: sch_htb: limit htb_classify inner-class filter hops
First Time appeared		Linux Linux linux Kernel
CPEs		cpe:2.3:o:linux:linux_kernel::::::::
Vendors & Products		Linux Linux linux Kernel
References		https://git.kernel.org/stable/c/010d05df52cec053bdc67d82c5b61cc4996f3fde https://git.kernel.org/stable/c/729c4896ab829169f95915d65edd530325910b37 https://git.kernel.org/stable/c/83bb11805b620c498d6417e0e409f26887981489 https://git.kernel.org/stable/c/872317e84fd019ceffba125e62d31586e3336d41 https://git.kernel.org/stable/c/8c9611f7ba3ea5594e41bac952363aa0966d3280 https://git.kernel.org/stable/c/90a149ab1f8344b7f713e9a838f69fde9e8d2a51 https://git.kernel.org/stable/c/e54be9aa503a0297b63aec530fbf760350effd45