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

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-90053
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-17 17:16:53 UTC
Updated2026-09-17 17:16:53 UTC
DescriptionIn 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).

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 83bb11805b620c498d6417e0e409f26887981489 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 90a149ab1f8344b7f713e9a838f69fde9e8d2a51 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 010d05df52cec053bdc67d82c5b61cc4996f3fde git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 8c9611f7ba3ea5594e41bac952363aa0966d3280 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 e54be9aa503a0297b63aec530fbf760350effd45 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 872317e84fd019ceffba125e62d31586e3336d41 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 729c4896ab829169f95915d65edd530325910b37 git Not specified
CNA Linux Linux affected 2.6.12 Not specified
CNA Linux Linux unaffected 2.6.12 semver Not specified
CNA Linux Linux unaffected 5.15.221 5.15.* semver Not specified
CNA Linux Linux unaffected 6.1.188 6.1.* semver Not specified
CNA Linux Linux unaffected 6.6.157 6.6.* semver Not specified
CNA Linux Linux unaffected 6.12.110 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.52 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.6 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3-rc1 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/010d05df52cec053bdc67d82c5b61cc4996f3fde 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/90a149ab1f8344b7f713e9a838f69fde9e8d2a51 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/8c9611f7ba3ea5594e41bac952363aa0966d3280 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/729c4896ab829169f95915d65edd530325910b37 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/872317e84fd019ceffba125e62d31586e3336d41 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/e54be9aa503a0297b63aec530fbf760350effd45 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/83bb11805b620c498d6417e0e409f26887981489 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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