CVE-2026-90053: net/sched: sch_htb: limit htb_classify inner-class filter hops
In the Linux kernel, the following vulnerability has been resolved:
net/sched: schhtb: limit htbclassify inner-class filter hops
htbclassify() follows each filter-selected inner class by switching to cl->filterlist, 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 TCACT switch has consumed the classifier verdict, so a terminal TCACTQUEUED/STOLEN/TRAP on the last permitted chain still sets qerr to NETXMITSTOLEN and the packet is not charged as a drop by this qdisc or its parent.
The bound is TCHTBMAXDEPTH, taken from HTB's own parameters rather than from the qdisc hierarchy depth limit. Class levels run from 0 to TCHTBMAXDEPTH - 1, so a traversal that strictly descends in level can take at most TCHTBMAXDEPTH hops. That descent is what a sane configuration does, but it is assumed here rather than enforced: htbfind() 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.
htbclassify() can now return NULL irrespective of CONFIGNETCLSACT, whereas previously every NULL return sat inside that ifdef. The NULL handler in htbenqueue() therefore cannot stay conditional either, so drop the ifdef around it. This matches hfscenqueue(), 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: - CONFIGNETSCHED, CONFIGNETSCHHTB, CONFIGNETCLSU32, CONFIGLOCKUPDETECTOR. - 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 softlockuppanic=1 it panics. - Reachable from unprivileged user via unshare -Urn (CAPNETADMIN).
Affected Software
Remediation
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In htb_classify(), bound inner-class filter traversal at TC_HTB_MAXDEPTH; when the bound is exceeded, drop the packet and issue a rate-limited warning.
Event History
Frequently Asked Questions
What traffic-control configuration is required to trigger the issue?
An HTB inner-class filter must select itself or participate in a cycle with another inner class. This causes classification to repeatedly switch to the selected class's filter list without terminating.
What is the impact once a triggering configuration is present?
A single packet reaching the cyclic classification path can cause an infinite loop while the qdisc lock is held and bottom halves are disabled. This can result in a soft lockup or panic.
How does the fix change behavior for cyclic filter paths?
The fix limits inner-class filter traversal to TC_HTB_MAXDEPTH hops. When that limit is exceeded, the packet is dropped and a rate-limited warning is emitted; terminal queued, stolen, or trap verdicts on the final permitted chain retain their existing handling.