UBUNTU-CVE-2026-98610: Medium severity ubuntu/linux vulnerability
In the Linux kernel, the following vulnerability has been resolved: net/sched: schhfsc: bound the classify inner-filter walk with a drift budget hfscclassify() applies the "filter may only point downwards" level check only when the filter result carries no bound class. A filter created with a flowid gets res.class set once at bind time, so the check never runs for it during classification. hfscadjustlevels() can later raise a class's level without revalidating existing bindings, leaving two binds that were each legal at bind time pointing at each other; the classify walk then bounces between two interior classes forever with the qdisc lock held and BH disabled — a soft lockup from a single packet. The stuck walk trips the watchdog: watchdog: BUG: soft lockup - CPU#3 stuck for 13s! [ping:444] RIP: 0010:u32classify+0x542/0x17f0 ... tcfclassify+0x66/0xa0 hfscenqueue+0x166/0xdf0 Bound the traversal with a budget of non-descending hops, the only way a configured walk can move without descending the class tree once levels drift after bind time. The budget is cumulative over the whole walk and is deliberately not reset on a descending hop: a chain that alternates a descent with a lateral hop would return the budget every lap and never trip. Descending hops never decrement it, so legitimately deep trees are unaffected and a terminating lateral chain still classifies normally. Drop the packet with a rate-limited warning once the budget is exhausted, mirroring the merged HTB fix. This is a follow-up to commit 729c4896ab82 ("net/sched: schhtb: limit htbclassify inner-class filter hops"), which bounded the same classify loop on the HTB side but left the HFSC walk unbounded. Conditions to recreate the bug: - CONFIGNETSCHED, CONFIGNETSCHHFSC, CONFIGNETCLSU32, CONFIGLOCKUPDETECTOR. - Build a cycle with two legal-at-bind-time flowid binds and a level drift: class X 1:1 (child of root) with leaf child 1:10; class Y 1:2 (sibling of X) with children 1:20 and 1:200; root u32 filter flowid 1:1; filter on X flowid 1:2 (legal when Y is a leaf); after Y's level rises to 2, filter on Y flowid 1:1 (legal then). Send one packet (ping on the device). Unfixed kernel: classify spins with the qdisc lock held; with softlockuppanic=1 it panics. - Reachable from unprivileged user via unshare -Urn (CAPNETADMIN).
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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ubuntu/linux-aws-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1119.126~20.04.1 - Upgrade
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ubuntu/linux-azure-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1125.134~20.04.1 - Upgrade
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ubuntu/linux-azure-fde-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1124.133~20.04.1 - Upgrade
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ubuntu/linux-gcp-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1120.130~20.04.1 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 7.0.0-1016.17 - Upgrade
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ubuntu/linux-ibmto a version that resolves this vulnerability.Fixed in 7.0.0-1017.17 - Upgrade
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Event History
Frequently Asked Questions
Who can trigger the denial of service?
An attacker needs local low-privilege access and the ability to create or alter the relevant HFSC traffic-control classes and filters. After the cyclic bindings are present, a single packet can trigger the soft lockup.
What configurations are exposed?
Exposure requires HFSC filters bound with a flowid, followed by class-level changes that leave bindings between interior classes pointing at each other. The classification walk can then repeatedly traverse the non-descending cycle while holding the qdisc lock with bottom halves disabled.
How can I tell whether the issue has been triggered?
The kernel watchdog may report a soft lockup, such as a CPU stuck in u32_classify, with tcf_classify and hfsc_enqueue in the call trace. The description gives an example of a watchdog message reporting a CPU stuck for 13 seconds.