CVE-2026-98610: net/sched: sch_hfsc: bound the classify inner-filter walk with a drift budget
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).