CVE-2026-74284: net/sched: sch_hfsc: Don't make class passive twice
In the Linux kernel, the following vulnerability has been resolved:
net/sched: schhfsc: Don't make class passive twice
updatevf() is called from two places for the same class during a single dequeue when the class's child qdisc (e.g. codel/fqcodel) drops its last packets while dequeuing:
1. The child calls qdisctreereducebacklog(), which, now that the child is empty, invokes hfscqlennotify() -> updatevf(cl, 0, 0) and turns the class passive (clnactive is decremented up the hierarchy).
2. hfscdequeue() then calls updatevf(cl, qdiscpktlen(skb), curtime) to charge the dequeued bytes.
On the second call the class is already passive, but its child qdisc is still empty, so updatevf() arms gopassive again:
if (cl->qdisc->q.qlen == 0 && cl->clflags & HFSCFSC) gopassive = 1;
The leaf is then skipped by the clnactive == 0 check inside the loop, which does not clear gopassive, so the stale gopassive propagates to the parent and decrements its clnactive a second time. A parent that still has other active children is driven to clnactive == 0 and removed from the vttree, even though those siblings are still backlogged. They are never dequeued again and the qdisc stalls.
Fix this by only arming gopassive when the class is actually active, so an already-passive class no longer triggers a second passive transition. The byte accounting (cl->cltotal += len) still runs for every ancestor, so dequeued bytes continue to be counted exactly once.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-38.38.1 - Compensating control
In Linux kernel net/sched sch_hfsc, modify update_vf() so go_passive is armed only when the class is actually active, preventing an already-passive class from triggering a second passive transition.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-74284?
The severity of CVE-2026-74284 is rated at risk level 23.
How do I fix CVE-2026-74284?
To fix CVE-2026-74284, you should update to the patched version of the Linux kernel that addresses this vulnerability.
What are the potential impacts of CVE-2026-74284?
The potential impacts of CVE-2026-74284 may include unexpected behavior in traffic scheduling due to improper class management.
Is CVE-2026-74284 exploitable remotely?
CVE-2026-74284 primarily affects the internal scheduling of traffic in the Linux kernel, making it less likely to be directly exploitable remotely.
Which versions of Linux are affected by CVE-2026-74284?
CVE-2026-74284 affects specific versions of the Linux kernel that contain the vulnerable sch_hfsc scheduler.