CVE-2026-74284
Low risk· EPSS 12%Exploitation Probability (EPSS)
Low risk12th percentile - higher than 12% of all known CVEs
Summary
A vulnerability was found in the Linux kernel's HFSC network scheduler (sch_hfsc). During a single dequeue, a class can be marked passive twice, leading to incorrect decrementing of the parent's active class counter. As a result, a parent with active child classes is removed from the vttree, and queues stop being processed (stall).
Risk Assessment
The vulnerability can cause a complete network traffic stall on interfaces using the HFSC discipline, resulting in unavailability of network services. An attacker can intentionally trigger this scenario by sending crafted traffic, causing a denial of service.
Recommendation
Immediately update the Linux kernel to a version containing the fix (commit resolving the issue). If updating is not possible, consider temporarily disabling the HFSC discipline or limiting its use until the patch is applied.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: net/sched: sch_hfsc: Don't make class passive twice update_vf() is called from two places for the same class during a single dequeue when the class's child qdisc (e.g. codel/fq_codel) drops its last packets while dequeuing: 1. The child calls qdisc_tree_reduce_backlog(), which, now that the child is empty, invokes hfsc_qlen_notify() -> update_vf(cl, 0, 0) and turns the class passive (cl_nactive is decremented up the hierarchy). 2. hfsc_dequeue() then calls update_vf(cl, qdisc_pkt_len(skb), cur_time) to charge the dequeued bytes. On the second call the class is already passive, but its child qdisc is still empty, so update_vf() arms go_passive again: if (cl->qdisc->q.qlen == 0 && cl->cl_flags & HFSC_FSC) go_passive = 1; The leaf is then skipped by the cl_nactive == 0 check inside the loop, which does not clear go_passive, so the stale go_passive propagates to the parent and decrements its cl_nactive a second time. A parent that still has other active children is driven to cl_nactive == 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 go_passive when the class is actually active, so an already-passive class no longer triggers a second passive transition. The byte accounting (cl->cl_total += len) still runs for every ancestor, so dequeued bytes continue to be counted exactly once.