CVE-2026-98622: net/sched: act_ct: don't WARN on benign flow_offload_alloc() failure
In the Linux kernel, the following vulnerability has been resolved:
net/sched: actct: don't WARN on benign flowoffloadalloc() failure
flowoffloadalloc() returns NULL when the conntrack entry is dying (e.g. raced with a conntrack flush) or when the GFPATOMIC allocation fails; both are expected under load and neither is a kernel bug. This path runs from softirq on every committed packet, so with paniconwarn=1 an unprivileged user can panic the box just by racing a conntrack flush against a tc ... action ct commit classifier.
Reproduced with a custom repro under QEMU: a small, fixed set of UDP flows through tc filter ... action ct commit on lo, raced against threads flooding bare ctnetlink CTDELETE (flush) requests. Hits WARNING: net/sched/actct.c:437 (tcfctflowtableadd(), inlined into tcfctact() in this build) within ~15s on the unpatched kernel; same setup is clean on the patched kernel. The fix itself is behavior-preserving: both branches already did goto erralloc before and after, only the WARN is removed.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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Fixed in 6.6.159.1-1 - Upgrade
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Event History
Frequently Asked Questions
Under what conditions can this lead to a system panic?
A panic requires panic_on_warn=1. An unprivileged user must be able to race conntrack flush or CT_DELETE activity against traffic matching a tc classifier using action ct commit, causing flow_offload_alloc() to fail and trigger the warning on an unpatched kernel.
Are failures from flow_offload_alloc() necessarily signs of a kernel fault?
No. The allocation can return NULL when a conntrack entry is dying, such as during a conntrack flush, or when a GFP_ATOMIC allocation fails under load; both conditions are expected.
How can I determine whether a system is affected?
On an unpatched kernel, reproducing the race can produce a WARNING in net/sched/act_ct.c, in tcf_ct_flow_table_add() or tcf_ct_act(). The reported reproduction used UDP flows through tc action ct commit on the loopback interface while flooding ctnetlink CT_DELETE requests.
What can be done if the fix cannot be deployed immediately?
Avoid using panic_on_warn=1 where this risk is unacceptable, and restrict unprivileged users' ability to create or manipulate tc ct commit classifiers and to issue conntrack deletion or flush requests.