CVE-2026-99025: openvswitch: fix soft lockup in the netlink flow dump
In the Linux kernel, the following vulnerability has been resolved:
openvswitch: fix soft lockup in the netlink flow dump
A production compute node carrying a few thousand datapath flows hit a soft lockup inside a single netlink flow dump and panicked.
ovsflowcmddump() calls ovsflowstatsget() for every flow it emits, and that releases stats->lock with spinunlockbh() once per CPU that has touched the flow. Every release is a localbhenable(), and each one runs the pending softirq backlog in the dumping thread's own context.
The skb bounds the flows one callback emits, but not the softirq work it absorbs. On a CPU that carries the box's packet load the backlog refills as fast as it drains, so the dumping thread becomes that CPU's softirq engine. It never sleeps and it has no reschedule point, so under voluntary preemption nothing can take the CPU away from it: neither the ksoftirqd the kernel woke to take the work over, nor the stopper thread the softlockup detector dispatches to refresh its timestamp.
Hold BH off across the whole callback instead, the way ctnetlinkdumptable() does, so the nested spinunlockbh() stop draining softirqs. The loop already runs under rcureadlock() and cannot sleep. What it gives up is preemption under CONFIGPREEMPT, since a BH-off region is not preemptible outside PREEMPTRT. The region stays short: the skb caps the flows one callback emits, and empty buckets cost no skb space but are each visited once per dump, as the cursor only moves forward. The table grows on insert and shrinks only on flush, so the walk is bounded by the largest flow count the datapath has held. The softirq backlog the callback used to absorb has no bound at all.
Affected Software
Remediation
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Hold bottom halves (BH) disabled across the entire netlink flow-dump callback, as ctnetlink_dump_table() does, so nested spin_unlock_bh() calls do not drain pending softirqs in the dumping thread's context.
Event History
Frequently Asked Questions
Which systems are most likely to experience this issue?
Production compute nodes with Open vSwitch datapaths carrying thousands of flows are implicated. The described failure is most likely when the CPU performing a netlink flow dump also carries substantial packet-processing load, allowing the softirq backlog to refill while the dump runs.
What activity triggers the failure?
A netlink flow dump triggers the affected path because it collects statistics for every emitted flow. During the dump, repeated softirq processing can monopolize the dumping CPU, leading to a soft lockup and potentially a panic.
Are systems affected by default?
The data does not establish that every default Linux kernel or Open vSwitch configuration is affected. Exploitation requires the relevant Open vSwitch netlink flow-dump path together with sufficient datapath flow and packet load conditions.
What can be done if the fix cannot be applied immediately?
The provided data does not specify a supported mitigation. Reducing or avoiding netlink flow dumps on heavily loaded CPUs may reduce exposure to the described trigger, but this is an inference from the reported failure conditions rather than a documented workaround.