Cloudanix VulnDB

CVE-2026-98811

None

CVSS

—

EPSS

0.2%

Published

Oct 18, 2026

CISA due date

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Cloudanix analysis

Schedule a fix SSVC: Track

A Linux kernel networking bug can leave behind wasted resources when multiple traffic-filter changes happen simultaneously, which might eventually slow down or disrupt network handling.

Exploitation
Local Privilege
Attacker needs
Low Privilege
Complexity
Moderate
Impact
Denial Of Service
Component
OS kernel
Class
Race Condition
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

A race condition in the classifier API allows concurrent filter addition requests to leave an empty protocol structure linked in the chain, retaining references to the chain, block, and module without reclamation on error paths.

Why it matters to the business

Networked Linux systems may suffer gradual resource leaks during concurrent traffic control operations, risking memory exhaustion, module reference leaks, or degraded performance in high-throughput environments.

How an attack unfolds

An attacker with network admin privileges issues concurrent tc filter add requests against the same chain and priority on an unlocked classifier; both fail after handle publication, but the error path omits reclamation, leaving the empty proto resident.

How to fix it

Apply the upstream kernel patch that reclaims the proto on any failed request holding a reference and ensure emptiness-gated deletion occurs for all classifier types.

How to confirm the fix worked

After patching, repeat the concurrent filter-add reproducer and confirm no bare filter headers remain visible in tc filter show output; verify absence of residual module or block references via kernel memory accounting.

Typical attack path

  1. Obtain CAP_NET_ADMIN capability in a network namespace
  2. Configure a qdisc and chain on a network interface
  3. Launch two concurrent tc filter add commands targeting identical chain/prio
  4. Repeat the racing operations to accumulate unreclaimed protos

How to detect it

Search tc filter show output for bare protocol/pref headers lacking any filter entries; examine kernel logs for cls_api or tcf_chain_tp_delete_empty activity during filter operations.

  • bare `filter protocol ip pref 1 flower chain 0` header
  • empty flower tp linked after concurrent tc filter add failures

What your security team should do

  • Prioritize kernel updates on systems exposing tc filter configuration to untrusted namespaces
  • Restrict CAP_NET_ADMIN to minimal necessary processes and containers
  • Audit network namespace creation and tc usage in production workloads

The record

In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_api: reclaim an empty proto on the error path Two racing tc filter add requests on the same chain/prio of an unlocked classifier both run change() on the shared proto and both can fail: the winner's tcf_chain_tp_delete_empty() attempt gives up because the loser's handle is still in the idr, and the loser's error path drops only its own reference without a second reclamation attempt. The empty proto stays linked in the chain, holding the chain reference, a block reference and the classifier module reference until the chain or block is torn down. Reclaim the proto on the error path of any failed request that holds a proto reference. The reclamation is emptiness-gated: tcf_chain_tp_delete_empty() unlinks the proto only when delete_empty() admits it is empty, so a live shared proto is never unlinked. A proto the request created is reclaimed unconditionally - it is the only owner, so marking it for deletion is safe even without a delete_empty callback. Classifiers without one (the check marks the proto unconditionally) are rtnl-serialized, so the raced window this guard closes cannot arise for them. This is a follow-up to commit d4e359b3608a ("net/sched: cls_api: fix teardown of an adopted proto on insert-race loss"), which stopped the loser of the insert race from unlinking the winner's live proto but left the empty-proto residual in place. Conditions to recreate: - CONFIG_NET_CLS_FLOWER=y; veth pair - tc qdisc add dev veth0 ingress - two concurrent `tc filter add dev veth0 ingress protocol ip pref 1 flower skip_sw ... action drop` (both fail in fl_hw_replace_filter after publishing their handle in the idr); repeat in a loop - an empty flower tp stays linked after both requests fail; visible as a bare `filter protocol ip pref 1 flower chain 0` header in `tc filter show` with no filter entries - CAP_NET_ADMIN (namespace-local via unshare -Urn suffices)

Affected packages

Ecosystem Package Introduced Fixed