CVE-2026-98021

Overview

CVE-2026-98021 is a Linux kernel vulnerability that allowed oversized tx_queue_len values to bypass the S16_MAX cap during netlink link creation or modification. This could lead to excessive memory allocation in pfifo_fast, tun, and tap devices. The issue has been fixed by enforcing the cap at parse time.

Description

In the Linux kernel, the following vulnerability has been resolved: net: reject oversized tx_queue_len at netlink parse time rtnl_create_link() assigns IFLA_TXQLEN directly to dev->tx_queue_len without going through netif_change_tx_queue_len(), so a device created with "ip link add ... txqueuelen 500000" bypasses the S16_MAX cap and still triggers the oversized ring allocations in pfifo_fast, tun and tap. The veth peer nest (rtnl_nla_parse_ifinfomsg()) and the RTM_NEWLINK-on-existing-device path reach the same sinks. Enforce the cap in ifla_policy instead: IFLA_TXQLEN becomes NLA_POLICY_FULL_RANGE(NLA_U32, &txqlen_range) with txqlen_range = { .min = 0, .max = S16_MAX }. All netlink consumers parse against this policy - rtnl_setlink(), rtnl_newlink() (create and change), and the veth peer nest - so every netlink path is capped at parse time and rejects the attribute with -ERANGE plus a proper "integer out of range" extack message before any device state is modified (the RTM_SETLINK half-application wart is gone with it). Document the bound in the rt-link.yaml netlink spec. Conditions to recreate the bug: - CONFIG_NET_SCHED=y, CONFIG_VETH=y, CONFIG_USER_NS=y, CONFIG_NET_NS=y. - Unprivileged user in a fresh user+net namespace (unshare -Urn): ip link add v0 txqueuelen 500000 type veth peer name v1 -> on the fixed kernel this is rejected with -ERANGE ("integer out of range" extack) instead of installing an oversized tx_queue_len that later inflates pfifo_fast/tun/tap ring allocations. - ip link set v0 txqueuelen 500000 is likewise rejected at parse time.

Impact

Confidentiality: no direct data exposure. Integrity: potential denial of service via memory exhaustion. Availability: attackers could trigger OOM or crash the kernel. Defenders: unprivileged users in user namespaces are the primary threat actors.

Remediation

Upgrade to a kernel version that includes the CVE-2026-98021 fix. If upgrading is not immediately possible, disable CONFIG_NET_SCHED, CONFIG_VETH, CONFIG_USER_NS, and CONFIG_NET_NS in the kernel configuration or apply the upstream patch that enforces the tx_queue_len cap at parse time. Verify that netlink commands reject oversized txqueuelen values with an ERANGE error.

Risk context

High severity (CVSS 7.1). No EPSS data available. Defenders should treat this as a high‑priority issue and apply the fix promptly.

Affected products

• Linux kernel