CVE-2026-98022net: cap tx_queue_len at S16_MAX to prevent oversized ring allocations

Official description

In the Linux kernel, the following vulnerability has been resolved: net: cap tx_queue_len at S16_MAX to prevent oversized ring allocations Several subsystems allocate ring buffers sized by dev->tx_queue_len with no upper bound. An unprivileged user (via unshare -Urn) can set a huge tx_queue_len and exhaust global memory with ring allocations: - pfifo_fast: pfifo_fast_init() and pfifo_fast_change_tx_queue_len() allocate 3 skb_array rings of tx_queue_len entries each. - tun: tun_queue_resize() and the queue-attach path resize ptr_rings to tx_queue_len on the NETDEV_CHANGE_TX_QUEUE_LEN notifier. - tap (macvtap/ipvtap): tap_queue_resize() and tap_init() resize/init ptr_rings to tx_queue_len on the same notifier. netif_change_tx_queue_len() is the single entry point for IFLA_TXQLEN, sysfs, and the SIOCSIFTXQLEN ioctl. Cap new_len at S16_MAX (32767) there so the oversized value is rejected at set time. This takes effect whether the device is up or down, before dev->tx_queue_len is written, before any notifier fires, and before any ring is allocated. The "> S16_MAX" check also subsumes the previous unsigned-long truncation test, and a negative ifr_qlen from the ioctl lands far above the cap after conversion, so both old failure modes are covered by the one comparison. tx_queue_len is ambigious: both a per-ring sizing multiplier and a default queue-length/limit knob for consumers that allocate nothing at set time (pfifo/bfifo/gred/plug/sfb limits, htb direct_qlen, qfq max_classes, teql). 32767 is chosen as the largest value NLA_POLICY_FULL_RANGE can express for the u32 IFLA_TXQLEN policy in patch 2/3 while staying a legitimate queue length on high-BDP paths; the ring-memory trade-off of a shared knob is disclosed below. 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). - pfifo_fast: create veth pairs, set tx_queue_len to 500000, attach mq+pfifo_fast. ~28 iterations OOMs a 2GB guest. - tun: create 50 tun devices with IFF_MULTI_QUEUE, set tx_queue_len to 500000, open 8 queues each. ~1.6GB of ptr_ring allocations OOMs a 512MB guest. - tap: same as tun with IFF_TAP. ~960MB OOMs a 512MB guest. - On the fixed kernel the oversized tx_queue_len is rejected with -ERANGE at set time (all four paths: RTM_SETLINK, RTM_NEWLINK create, sysfs, ioctl - the latter two via this check, the former two via this check and the 2/3 parse policy respectively).

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/24d02c909a1725ab9eceb92db7d02e4e841f17ca
• https://git.kernel.org/stable/c/5e5591a0f33f7c11bf1d338dbb6c5072575de64c
• https://git.kernel.org/stable/c/c8d4060610c69645733c92d94fceb4b93808ac4d
• https://git.kernel.org/stable/c/c2362c500931e5bd1826bce34e9ce28af80e7541
• https://git.kernel.org/stable/c/81c0e02076989743a603806a5a4f44d2c40dc01f
• https://git.kernel.org/stable/c/081340cf70a789b94b4174af54a63ffd49677a4e
• https://git.kernel.org/stable/c/a03b927bae4af5c23fd5315a237855c17cc8f422
• https://git.kernel.org/stable/c/66ab4c59b74db7ab53a1c9083feaaede393a96a0