CVE-2026-23340

High · 7.8

CVSS

7.8

EPSS

0.1%

Published

Mar 25, 2026

CISA due date

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Cloudanix analysis

Monitor only SSVC: Track

A flaw in the Linux kernel's network handling can let a local user crash the system or potentially run harmful code when network queues are frequently resized under load.

Exploitation: Local Privilege
Attacker needs: Low Privilege
Complexity: Moderate
Impact: Full Compromise
Component: OS kernel
Class: Use After Free
Remediation: Patch
Effort: Medium
Patch risk: Medium

What it actually does

Race condition between qdisc_reset_all_tx_gt() and the lockless dequeue path in net/sched allows qdisc_reset() to free skbs concurrently with __qdisc_run(), producing a use-after-free on lockless qdiscs.

Why it matters to the business

Local compromise or kernel panic is possible on Linux systems that dynamically change TX queue counts under network load, increasing downtime and potential data exposure risk on servers and virtualized environments.

How an attack unfolds

An attacker with local access generates sustained network traffic while repeatedly invoking ethtool or netlink operations to shrink and expand the number of real TX queues, racing qdisc_reset against active dequeuing and freeing in-use socket buffers.

How to fix it

Apply a kernel update containing the fix that serializes qdisc_reset_all_tx_gt() by taking qdisc->seqlock for TCQ_F_NOLOCK qdiscs and clears QDISC_STATE_NON_EMPTY after reset; rebuild and reboot with the patched kernel.

How to confirm the fix worked

Confirm the running kernel source contains the seqlock acquisition around qdisc_reset for lockless qdiscs and the QDISC_STATE_NON_EMPTY clearing logic; re-test the reproducer with KASAN disabled to verify absence of UAF.

Typical attack path

Obtain local user access on an affected Linux system
Initiate heavy outbound network traffic
Repeatedly call netif_set_real_num_tx_queues via ethtool or netlink
Trigger concurrent qdisc reset and dequeue leading to UAF

How to detect it

Inspect kernel logs and KASAN output for slab-use-after-free in __qdisc_run or pfifo_fast_reset during netif_set_real_num_tx_queues operations; correlate with ethtool channel change events.

What your security team should do

Apply the kernel fix to all systems that perform runtime TX queue changes
Restrict unprivileged use of ethtool -L and equivalent netlink operations where possible
Monitor kernel logs for slab-use-after-free reports involving __qdisc_run

The record

In the Linux kernel, the following vulnerability has been resolved: net: sched: avoid qdisc_reset_all_tx_gt() vs dequeue race for lockless qdiscs When shrinking the number of real tx queues, netif_set_real_num_tx_queues() calls qdisc_reset_all_tx_gt() to flush qdiscs for queues which will no longer be used. qdisc_reset_all_tx_gt() currently serializes qdisc_reset() with qdisc_lock(). However, for lockless qdiscs, the dequeue path is serialized by qdisc_run_begin/end() using qdisc->seqlock instead, so qdisc_reset() can run concurrently with __qdisc_run() and free skbs while they are still being dequeued, leading to UAF. This can easily be reproduced on e.g. virtio-net by imposing heavy traffic while frequently changing the number of queue pairs: iperf3 -ub0 -c $peer -t 0 & while :; do ethtool -L eth0 combined 1 ethtool -L eth0 combined 2 done With KASAN enabled, this leads to reports like: BUG: KASAN: slab-use-after-free in __qdisc_run+0x133f/0x1760 ... Call Trace: <TASK> ... __qdisc_run+0x133f/0x1760 __dev_queue_xmit+0x248f/0x3550 ip_finish_output2+0xa42/0x2110 ip_output+0x1a7/0x410 ip_send_skb+0x2e6/0x480 udp_send_skb+0xb0a/0x1590 udp_sendmsg+0x13c9/0x1fc0 ... </TASK> Allocated by task 1270 on cpu 5 at 44.558414s: ... alloc_skb_with_frags+0x84/0x7c0 sock_alloc_send_pskb+0x69a/0x830 __ip_append_data+0x1b86/0x48c0 ip_make_skb+0x1e8/0x2b0 udp_sendmsg+0x13a6/0x1fc0 ... Freed by task 1306 on cpu 3 at 44.558445s: ... kmem_cache_free+0x117/0x5e0 pfifo_fast_reset+0x14d/0x580 qdisc_reset+0x9e/0x5f0 netif_set_real_num_tx_queues+0x303/0x840 virtnet_set_channels+0x1bf/0x260 [virtio_net] ethnl_set_channels+0x684/0xae0 ethnl_default_set_doit+0x31a/0x890 ... Serialize qdisc_reset_all_tx_gt() against the lockless dequeue path by taking qdisc->seqlock for TCQ_F_NOLOCK qdiscs, matching the serialization model already used by dev_reset_queue(). Additionally clear QDISC_STATE_NON_EMPTY after reset so the qdisc state reflects an empty queue, avoiding needless re-scheduling.

Affected packages

Ecosystem	Package	Introduced	Fixed