HIGH
net sched Qdisc Reset UAF
CVE-2026-23340
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
KernelScan AI7.8HIGH
01Description
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.
02KernelScan AI Analysis
Risk summary
A race condition in the network scheduler allows concurrent access to freed memory when changing the number of TX queues on network interfaces. An attacker with network administration privileges could trigger this by rapidly changing queue configurations during heavy network traffic, potentially causing system crashes or memory corruption.
Vulnerability analysis
Root Cause: Race condition between qdisc_reset_all_tx_gt() and __qdisc_run() in lockless qdiscs. When shrinking TX queues, qdisc_reset_all_tx_gt() uses qdisc_lock() for serialization, but lockless qdiscs use qdisc->seqlock in their dequeue path. This allows qdisc_reset() to free skbs concurrently with __qdisc_run() still dequeuing them.
Attack Surface: Local attack surface requiring ability to change network interface queue configuration (ethtool -L) while network traffic is active. Typically requires CAP_NET_ADMIN or root privileges to modify interface settings.
Fix Mechanism: The patch adds proper serialization for lockless qdiscs by taking qdisc->seqlock in qdisc_reset_all_tx_gt() when TCQ_F_NOLOCK is set, matching the locking model used by the dequeue path. It also clears QDISC_STATE_MISSED and QDISC_STATE_DRAINING flags to ensure proper queue state after reset.
03Fix Versions
| Branch | Fixed in | Patch commit |
|---|---|---|
| 5.15 | 5.15.203 | 5bb27ad54d12 |
| 6.1 | 6.1.167 | 7594467c49bf |
| 6.12 | 6.12.77 | 5bc4e69306ed |
| 6.18 | 6.18.17 | 8314944cc3bd |
| 6.19 | 6.19.7 | c69df4e0524f |
| 6.6 | 6.6.130 | dbd58b0730aa |
| mainline | 7.0 | 7f083faf59d1 |