AZL-80666

In the Linux kernel, the following vulnerability has been resolved:

net: sched: avoid qdiscresetalltxgt() vs dequeue race for lockless qdiscs

When shrinking the number of real tx queues, netifsetrealnumtxqueues() calls qdiscresetalltx_gt() to flush qdiscs for queues which will no longer be used.

qdiscresetalltxgt() currently serializes qdiscreset() with qdisclock(). However, for lockless qdiscs, the dequeue path is serialized by qdiscrunbegin/end() using qdisc->seqlock instead, so qdisc_reset() can run concurrently with _qdiscrun() 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 __devqueuexmit+0x248f/0x3550 ipfinishoutput2+0xa42/0x2110 ipoutput+0x1a7/0x410 ipsendskb+0x2e6/0x480 udpsendskb+0xb0a/0x1590 udpsendmsg+0x13c9/0x1fc0 ... </TASK>

Allocated by task 1270 on cpu 5 at 44.558414s: ... allocskbwithfrags+0x84/0x7c0 sockallocsendpskb+0x69a/0x830 _ipappenddata+0x1b86/0x48c0 ipmakeskb+0x1e8/0x2b0 udpsendmsg+0x13a6/0x1fc0 ...

Freed by task 1306 on cpu 3 at 44.558445s: ... kmemcachefree+0x117/0x5e0 pfifofastreset+0x14d/0x580 qdiscreset+0x9e/0x5f0 netifsetrealnumtxqueues+0x303/0x840 virtnetsetchannels+0x1bf/0x260 [virtionet] ethnlsetchannels+0x684/0xae0 ethnldefaultsetdoit+0x31a/0x890 ...

Serialize qdiscresetalltxgt() against the lockless dequeue path by taking qdisc->seqlock for TCQFNOLOCK qdiscs, matching the serialization model already used by devresetqueue().

Additionally clear QDISCSTATENON_EMPTY after reset so the qdisc state reflects an empty queue, avoiding needless re-scheduling.