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

net/sched: sch_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen

Whenever codel drops packets during peek, it calls
qdisc_tree_reduce_backlog. An issue arises because it calls
qdisc_tree_reduce_backlog before it reincrements the qlen. If qlen drops
to zero, but peek returns an skb, the parent's qlen_notify callback will
be executed even though codel still has 1 packet on the queue and, thus,
will mistakenly deactivate the parent's class causing issues like a wild
memory access when qfq has codel as a child:

[   36.339843][  T370] Oops: general protection fault, probably for non-canonical address 0xfbd59c0000000024: 0000 [#1] SMP KASAN NOPTI
[   36.340408][  T370] KASAN: maybe wild-memory-access in range [0xdead000000000120-0xdead000000000127]
[   36.340737][  T370] CPU: 2 UID: 0 PID: 370 Comm: tc Not tainted 7.1.0-rc5-00287-g66e13b626592 #87 PREEMPT(full)
[   36.341113][  T370] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   36.341357][  T370] RIP: 0010:qfq_deactivate_agg (include/linux/list.h:1029 (discriminator 2) include/linux/list.h:1043 (discriminator 2) net/sched/sch_qfq.c:1369 (discriminator 2) net/sched/sch_qfq.c:1395 (discriminator 2)) sch_qfq
[   36.342221][  T370] RSP: 0018:ffff8881100ef370 EFLAGS: 00010216
[   36.342422][  T370] RAX: 0000000000000000 RBX: ffff8881058a9568 RCX: dffffc0000000000
[   36.342664][  T370] RDX: 1ffff11021064dc3 RSI: ffff888108326e00 RDI: dffffc0000000000
[   36.342905][  T370] RBP: ffff8881058a8280 R08: dead000000000122 R09: 1bd5a00000000024
[   36.343140][  T370] R10: fffffbfff2940329 R11: fffffbfff2940329 R12: 0000000000000000
[   36.343383][  T370] R13: dead000000000100 R14: ffff8881058a9580 R15: ffff8881058a9578
[   36.343631][  T370] FS:  00007fc04b0ca780(0000) GS:ffff888184fef000(0000) knlGS:0000000000000000
[   36.343911][  T370] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   36.344116][  T370] CR2: 0000557c02c02000 CR3: 000000010e0ba000 CR4: 0000000000750ef0
[   36.344359][  T370] PKRU: 55555554
[   36.344481][  T370] Call Trace:
...
[   36.345054][  T370] qfq_reset_qdisc (net/sched/sch_qfq.c:357 net/sched/sch_qfq.c:1487) sch_qfq
[   36.345222][  T370]  qdisc_reset (net/sched/sch_generic.c:1057)
[   36.345503][  T370]  __qdisc_destroy (net/sched/sch_generic.c:1096)
[   36.345677][  T370]  qdisc_graft (net/sched/sch_api.c:1062 net/sched/sch_api.c:1053 net/sched/sch_api.c:1159)
[   36.346335][  T370]  tc_get_qdisc (net/sched/sch_api.c:1528 net/sched/sch_api.c:1556)

Fix this by only calling qdisc_tree_reduce_backlog in peek after the
qlen is restored.

https://git.kernel.org/stable/c/52f1da34c9f4d5bdc1e8b44242da5c7ba8db85f3

https://git.kernel.org/stable/c/755108bb7a5083e911294c416cfea605dc75632f

https://git.kernel.org/stable/c/7a05af7f58566682b73578b72ca8e53a268c43bd

https://git.kernel.org/stable/c/91e0a793a72374c20ab31a40ccec21373e82e973

https://git.kernel.org/stable/c/bb9cfd874ee884117b33e92a002b9a45b48202f4

https://git.kernel.org/stable/c/e4615aa6bb7802944ae790cb4b3ef8c1b7491af3

https://git.kernel.org/stable/c/e8c6dbadf139cb14ea6ed14add6ed6e88504dedd

https://git.kernel.org/stable/c/eba49fd85995a3851c597fa2d214f8d21736e1d8

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