net/sched: ets: use old 'nbands' while purging unused classes

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

net/sched: ets: use old 'nbands' while purging unused classes

Shuang reported sch_ets test-case [1] crashing in ets_class_qlen_notify()
after recent changes from Lion [2]. The problem is: in ets_qdisc_change()
we purge unused DWRR queues; the value of 'q->nbands' is the new one, and
the cleanup should be done with the old one. The problem is here since my
first attempts to fix ets_qdisc_change(), but it surfaced again after the
recent qdisc len accounting fixes. Fix it purging idle DWRR queues before
assigning a new value of 'q->nbands', so that all purge operations find a
consistent configuration:

- old 'q->nbands' because it's needed by ets_class_find()
- old 'q->nstrict' because it's needed by ets_class_is_strict()

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 62 UID: 0 PID: 39457 Comm: tc Kdump: loaded Not tainted 6.12.0-116.el10.x86_64 #1 PREEMPT(voluntary)
Hardware name: Dell Inc. PowerEdge R640/06DKY5, BIOS 2.12.2 07/09/2021
RIP: 0010:__list_del_entry_valid_or_report+0x4/0x80
Code: ff 4c 39 c7 0f 84 39 19 8e ff b8 01 00 00 00 c3 cc cc cc cc 66 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa <48> 8b 17 48 8b 4f 08 48 85 d2 0f 84 56 19 8e ff 48 85 c9 0f 84 ab
RSP: 0018:ffffba186009f400 EFLAGS: 00010202
RAX: 00000000000000d6 RBX: 0000000000000000 RCX: 0000000000000004
RDX: ffff9f0fa29b69c0 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffffffc12c2400 R08: 0000000000000008 R09: 0000000000000004
R10: ffffffffffffffff R11: 0000000000000004 R12: 0000000000000000
R13: ffff9f0f8cfe0000 R14: 0000000000100005 R15: 0000000000000000
FS: 00007f2154f37480(0000) GS:ffff9f269c1c0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 00000001530be001 CR4: 00000000007726f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
<TASK>
ets_class_qlen_notify+0x65/0x90 [sch_ets]
qdisc_tree_reduce_backlog+0x74/0x110
ets_qdisc_change+0x630/0xa40 [sch_ets]
__tc_modify_qdisc.constprop.0+0x216/0x7f0
tc_modify_qdisc+0x7c/0x120
rtnetlink_rcv_msg+0x145/0x3f0
netlink_rcv_skb+0x53/0x100
netlink_unicast+0x245/0x390
netlink_sendmsg+0x21b/0x470
____sys_sendmsg+0x39d/0x3d0
___sys_sendmsg+0x9a/0xe0
__sys_sendmsg+0x7a/0xd0
do_syscall_64+0x7d/0x160
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f2155114084
Code: 89 02 b8 ff ff ff ff eb bb 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 f3 0f 1e fa 80 3d 25 f0 0c 00 00 74 13 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 48 83 ec 28 89 54 24 1c 48 89
RSP: 002b:00007fff1fd7a988 EFLAGS: 00000202 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000560ec063e5e0 RCX: 00007f2155114084
RDX: 0000000000000000 RSI: 00007fff1fd7a9f0 RDI: 0000000000000003
RBP: 00007fff1fd7aa60 R08: 0000000000000010 R09: 000000000000003f
R10: 0000560ee9b3a010 R11: 0000000000000202 R12: 00007fff1fd7aae0
R13: 000000006891ccde R14: 0000560ec063e5e0 R15: 00007fff1fd7aad0
</TASK>

[1] https://lore.kernel.org/netdev/e08c7f4a6882f260011909a868311c6e9b54f3e4.1639153474.git.dcaratti@redhat.com/
[2] https://lore.kernel.org/netdev/d912cbd7-193b-4269-9857-525bee8bbb6a@gmail.com/