net/sched: sch_cbs: Call qdisc_reset for child qdisc

net/sched: sch_cbs: Call qdisc_reset for child qdisc

During a reset, CBS is not calling reset on its child qdisc, which
might cause qlen/backlog accounting issues. For example, if we have CBS
with a QFQ parent and a netem child with delay, we can create a scenario
where the parent's qlen underflows. QFQ, specifically, uses qlen to
check whether it should deference a pointer, so this scenario may cause
a null-ptr deref in QFQ:

[   43.875639][  T319] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000009: 0000 [#1] SMP KASAN NOPTI
[   43.876124][  T319] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
[   43.876417][  T319] CPU: 10 UID: 0 PID: 319 Comm: ping Not tainted 7.0.0-13039-ge728258debd5 #773 PREEMPT(full)
[   43.876751][  T319] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   43.876949][  T319] RIP: 0010:qfq_dequeue+0x35c/0x1650
[   43.877123][  T319] Code: 00 fc ff df 80 3c 02 00 0f 85 17 0e 00 00 4c 8d 73 48 48 89 9d b8 02 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 0f 85 76 0c 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b
[   43.877648][  T319] RSP: 0018:ffff8881017ef4f0 EFLAGS: 00010216
[   43.877845][  T319] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: dffffc0000000000
[   43.878073][  T319] RDX: 0000000000000009 RSI: 0000000c40000000 RDI: ffff88810eef02b0
[   43.878306][  T319] RBP: ffff88810eef0000 R08: ffff88810eef0280 R09: 1ffff1102120fd63
[   43.878523][  T319] R10: 1ffff1102120fd66 R11: 1ffff1102120fd67 R12: 0000000c40000000
[   43.878742][  T319] R13: ffff88810eef02b8 R14: 0000000000000048 R15: 0000000020000000
[   43.878959][  T319] FS:  00007f9c51c47c40(0000) GS:ffff88817a0be000(0000) knlGS:0000000000000000
[   43.879214][  T319] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   43.879403][  T319] CR2: 000055e69a2230a8 CR3: 000000010c07a000 CR4: 0000000000750ef0
[   43.879621][  T319] PKRU: 55555554
[   43.879735][  T319] Call Trace:
[   43.879844][  T319]  <TASK>
[   43.879924][  T319]  __qdisc_run+0x169/0x1900
[   43.880075][  T319]  ? dev_qdisc_enqueue+0x8b/0x210
[   43.880222][  T319]  __dev_queue_xmit+0x2346/0x37a0
[   43.880376][  T319]  ? register_lock_class+0x3f/0x800
[   43.880531][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.880684][  T319]  ? __pfx___dev_queue_xmit+0x10/0x10
[   43.880834][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.880977][  T319]  ? __lock_acquire+0x819/0x1df0
[   43.881124][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.881275][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.881418][  T319]  ? __asan_memcpy+0x3c/0x60
[   43.881563][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.881708][  T319]  ? eth_header+0x165/0x1a0
[   43.881853][  T319]  ? lockdep_hardirqs_on_prepare+0xdb/0x1a0
[   43.882031][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.882174][  T319]  ? neigh_resolve_output+0x3cc/0x7e0
[   43.882325][  T319]  ? srso_alias_return_thunk+0x5/0xfbef5
[   43.882471][  T319]  ip_finish_output2+0x6b6/0x1e10

Fix this by calling qdisc_reset for CBS' child qdisc.
Sashiko caught an issue which could result in a null ptr deref if
qdisc_create_dflt() is invoked on an unitialised cbs qdisc which is exposed
by this patch. We add an early return if the qdisc is null to address this.
This is a similar approach used by two other fixes[1][2].

The proper fix for this specific issue elucidated by sashiko is to remove
the call to qdisc_reset when qdisc_create_dflt fails. Since the dflt qdisc
isn't attached anywhere yet at that point, calling the reset callback doesn't
make much sense (and as stated has been a source of two other bugs).
We plan on  submitting this fix in a later patch.
[1] https://lore.kernel.org/netdev/20221018063201.306474-2-shaozhengchao@huawei.com/
[2] https://lore.kernel.org/netdev/20221018063201.306474-4-shaozhengchao@huawei.com/

Fixes: 585d763af09c ("net/sched: Introduce Credit Based Shaper (CBS) qdisc")
Reported-by: Junyoung Jang <graypanda.inzag@gmail.com>
Tested-by: Junyoung Jang <graypanda.inzag@gmail.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>