net/sched: pfifo_fast: fix wrong dereference in pfifo_fast_enqueue

Now that 'TCQ_F_CPUSTATS' bit can be cleared, depending on the value of
'TCQ_F_NOLOCK' bit in the parent qdisc, we can't assume anymore that
per-cpu counters are there in the error path of skb_array_produce().
Otherwise, the following splat can be seen:

 Unable to handle kernel paging request at virtual address 0000600dea430008
 Mem abort info:
   ESR = 0x96000005
   Exception class = DABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
 Data abort info:
   ISV = 0, ISS = 0x00000005
   CM = 0, WnR = 0
 user pgtable: 64k pages, 48-bit VAs, pgdp = 000000007b97530e
 [0000600dea430008] pgd=0000000000000000, pud=0000000000000000
 Internal error: Oops: 96000005 [#1] SMP
[...]
 pstate: 10000005 (nzcV daif -PAN -UAO)
 pc : pfifo_fast_enqueue+0x524/0x6e8
 lr : pfifo_fast_enqueue+0x46c/0x6e8
 sp : ffff800d39376fe0
 x29: ffff800d39376fe0 x28: 1ffff001a07d1e40
 x27: ffff800d03e8f188 x26: ffff800d03e8f200
 x25: 0000000000000062 x24: ffff800d393772f0
 x23: 0000000000000000 x22: 0000000000000403
 x21: ffff800cca569a00 x20: ffff800d03e8ee00
 x19: ffff800cca569a10 x18: 00000000000000bf
 x17: 0000000000000000 x16: 0000000000000000
 x15: 0000000000000000 x14: ffff1001a726edd0
 x13: 1fffe4000276a9a4 x12: 0000000000000000
 x11: dfff200000000000 x10: ffff800d03e8f1a0
 x9 : 0000000000000003 x8 : 0000000000000000
 x7 : 00000000f1f1f1f1 x6 : ffff1001a726edea
 x5 : ffff800cca56a53c x4 : 1ffff001bf9a8003
 x3 : 1ffff001bf9a8003 x2 : 1ffff001a07d1dcb
 x1 : 0000600dea430000 x0 : 0000600dea430008
 Process ping (pid: 6067, stack limit = 0x00000000dc0aa557)
 Call trace:
  pfifo_fast_enqueue+0x524/0x6e8
  htb_enqueue+0x660/0x10e0 [sch_htb]
  __dev_queue_xmit+0x123c/0x2de0
  dev_queue_xmit+0x24/0x30
  ip_finish_output2+0xc48/0x1720
  ip_finish_output+0x548/0x9d8
  ip_output+0x334/0x788
  ip_local_out+0x90/0x138
  ip_send_skb+0x44/0x1d0
  ip_push_pending_frames+0x5c/0x78
  raw_sendmsg+0xed8/0x28d0
  inet_sendmsg+0xc4/0x5c0
  sock_sendmsg+0xac/0x108
  __sys_sendto+0x1ac/0x2a0
  __arm64_sys_sendto+0xc4/0x138
  el0_svc_handler+0x13c/0x298
  el0_svc+0x8/0xc
 Code: f9402e80 d538d081 91002000 8b010000 (885f7c03)

Fix this by testing the value of 'TCQ_F_CPUSTATS' bit in 'qdisc->flags',
before dereferencing 'qdisc->cpu_qstats'.

Fixes: 8a53e616de ("net: sched: when clearing NOLOCK, clear TCQ_F_CPUSTATS, too")
CC: Paolo Abeni <pabeni@redhat.com>
CC: Stefano Brivio <sbrivio@redhat.com>
Reported-by: Li Shuang <shuali@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -624,8 +624,12 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc,
 
 	err = skb_array_produce(q, skb);
 
-	if (unlikely(err))
-		return qdisc_drop_cpu(skb, qdisc, to_free);
+	if (unlikely(err)) {
+		if (qdisc_is_percpu_stats(qdisc))
+			return qdisc_drop_cpu(skb, qdisc, to_free);
+		else
+			return qdisc_drop(skb, qdisc, to_free);
+	}
 
 	qdisc_update_stats_at_enqueue(qdisc, pkt_len);
 	return NET_XMIT_SUCCESS;