net: sched: red: avoid hashing NULL child

Hangbin reported an Oops triggered by the syzkaller qdisc rules:

 kasan: GPF could be caused by NULL-ptr deref or user memory access
 general protection fault: 0000 [#1] SMP KASAN PTI
 Modules linked in: sch_red
 CPU: 0 PID: 28699 Comm: syz-executor5 Not tainted 4.17.0-rc4.kcov #1
 Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
 RIP: 0010:qdisc_hash_add+0x26/0xa0
 RSP: 0018:ffff8800589cf470 EFLAGS: 00010203
 RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff824ad971
 RDX: 0000000000000007 RSI: ffffc9000ce9f000 RDI: 000000000000003c
 RBP: 0000000000000001 R08: ffffed000b139ea2 R09: ffff8800589cf4f0
 R10: ffff8800589cf50f R11: ffffed000b139ea2 R12: ffff880054019fc0
 R13: ffff880054019fb4 R14: ffff88005c0af600 R15: ffff880054019fb0
 FS:  00007fa6edcb1700(0000) GS:ffff88005ce00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000020000740 CR3: 000000000fc16000 CR4: 00000000000006f0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  red_change+0x2d2/0xed0 [sch_red]
  qdisc_create+0x57e/0xef0
  tc_modify_qdisc+0x47f/0x14e0
  rtnetlink_rcv_msg+0x6a8/0x920
  netlink_rcv_skb+0x2a2/0x3c0
  netlink_unicast+0x511/0x740
  netlink_sendmsg+0x825/0xc30
  sock_sendmsg+0xc5/0x100
  ___sys_sendmsg+0x778/0x8e0
  __sys_sendmsg+0xf5/0x1b0
  do_syscall_64+0xbd/0x3b0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
 RIP: 0033:0x450869
 RSP: 002b:00007fa6edcb0c48 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
 RAX: ffffffffffffffda RBX: 00007fa6edcb16b4 RCX: 0000000000450869
 RDX: 0000000000000000 RSI: 00000000200000c0 RDI: 0000000000000013
 RBP: 000000000072bea0 R08: 0000000000000000 R09: 0000000000000000
 R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
 R13: 0000000000008778 R14: 0000000000702838 R15: 00007fa6edcb1700
 Code: e9 0b fe ff ff 0f 1f 44 00 00 55 53 48 89 fb 89 f5 e8 3f 07 f3 fe 48 8d 7b 3c 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 04 84 d2 75 51
 RIP: qdisc_hash_add+0x26/0xa0 RSP: ffff8800589cf470

When a red qdisc is updated with a 0 limit, the child qdisc is left
unmodified, no additional scheduler is created in red_change(),
the 'child' local variable is rightfully NULL and must not add it
to the hash table.

This change addresses the above issue moving qdisc_hash_add() right
after the child qdisc creation. It additionally removes unneeded checks
for noop_qdisc.

Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Fixes: 49b499718fa1 ("net: sched: make default fifo qdiscs appear in the dump")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_red.c

@@ -222,10 +222,11 @@ static int red_change(struct Qdisc *sch, struct nlattr *opt,
 					 extack);
 		if (IS_ERR(child))
 			return PTR_ERR(child);
-	}
 
-	if (child != &noop_qdisc)
+		/* child is fifo, no need to check for noop_qdisc */
 		qdisc_hash_add(child, true);
+	}
+
 	sch_tree_lock(sch);
 	q->flags = ctl->flags;
 	q->limit = ctl->limit;

net/sched/sch_tbf.c

@@ -383,6 +383,9 @@ static int tbf_change(struct Qdisc *sch, struct nlattr *opt,
 			err = PTR_ERR(child);
 			goto done;
 		}
+
+		/* child is fifo, no need to check for noop_qdisc */
+		qdisc_hash_add(child, true);
 	}
 
 	sch_tree_lock(sch);
@@ -391,8 +394,6 @@ static int tbf_change(struct Qdisc *sch, struct nlattr *opt,
 					  q->qdisc->qstats.backlog);
 		qdisc_destroy(q->qdisc);
 		q->qdisc = child;
-		if (child != &noop_qdisc)
-			qdisc_hash_add(child, true);
 	}
 	q->limit = qopt->limit;
 	if (tb[TCA_TBF_PBURST])