sctp: not copy sctp_sock pd_lobby in sctp_copy_descendant

Now sctp_copy_descendant() copies pd_lobby from old sctp scok to new
sctp sock. If sctp_sock_migrate() returns error, it will panic when
releasing new sock and trying to purge pd_lobby due to the incorrect
pointers in pd_lobby.

  [  120.485116] kasan: CONFIG_KASAN_INLINE enabled
  [  120.486270] kasan: GPF could be caused by NULL-ptr deref or user
  [  120.509901] Call Trace:
  [  120.510443]  sctp_ulpevent_free+0x1e8/0x490 [sctp]
  [  120.511438]  sctp_queue_purge_ulpevents+0x97/0xe0 [sctp]
  [  120.512535]  sctp_close+0x13a/0x700 [sctp]
  [  120.517483]  inet_release+0xdc/0x1c0
  [  120.518215]  __sock_release+0x1d2/0x2a0
  [  120.519025]  sctp_do_peeloff+0x30f/0x3c0 [sctp]

We fix it by not copying sctp_sock pd_lobby in sctp_copy_descendan(),
and skb_queue_head_init() can also be removed in sctp_sock_migrate().

Reported-by: syzbot+85e0b422ff140b03672a@syzkaller.appspotmail.com
Fixes: 89664c62 ("sctp: sctp_sock_migrate() returns error if sctp_bind_addr_dup() fails")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -9169,7 +9169,7 @@ static inline void sctp_copy_descendant(struct sock *sk_to,
 {
 	int ancestor_size = sizeof(struct inet_sock) +
 			    sizeof(struct sctp_sock) -
-			    offsetof(struct sctp_sock, auto_asconf_list);
+			    offsetof(struct sctp_sock, pd_lobby);
 
 	if (sk_from->sk_family == PF_INET6)
 		ancestor_size += sizeof(struct ipv6_pinfo);
@@ -9253,7 +9253,6 @@ static int sctp_sock_migrate(struct sock *oldsk, struct sock *newsk,
 	 * 2) Peeling off partial delivery; keep pd_lobby in new pd_lobby.
 	 * 3) Peeling off non-partial delivery; move pd_lobby to receive_queue.
 	 */
-	skb_queue_head_init(&newsp->pd_lobby);
 	atomic_set(&sctp_sk(newsk)->pd_mode, assoc->ulpq.pd_mode);
 
 	if (atomic_read(&sctp_sk(oldsk)->pd_mode)) {