sctp: add chunks to sk_backlog when the newsk sk_socket is not set

This patch is to fix a NULL-ptr deref in selinux_socket_connect_helper:

[...] kasan: GPF could be caused by NULL-ptr deref or user memory access
[...] RIP: 0010:selinux_socket_connect_helper+0x94/0x460
[...] Call Trace:
[...] selinux_sctp_bind_connect+0x16a/0x1d0
[...] security_sctp_bind_connect+0x58/0x90
[...] sctp_process_asconf+0xa52/0xfd0 [sctp]
[...] sctp_sf_do_asconf+0x785/0x980 [sctp]
[...] sctp_do_sm+0x175/0x5a0 [sctp]
[...] sctp_assoc_bh_rcv+0x285/0x5b0 [sctp]
[...] sctp_backlog_rcv+0x482/0x910 [sctp]
[...] __release_sock+0x11e/0x310
[...] release_sock+0x4f/0x180
[...] sctp_accept+0x3f9/0x5a0 [sctp]
[...] inet_accept+0xe7/0x720

It was caused by that the 'newsk' sk_socket was not set before going to
security sctp hook when processing asconf chunk with SCTP_PARAM_ADD_IP
or SCTP_PARAM_SET_PRIMARY:

inet_accept()->
sctp_accept():
lock_sock():
lock listening 'sk'
do_softirq():
sctp_rcv(): <-- [1]
asconf chunk arrives and
enqueued in 'sk' backlog
sctp_sock_migrate():
set asoc's sk to 'newsk'
release_sock():
sctp_backlog_rcv():
lock 'newsk'
sctp_process_asconf() <-- [2]
unlock 'newsk'
sock_graft():
set sk_socket <-- [3]

As it shows, at [1] the asconf chunk would be put into the listening 'sk'
backlog, as accept() was holding its sock lock. Then at [2] asconf would
get processed with 'newsk' as asoc's sk had been set to 'newsk'. However,
'newsk' sk_socket is not set until [3], while selinux_sctp_bind_connect()
would deref it, then kernel crashed.

Here to fix it by adding the chunk to sk_backlog until newsk sk_socket is
set when .accept() is done.

Note that sk->sk_socket can be NULL when the sock is closed, so SOCK_DEAD
flag is also needed to check in sctp_newsk_ready().

Thanks to Ondrej for reviewing the code.

Fixes: d452930fd3b9 ("selinux: Add SCTP support")
Reported-by: Ying Xu <[e-mail]>
Suggested-by: Marcelo Ricardo Leitner <[e-mail]>
Signed-off-by: Xin Long <[e-mail]>
Acked-by: Marcelo Ricardo Leitner <[e-mail]>
Acked-by: Neil Horman <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/sctp/sctp.h

···
 	return sctp_mtu_payload(sp, SCTP_DEFAULT_MINSEGMENT, datasize);
 }
 
+static inline bool sctp_newsk_ready(const struct sock *sk)
+{
+	return sock_flag(sk, SOCK_DEAD) || sk->sk_socket;
+}
+
 #endif /* __net_sctp_h__ */

net/sctp/input.c

···
 		bh_lock_sock(sk);
 	}
 
-	if (sock_owned_by_user(sk)) {
+	if (sock_owned_by_user(sk) || !sctp_newsk_ready(sk)) {
 		if (sctp_add_backlog(sk, skb)) {
 			bh_unlock_sock(sk);
 			sctp_chunk_free(chunk);
···
 		local_bh_disable();
 		bh_lock_sock(sk);
 
-		if (sock_owned_by_user(sk)) {
+		if (sock_owned_by_user(sk) || !sctp_newsk_ready(sk)) {
 			if (sk_add_backlog(sk, skb, sk->sk_rcvbuf))
 				sctp_chunk_free(chunk);
 			else
···
 		if (backloged)
 			return 0;
 	} else {
-		sctp_inq_push(inqueue, chunk);
+		if (!sctp_newsk_ready(sk)) {
+			if (!sk_add_backlog(sk, skb, sk->sk_rcvbuf))
+				return 0;
+			sctp_chunk_free(chunk);
+		} else {
+			sctp_inq_push(inqueue, chunk);
+		}
 	}
 
 done: