llc: hold llc_sap before release_sock()

syzbot reported we still access llc->sap in llc_backlog_rcv()
after it is freed in llc_sap_remove_socket():

Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1b9/0x294 lib/dump_stack.c:113
 print_address_description+0x6c/0x20b mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
 __asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:430
 llc_conn_ac_send_sabme_cmd_p_set_x+0x3a8/0x460 net/llc/llc_c_ac.c:785
 llc_exec_conn_trans_actions net/llc/llc_conn.c:475 [inline]
 llc_conn_service net/llc/llc_conn.c:400 [inline]
 llc_conn_state_process+0x4e1/0x13a0 net/llc/llc_conn.c:75
 llc_backlog_rcv+0x195/0x1e0 net/llc/llc_conn.c:891
 sk_backlog_rcv include/net/sock.h:909 [inline]
 __release_sock+0x12f/0x3a0 net/core/sock.c:2335
 release_sock+0xa4/0x2b0 net/core/sock.c:2850
 llc_ui_release+0xc8/0x220 net/llc/af_llc.c:204

llc->sap is refcount'ed and llc_sap_remove_socket() is paired
with llc_sap_add_socket(). This can be amended by holding its refcount
before llc_sap_remove_socket() and releasing it after release_sock().

Reported-by: <syzbot+6e181fc95081c2cf9051@syzkaller.appspotmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/llc/af_llc.c

@@ -189,6 +189,7 @@ static int llc_ui_release(struct socket *sock)
 {
 	struct sock *sk = sock->sk;
 	struct llc_sock *llc;
+	struct llc_sap *sap;
 
 	if (unlikely(sk == NULL))
 		goto out;
@@ -199,9 +200,15 @@ static int llc_ui_release(struct socket *sock)
 		llc->laddr.lsap, llc->daddr.lsap);
 	if (!llc_send_disc(sk))
 		llc_ui_wait_for_disc(sk, sk->sk_rcvtimeo);
+	sap = llc->sap;
+	/* Hold this for release_sock(), so that llc_backlog_rcv() could still
+	 * use it.
+	 */
+	llc_sap_hold(sap);
 	if (!sock_flag(sk, SOCK_ZAPPED))
 		llc_sap_remove_socket(llc->sap, sk);
 	release_sock(sk);
+	llc_sap_put(sap);
 	if (llc->dev)
 		dev_put(llc->dev);
 	sock_put(sk);