net: rds: Fix NULL ptr use in rds_tcp_kill_sock

BugLink: https://bugs.launchpad.net/bugs/1848589

After the commit c4e97b06cfdc ("net: rds: force to destroy
connection if t_sock is NULL in rds_tcp_kill_sock()."),
it introduced null-ptr-deref in rds_tcp_kill_sock as below:

BUG: KASAN: null-ptr-deref on address 0000000000000020
Read of size 8 by task kworker/u16:10/910
CPU: 3 PID: 910 Comm: kworker/u16:10 Not tainted 4.4.178+ #3
Hardware name: linux,dummy-virt (DT)
Workqueue: netns cleanup_net
Call trace:
[<ffffff90080abb50>] dump_backtrace+0x0/0x618
[<ffffff90080ac1a0>] show_stack+0x38/0x60
[<ffffff9008c42b78>] dump_stack+0x1a8/0x230
[<ffffff90085d469c>] kasan_report_error+0xc8c/0xfc0
[<ffffff90085d54a4>] kasan_report+0x94/0xd8
[<ffffff90085d1b28>] __asan_load8+0x88/0x150
[<ffffff9009c9cc2c>] rds_tcp_dev_event+0x734/0xb48
[<ffffff90081eacb0>] raw_notifier_call_chain+0x150/0x1e8
[<ffffff900973fec0>] call_netdevice_notifiers_info+0x90/0x110
[<ffffff9009764874>] netdev_run_todo+0x2f4/0xb08
[<ffffff9009796d34>] rtnl_unlock+0x2c/0x48
[<ffffff9009756484>] default_device_exit_batch+0x444/0x528
[<ffffff9009720498>] ops_exit_list+0x1c0/0x240
[<ffffff9009724a80>] cleanup_net+0x738/0xbf8
[<ffffff90081ca6cc>] process_one_work+0x96c/0x13e0
[<ffffff90081cf370>] worker_thread+0x7e0/0x1910
[<ffffff90081e7174>] kthread+0x304/0x390
[<ffffff9008094280>] ret_from_fork+0x10/0x50

If the first loop add the tc->t_sock = NULL to the tmp_list,
1). list_for_each_entry_safe(tc, _tc, &rds_tcp_conn_list, t_tcp_node)

then the second loop is to find connections to destroy, tc->t_sock
might equal NULL, and tc->t_sock->sk happens null-ptr-deref.
2). list_for_each_entry_safe(tc, _tc, &tmp_list, t_tcp_node)

Fixes: c4e97b06cfdc ("net: rds: force to destroy connection if t_sock is NULL in rds_tcp_kill_sock().")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/rds/tcp.c

@@ -352,9 +352,11 @@ static void rds_tcp_kill_sock(struct net *net)
 	}
 	spin_unlock_irq(&rds_tcp_conn_lock);
 	list_for_each_entry_safe(tc, _tc, &tmp_list, t_tcp_node) {
+		if (tc->t_sock) {
 			sk = tc->t_sock->sk;
 			sk->sk_prot->disconnect(sk, 0);
 			tcp_done(sk);
+		}
 		if (tc->conn->c_passive)
 			rds_conn_destroy(tc->conn->c_passive);
 		rds_conn_destroy(tc->conn);