net: fix memory leak in tcp_conn_request()

If inet_csk_reqsk_queue_hash_add() return false, tcp_conn_request() will
return without free the dst memory, which allocated in af_ops->route_req.

Here is the kmemleak stack:

unreferenced object 0xffff8881198631c0 (size 240):
  comm "softirq", pid 0, jiffies 4299266571 (age 1802.392s)
  hex dump (first 32 bytes):
    00 10 9b 03 81 88 ff ff 80 98 da bc ff ff ff ff  ................
    81 55 18 bb ff ff ff ff 00 00 00 00 00 00 00 00  .U..............
  backtrace:
    [<ffffffffb93e8d4c>] kmem_cache_alloc+0x60c/0xa80
    [<ffffffffba11b4c5>] dst_alloc+0x55/0x250
    [<ffffffffba227bf6>] rt_dst_alloc+0x46/0x1d0
    [<ffffffffba23050a>] __mkroute_output+0x29a/0xa50
    [<ffffffffba23456b>] ip_route_output_key_hash+0x10b/0x240
    [<ffffffffba2346bd>] ip_route_output_flow+0x1d/0x90
    [<ffffffffba254855>] inet_csk_route_req+0x2c5/0x500
    [<ffffffffba26b331>] tcp_conn_request+0x691/0x12c0
    [<ffffffffba27bd08>] tcp_rcv_state_process+0x3c8/0x11b0
    [<ffffffffba2965c6>] tcp_v4_do_rcv+0x156/0x3b0
    [<ffffffffba299c98>] tcp_v4_rcv+0x1cf8/0x1d80
    [<ffffffffba239656>] ip_protocol_deliver_rcu+0xf6/0x360
    [<ffffffffba2399a6>] ip_local_deliver_finish+0xe6/0x1e0
    [<ffffffffba239b8e>] ip_local_deliver+0xee/0x360
    [<ffffffffba239ead>] ip_rcv+0xad/0x2f0
    [<ffffffffba110943>] __netif_receive_skb_one_core+0x123/0x140

Call dst_release() to free the dst memory when
inet_csk_reqsk_queue_hash_add() return false in tcp_conn_request().

Fixes: ff46e3b ("Fix race for duplicate reqsk on identical SYN")
Signed-off-by: Wang Liang <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Wang Liang

net/ipv4/tcp_input.c

@@ -7328,6 +7328,7 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 			if (unlikely(!inet_csk_reqsk_queue_hash_add(sk, req,
 								    req->timeout))) {
 				reqsk_free(req);
+				dst_release(dst);
 				return 0;
 			}