tcp: Fix a data-race around sysctl_tcp_comp_sack_nr.

stable inclusion
from stable-v4.19.255
commit 37ef2ea54885ffebc45fae73f04352bab0ffb367
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ
CVE: NA

--------------------------------

[ Upstream commit 79f55473 ]

While reading sysctl_tcp_comp_sack_nr, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its reader.

Fixes: 9c21d2fc ("tcp: add tcp_comp_sack_nr sysctl")
Signed-off-by: NKuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

net/ipv4/tcp_input.c

@@ -5274,7 +5274,7 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
 	}
 
 	if (!tcp_is_sack(tp) ||
-	    tp->compressed_ack >= sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr)
+	    tp->compressed_ack >= READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr))
 		goto send_now;
 
 	if (tp->compressed_ack_rcv_nxt != tp->rcv_nxt) {