tcp: Fix a data-race around sysctl_tcp_retrans_collapse.

stable inclusion
from stable-v4.19.254
commit d4a90eb32681c79a140ff2600b52fce291cf4e91
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

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

[ Upstream commit 1a63cb91f0c2fcdeced6d6edee8d1d886583d139 ]

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

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>

net/ipv4/tcp_output.c

@@ -2869,7 +2869,7 @@ static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
 	struct sk_buff *skb = to, *tmp;
 	bool first = true;
 
-	if (!sock_net(sk)->ipv4.sysctl_tcp_retrans_collapse)
+	if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_retrans_collapse))
 		return;
 	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
 		return;