tcp: Fix a data-race around sysctl_tcp_comp_sack_slack_ns.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2160073
Upstream Status: linux.git

commit 2239694
Author: Kuniyuki Iwashima <kuniyu@amazon.com>
Date:   Fri Jul 22 11:22:02 2022 -0700

    tcp: Fix a data-race around sysctl_tcp_comp_sack_slack_ns.

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

    Fixes: a70437c ("tcp: add hrtimer slack to sack compression")
    Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Signed-off-by: Guillaume Nault <gnault@redhat.com>

Author: Guillaume Nault

net/ipv4/tcp_input.c

@@ -5504,7 +5504,7 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
 		      rtt * (NSEC_PER_USEC >> 3)/20);
 	sock_hold(sk);
 	hrtimer_start_range_ns(&tp->compressed_ack_timer, ns_to_ktime(delay),
-			       sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns,
+			       READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns),
 			       HRTIMER_MODE_REL_PINNED_SOFT);
 }