net/tcp_fastopen: fix data races around tfo_active_disable_stamp

tfo_active_disable_stamp is read and written locklessly.
We need to annotate these accesses appropriately.

Then, we need to perform the atomic_inc(tfo_active_disable_times)
after the timestamp has been updated, and thus add barriers
to make sure tcp_fastopen_active_should_disable() wont read
a stale timestamp.

Fixes: cf1ef3f0719b ("net/tcp_fastopen: Disable active side TFO in certain scenarios")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Wei Wang <weiwan@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c
index 47c32604d38fca960d2cd56f3588bfd2e390b789..b32af76e21325373126b51423496e3b8d47d97ff 100644 (file)
--- a/net/ipv4/tcp_fastopen.c
+++ b/net/ipv4/tcp_fastopen.c
@@ -507,8 +507,15 @@ void tcp_fastopen_active_disable(struct sock *sk)
 {
        struct net *net = sock_net(sk);
 
+       /* Paired with READ_ONCE() in tcp_fastopen_active_should_disable() */
+       WRITE_ONCE(net->ipv4.tfo_active_disable_stamp, jiffies);
+
+       /* Paired with smp_rmb() in tcp_fastopen_active_should_disable().
+        * We want net->ipv4.tfo_active_disable_stamp to be updated first.
+        */
+       smp_mb__before_atomic();
        atomic_inc(&net->ipv4.tfo_active_disable_times);
-       net->ipv4.tfo_active_disable_stamp = jiffies;
+
        NET_INC_STATS(net, LINUX_MIB_TCPFASTOPENBLACKHOLE);
 }
 
@@ -526,10 +533,16 @@ bool tcp_fastopen_active_should_disable(struct sock *sk)
        if (!tfo_da_times)
                return false;
 
+       /* Paired with smp_mb__before_atomic() in tcp_fastopen_active_disable() */
+       smp_rmb();
+
        /* Limit timeout to max: 2^6 * initial timeout */
        multiplier = 1 << min(tfo_da_times - 1, 6);
-       timeout = multiplier * tfo_bh_timeout * HZ;
-       if (time_before(jiffies, sock_net(sk)->ipv4.tfo_active_disable_stamp + timeout))
+
+       /* Paired with the WRITE_ONCE() in tcp_fastopen_active_disable(). */
+       timeout = READ_ONCE(sock_net(sk)->ipv4.tfo_active_disable_stamp) +
+                 multiplier * tfo_bh_timeout * HZ;
+       if (time_before(jiffies, timeout))
                return true;
 
        /* Mark check bit so we can check for successful active TFO