linux-kernel - [PATCH 4.19 082/120] net/tcp_fastopen: fix data races around tfo_active_disable

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Message-Id: <20210726153835.018403531@linuxfoundation.org>
Date:   Mon, 26 Jul 2021 17:38:54 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.19 082/120] net/tcp_fastopen: fix data races around tfo_active_disable_stamp

From: Eric Dumazet <edumazet@...gle.com>

[ Upstream commit 6f20c8adb1813467ea52c1296d52c4e95978cb2f ]

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@...gle.com>
Cc: Wei Wang <weiwan@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
Cc: Neal Cardwell <ncardwell@...gle.com>
Acked-by: Wei Wang <weiwan@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 net/ipv4/tcp_fastopen.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c
index 018a48477355..2ab371f55525 100644
--- a/net/ipv4/tcp_fastopen.c
+++ b/net/ipv4/tcp_fastopen.c
@@ -454,8 +454,15 @@ 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);
 }
 
@@ -473,10 +480,16 @@ 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 timout 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
-- 
2.30.2