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Date:   Fri, 6 Dec 2019 12:38:49 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net v4 3/3] tcp: Protect accesses to .ts_recent_stamp with
 {READ,WRITE}_ONCE()

Syncookies borrow the ->rx_opt.ts_recent_stamp field to store the
timestamp of the last synflood. Protect them with READ_ONCE() and
WRITE_ONCE() since reads and writes aren't serialised.

Use of .rx_opt.ts_recent_stamp for storing the synflood timestamp was
introduced by a0f82f64e269 ("syncookies: remove last_synq_overflow from
struct tcp_sock"). But unprotected accesses were already there when
timestamp was stored in .last_synq_overflow.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Guillaume Nault <gnault@...hat.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/net/tcp.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 43e04e14c41e..86b9a8766648 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -501,9 +501,9 @@ static inline void tcp_synq_overflow(const struct sock *sk)
 		}
 	}
 
-	last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
+	last_overflow = READ_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp);
 	if (!time_between32(now, last_overflow, last_overflow + HZ))
-		tcp_sk(sk)->rx_opt.ts_recent_stamp = now;
+		WRITE_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp, now);
 }
 
 /* syncookies: no recent synqueue overflow on this listening socket? */
@@ -524,7 +524,7 @@ static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
 		}
 	}
 
-	last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
+	last_overflow = READ_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp);
 
 	/* If last_overflow <= jiffies <= last_overflow + TCP_SYNCOOKIE_VALID,
 	 * then we're under synflood. However, we have to use
-- 
2.21.0

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