[PATCH 3.16 143/148] tcp: Protect accesses to .ts_recent_stamp with {READ,WRITE}_ONCE()

[Ben Hutchings]

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Guillaume Nault <gnault@xxxxxxxxxx>

commit 721c8dafad26ccfa90ff659ee19755e3377b829d upstream.

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@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: Use ACCESS_ONCE() instead of {READ,WRITE}_ONCE()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/net/tcp.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -482,17 +482,17 @@ struct sock *cookie_v4_check(struct sock
  */
 static inline void tcp_synq_overflow(struct sock *sk)
 {
-	unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
+	unsigned long last_overflow = ACCESS_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp);
 	unsigned long now = jiffies;
 
 	if (!time_between32(now, last_overflow, last_overflow + HZ))
-		tcp_sk(sk)->rx_opt.ts_recent_stamp = now;
+		ACCESS_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp) = now;
 }
 
 /* syncookies: no recent synqueue overflow on this listening socket? */
 static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
 {
-	unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
+	unsigned long last_overflow = ACCESS_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp);
 
 	return time_after(jiffies, last_overflow + TCP_SYNCOOKIE_VALID);
 }