Merge branch 'tcp-fix-handling-of-stale-syncookies-timestamps'

Guillaume Nault says:

====================
tcp: fix handling of stale syncookies timestamps

The synflood timestamps (->ts_recent_stamp and ->synq_overflow_ts) are
only refreshed when the syncookie protection triggers. Therefore, their
value can become very far apart from jiffies if no synflood happens for
a long time.

If jiffies grows too much and wraps while the synflood timestamp isn't
refreshed, then time_after32() might consider the later to be in the
future. This can trick tcp_synq_no_recent_overflow() into returning
erroneous values and rejecting valid ACKs.

Patch 1 handles the case of ACKs using legitimate syncookies.
Patch 2 handles the case of stray ACKs.
Patch 3 annotates lockless timestamp operations with READ_ONCE() and
WRITE_ONCE().

Changes from v3:
- Fix description of time_between32() (found by Eric Dumazet).
- Use more accurate Fixes tag in patch 3 (suggested by Eric Dumazet).

Changes from v2:
- Define and use time_between32() instead of a pair of
time_before32/time_after32 (suggested by Eric Dumazet).
- Use 'last_overflow - HZ' as lower bound in
tcp_synq_no_recent_overflow(), to accommodate for concurrent
timestamp updates (found by Eric Dumazet).
- Add a third patch to annotate lockless accesses to .ts_recent_stamp.

Changes from v1:
- Initialising timestamps at socket creation time is not enough
because jiffies wraps in 24 days with HZ=1000 (Eric Dumazet).
Handle stale timestamps in tcp_synq_overflow() and
tcp_synq_no_recent_overflow() instead.
- Rework commit description.
- Add a second patch to handle the case of stray ACKs.
====================

Signed-off-by: David S. Miller <[e-mail]>

include/linux/time.h

···
  */
 #define time_after32(a, b)	((s32)((u32)(b) - (u32)(a)) < 0)
 #define time_before32(b, a)	time_after32(a, b)
+
+/**
+ * time_between32 - check if a 32-bit timestamp is within a given time range
+ * @t:	the time which may be within [l,h]
+ * @l:	the lower bound of the range
+ * @h:	the higher bound of the range
+ *
+ * time_before32(t, l, h) returns true if @l <= @t <= @h. All operands are
+ * treated as 32-bit integers.
+ *
+ * Equivalent to !(time_before32(@t, @l) || time_after32(@t, @h)).
+ */
+#define time_between32(t, l, h) ((u32)(h) - (u32)(l) >= (u32)(t) - (u32)(l))
 #endif

include/net/tcp.h

···
 		reuse = rcu_dereference(sk->sk_reuseport_cb);
 		if (likely(reuse)) {
 			last_overflow = READ_ONCE(reuse->synq_overflow_ts);
-			if (time_after32(now, last_overflow + HZ))
+			if (!time_between32(now, last_overflow,
+					    last_overflow + HZ))
 				WRITE_ONCE(reuse->synq_overflow_ts, now);
 			return;
 		}
 	}
 
-	last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
-	if (time_after32(now, last_overflow + HZ))
-		tcp_sk(sk)->rx_opt.ts_recent_stamp = now;
+	last_overflow = READ_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp);
+	if (!time_between32(now, last_overflow, last_overflow + HZ))
+		WRITE_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp, now);
 }
 
 /* syncookies: no recent synqueue overflow on this listening socket? */
···
 		reuse = rcu_dereference(sk->sk_reuseport_cb);
 		if (likely(reuse)) {
 			last_overflow = READ_ONCE(reuse->synq_overflow_ts);
-			return time_after32(now, last_overflow +
-					    TCP_SYNCOOKIE_VALID);
+			return !time_between32(now, last_overflow - HZ,
+					       last_overflow +
+					       TCP_SYNCOOKIE_VALID);
 		}
 	}
 
-	last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
-	return time_after32(now, last_overflow + TCP_SYNCOOKIE_VALID);
+	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
+	 * 'last_overflow - HZ' as lower bound. That's because a concurrent
+	 * tcp_synq_overflow() could update .ts_recent_stamp after we read
+	 * jiffies but before we store .ts_recent_stamp into last_overflow,
+	 * which could lead to rejecting a valid syncookie.
+	 */
+	return !time_between32(now, last_overflow - HZ,
+			       last_overflow + TCP_SYNCOOKIE_VALID);
 }
 
 static inline u32 tcp_cookie_time(void)