[TCP]: secure_tcp_sequence_number() should not use a too fast clock

TCP V4 sequence numbers are 32bits, and RFC 793 assumed a 250 KHz clock.
In order to follow network speed increase, we can use a faster clock, but
we should limit this clock so that the delay between two rollovers is
greater than MSL (TCP Maximum Segment Lifetime : 2 minutes)

Choosing a 64 nsec clock should be OK, since the rollovers occur every
274 seconds.

Problem spotted by Denys Fedoryshchenko

[ This bug was introduced by f85958151900f9d30fa5ff941b0ce71eaa45a7de ]

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 4 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 397c714cf2ba..af274e5a25ee 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1550,11 +1550,13 @@ __u32 secure_tcp_sequence_number(__be32 saddr, __be32 daddr,
          *      As close as possible to RFC 793, which
          *      suggests using a 250 kHz clock.
          *      Further reading shows this assumes 2 Mb/s networks.
-         *      For 10 Gb/s Ethernet, a 1 GHz clock is appropriate.
-         *      That's funny, Linux has one built in!  Use it!
-         *      (Networks are faster now - should this be increased?)
+         *      For 10 Mb/s Ethernet, a 1 MHz clock is appropriate.
+         *      For 10 Gb/s Ethernet, a 1 GHz clock should be ok, but
+         *      we also need to limit the resolution so that the u32 seq
+         *      overlaps less than one time per MSL (2 minutes).
+         *      Choosing a clock of 64 ns period is OK. (period of 274 s)
          */
-        seq += ktime_get_real().tv64;
+        seq += ktime_get_real().tv64 >> 6;
 #if 0
         printk("init_seq(%lx, %lx, %d, %d) = %d\n",
                saddr, daddr, sport, dport, seq);