IPv6: fix DESYNC_FACTOR

The IPv6 temporary address generation uses a variable called DESYNC_FACTOR
to prevent hosts updating the addresses at the same time. Quoting RFC 4941:

   ... The value DESYNC_FACTOR is a random value (different for each
   client) that ensures that clients don't synchronize with each other and
   generate new addresses at exactly the same time ...

DESYNC_FACTOR is defined as:

   DESYNC_FACTOR -- A random value within the range 0 - MAX_DESYNC_FACTOR.
   It is computed once at system start (rather than each time it is used)
   and must never be greater than (TEMP_VALID_LIFETIME - REGEN_ADVANCE).

First, I believe the RFC has a typo in it and meant to say: "and must
never be greater than (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE)"

The reason is that at various places in the RFC, DESYNC_FACTOR is used in
a calculation like (TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR) or
(TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE - DESYNC_FACTOR). It needs to be
smaller than (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE) for the result of
these calculations to be larger than zero. It's never used in a
calculation together with TEMP_VALID_LIFETIME.

I already submitted an errata to the rfc-editor:
https://www.rfc-editor.org/errata_search.php?rfc=4941

The Linux implementation of DESYNC_FACTOR is very wrong:
max_desync_factor is used in places DESYNC_FACTOR should be used.
max_desync_factor is initialized to the RFC-recommended value for
MAX_DESYNC_FACTOR (600) but the whole point is to get a _random_ value.

And nothing ensures that the value used is not greater than
(TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE), which leads to underflows.  The
effect can easily be observed when setting the temp_prefered_lft sysctl
e.g. to 60. The preferred lifetime of the temporary addresses will be
bogus.

TEMP_PREFERRED_LIFETIME and REGEN_ADVANCE are not constants and can be
influenced by these three sysctls: regen_max_retry, dad_transmits and
temp_prefered_lft. Thus, the upper bound for desync_factor needs to be
re-calculated each time a new address is generated and if desync_factor is
larger than the new upper bound, a new random value needs to be
re-generated.

And since we already have max_desync_factor configurable per interface, we
also need to calculate and store desync_factor per interface.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

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