ipv6: Fix preferred_lft not updating in some cases

Consider the scenario where an IPv6 router is advertising a fixed
preferred_lft of 1800 seconds, while the valid_lft begins at 3600
seconds and counts down in realtime.

A client should reset its preferred_lft to 1800 every time the RA is
received, but a bug is causing Linux to ignore the update.

The core problem is here:
  if (prefered_lft != ifp->prefered_lft) {

Note that ifp->prefered_lft is an offset, so it doesn't decrease over
time.  Thus, the comparison is always (1800 != 1800), which fails to
trigger an update.

The most direct solution would be to compute a "stored_prefered_lft",
and use that value in the comparison.  But I think that trying to filter
out unnecessary updates here is a premature optimization.  In order for
the filter to apply, both of these would need to hold:

  - The advertised valid_lft and preferred_lft are both declining in
    real time.
  - No clock skew exists between the router & client.

So in this patch, I've set "update_lft = 1" unconditionally, which
allows the surrounding code to be greatly simplified.

Signed-off-by: Paul Marks <pmarks@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 37 deletions

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index a0c3abe72461..cd3fb301da38 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2220,43 +2220,21 @@ ok:
                         else
                                 stored_lft = 0;
                         if (!update_lft && !create && stored_lft) {
-                                if (valid_lft > MIN_VALID_LIFETIME ||
-                                    valid_lft > stored_lft)
-                                        update_lft = 1;
-                                else if (stored_lft <= MIN_VALID_LIFETIME) {
-                                        /* valid_lft <= stored_lft is always true */
-                                        /*
-                                         * RFC 4862 Section 5.5.3e:
-                                         * "Note that the preferred lifetime of
-                                         *  the corresponding address is always
-                                         *  reset to the Preferred Lifetime in
-                                         *  the received Prefix Information
-                                         *  option, regardless of whether the
-                                         *  valid lifetime is also reset or
-                                         *  ignored."
-                                         *
-                                         *  So if the preferred lifetime in
-                                         *  this advertisement is different
-                                         *  than what we have stored, but the
-                                         *  valid lifetime is invalid, just
-                                         *  reset prefered_lft.
-                                         *
-                                         *  We must set the valid lifetime
-                                         *  to the stored lifetime since we'll
-                                         *  be updating the timestamp below,
-                                         *  else we'll set it back to the
-                                         *  minimum.
-                                         */
-                                        if (prefered_lft != ifp->prefered_lft) {
-                                                valid_lft = stored_lft;
-                                                update_lft = 1;
-                                        }
-                                } else {
-                                        valid_lft = MIN_VALID_LIFETIME;
-                                        if (valid_lft < prefered_lft)
-                                                prefered_lft = valid_lft;
-                                        update_lft = 1;
-                                }
+                                const u32 minimum_lft = min(
+                                        stored_lft, (u32)MIN_VALID_LIFETIME);
+                                valid_lft = max(valid_lft, minimum_lft);
+
+                                /* RFC4862 Section 5.5.3e:
+                                 * "Note that the preferred lifetime of the
+                                 *  corresponding address is always reset to
+                                 *  the Preferred Lifetime in the received
+                                 *  Prefix Information option, regardless of
+                                 *  whether the valid lifetime is also reset or
+                                 *  ignored."
+                                 *
+                                 * So we should always update prefered_lft here.
+                                 */
+                                update_lft = 1;
                         }
 
                         if (update_lft) {