[PATCH net-next] ipv6: Honor all IPv6 PIO Valid Lifetime values

[Fernando Gont <fgont@si6networks.com>]

RFC4862 5.5.3 e) prevents received Router Advertisements from reducing
the Valid Lifetime of configured addresses to less than two hours, thus
preventing hosts from reacting to the information provided by a router
that has positive knowledge that a prefix has become invalid.

This patch makes hosts honor all Valid Lifetime values, as per
draft-gont-6man-slaac-renum-06, Section 4.2. This is meant to help
mitigate the problem discussed in draft-ietf-v6ops-slaac-renum.

Note: Attacks aiming at disabling an advertised prefix via a Valid
Lifetime of 0 are not really more harmful than other attacks
that can be performed via forged RA messages, such as those
aiming at completely disabling a next-hop router via an RA that
advertises a Router Lifetime of 0, or performing a Denial of
Service (DoS) attack by advertising illegitimate prefixes via
forged PIOs.  In scenarios where RA-based attacks are of concern,
proper mitigations such as RA-Guard [RFC6105] [RFC7113] should
be implemented.

Signed-off-by: Fernando Gont <fgont@si6networks.com>
---
 include/net/addrconf.h |  2 --
 net/ipv6/addrconf.c    | 27 +++++++--------------------
 2 files changed, 7 insertions(+), 22 deletions(-)

diff --git a/include/net/addrconf.h b/include/net/addrconf.h
index e0eabe58aa8b..fdb07105384c 100644
--- a/include/net/addrconf.h
+++ b/include/net/addrconf.h
@@ -6,8 +6,6 @@
 #define RTR_SOLICITATION_INTERVAL	(4*HZ)
 #define RTR_SOLICITATION_MAX_INTERVAL	(3600*HZ)	/* 1 hour */
 
-#define MIN_VALID_LIFETIME		(2*3600)	/* 2 hours */
-
 #define TEMP_VALID_LIFETIME		(7*86400)
 #define TEMP_PREFERRED_LIFETIME		(86400)
 #define REGEN_MAX_RETRY			(3)
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 24e319dfb510..27b4fb6e452b 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2564,7 +2564,7 @@ int addrconf_prefix_rcv_add_addr(struct net *net, struct net_device *dev,
 				 __u32 valid_lft, u32 prefered_lft)
 {
 	struct inet6_ifaddr *ifp = ipv6_get_ifaddr(net, addr, dev, 1);
-	int create = 0, update_lft = 0;
+	int create = 0;
 
 	if (!ifp && valid_lft) {
 		int max_addresses = in6_dev->cnf.max_addresses;
@@ -2608,32 +2608,19 @@ int addrconf_prefix_rcv_add_addr(struct net *net, struct net_device *dev,
 		unsigned long now;
 		u32 stored_lft;
 
-		/* update lifetime (RFC2462 5.5.3 e) */
+		/* Update lifetime (RFC4862 5.5.3 e)
+		 * We deviate from RFC4862 by honoring all Valid Lifetimes to
+		 * improve the reaction of SLAAC to renumbering events
+		 * (draft-gont-6man-slaac-renum-06, Section 4.2)
+		 */
 		spin_lock_bh(&ifp->lock);
 		now = jiffies;
 		if (ifp->valid_lft > (now - ifp->tstamp) / HZ)
 			stored_lft = ifp->valid_lft - (now - ifp->tstamp) / HZ;
 		else
 			stored_lft = 0;
-		if (!create && stored_lft) {
-			const u32 minimum_lft = min_t(u32,
-				stored_lft, 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) {
+		if (!create && stored_lft) {
 			ifp->valid_lft = valid_lft;
 			ifp->prefered_lft = prefered_lft;
 			ifp->tstamp = now;
-- 
2.26.0