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>

include/net/if_inet6.h

@@ -190,6 +190,7 @@ struct inet6_dev {
 	__u32			if_flags;
 	int			dead;
 
+	u32			desync_factor;
 	u8			rndid[8];
 	struct list_head	tempaddr_list;
 

net/ipv6/addrconf.c

@@ -422,6 +422,7 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev)
 #endif
 
 	INIT_LIST_HEAD(&ndev->tempaddr_list);
+	ndev->desync_factor = U32_MAX;
 	if ((dev->flags&IFF_LOOPBACK) ||
 	    dev->type == ARPHRD_TUNNEL ||
 	    dev->type == ARPHRD_TUNNEL6 ||
@@ -1183,6 +1184,7 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, struct inet6_ifaddr *i
 	int ret = 0;
 	u32 addr_flags;
 	unsigned long now = jiffies;
+	long max_desync_factor;
 
 	write_lock_bh(&idev->lock);
 	if (ift) {
@@ -1218,20 +1220,41 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, struct inet6_ifaddr *i
 	ipv6_try_regen_rndid(idev, tmpaddr);
 	memcpy(&addr.s6_addr[8], idev->rndid, 8);
 	age = (now - ifp->tstamp) / HZ;
+
+	regen_advance = idev->cnf.regen_max_retry *
+			idev->cnf.dad_transmits *
+			NEIGH_VAR(idev->nd_parms, RETRANS_TIME) / HZ;
+
+	/* recalculate max_desync_factor each time and update
+	 * idev->desync_factor if it's larger
+	 */
+	max_desync_factor = min_t(__u32,
+				  idev->cnf.max_desync_factor,
+				  idev->cnf.temp_prefered_lft - regen_advance);
+
+	if (unlikely(idev->desync_factor > max_desync_factor)) {
+		if (max_desync_factor > 0) {
+			get_random_bytes(&idev->desync_factor,
+					 sizeof(idev->desync_factor));
+			idev->desync_factor %= max_desync_factor;
+		} else {
+			idev->desync_factor = 0;
+		}
+	}
+
 	tmp_valid_lft = min_t(__u32,
 			      ifp->valid_lft,
 			      idev->cnf.temp_valid_lft + age);
-	tmp_prefered_lft = min_t(__u32,
-				 ifp->prefered_lft,
-				 idev->cnf.temp_prefered_lft + age -
-				 idev->cnf.max_desync_factor);
+	tmp_prefered_lft = idev->cnf.temp_prefered_lft + age -
+			    idev->desync_factor;
+	/* guard against underflow in case of concurrent updates to cnf */
+	if (unlikely(tmp_prefered_lft < 0))
+		tmp_prefered_lft = 0;
+	tmp_prefered_lft = min_t(__u32, ifp->prefered_lft, tmp_prefered_lft);
 	tmp_plen = ifp->prefix_len;
 	tmp_tstamp = ifp->tstamp;
 	spin_unlock_bh(&ifp->lock);
 
-	regen_advance = idev->cnf.regen_max_retry *
-			idev->cnf.dad_transmits *
-			NEIGH_VAR(idev->nd_parms, RETRANS_TIME) / HZ;
 	write_unlock_bh(&idev->lock);
 
 	/* A temporary address is created only if this calculated Preferred
@@ -2316,7 +2339,7 @@ static void manage_tempaddrs(struct inet6_dev *idev,
 			max_valid = 0;
 
 		max_prefered = idev->cnf.temp_prefered_lft -
-			       idev->cnf.max_desync_factor - age;
+			       idev->desync_factor - age;
 		if (max_prefered < 0)
 			max_prefered = 0;