net: ipv6/addrconf: ensure that regen_advance is at least 2 seconds

RFC 8981 defines REGEN_ADVANCE as follows:

REGEN_ADVANCE = 2 + (TEMP_IDGEN_RETRIES * DupAddrDetectTransmits * RetransTimer / 1000)

Thus, allowing it to be less than 2 seconds is technically a protocol
violation.

Link: https://datatracker.ietf.org/doc/html/rfc8981#name-defined-protocol-parametersSigned-off-by: Alex Henrie <alexhenrie24@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/ip-sysctl.rst

@@ -2503,7 +2503,7 @@ use_tempaddr - INTEGER
 
 temp_valid_lft - INTEGER
 	valid lifetime (in seconds) for temporary addresses. If less than the
-	minimum required lifetime (typically 5 seconds), temporary addresses
+	minimum required lifetime (typically 5-7 seconds), temporary addresses
 	will not be created.
 
 	Default: 172800 (2 days)
@@ -2511,7 +2511,7 @@ temp_valid_lft - INTEGER
 temp_prefered_lft - INTEGER
 	Preferred lifetime (in seconds) for temporary addresses. If
 	temp_prefered_lft is less than the minimum required lifetime (typically
-	5 seconds), temporary addresses will not be created. If
+	5-7 seconds), temporary addresses will not be created. If
 	temp_prefered_lft is greater than temp_valid_lft, the preferred lifetime
 	is temp_valid_lft.
 

net/ipv6/addrconf.c

@@ -1339,6 +1339,13 @@ static void ipv6_del_addr(struct inet6_ifaddr *ifp)
 	in6_ifa_put(ifp);
 }
 
+static unsigned long ipv6_get_regen_advance(struct inet6_dev *idev)
+{
+	return 2 + idev->cnf.regen_max_retry *
+			idev->cnf.dad_transmits *
+			max(NEIGH_VAR(idev->nd_parms, RETRANS_TIME), HZ/100) / HZ;
+}
+
 static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, bool block)
 {
 	struct inet6_dev *idev = ifp->idev;
@@ -1380,9 +1387,7 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, bool block)
 
 	age = (now - ifp->tstamp) / HZ;
 
-	regen_advance = idev->cnf.regen_max_retry *
-			idev->cnf.dad_transmits *
-			max(NEIGH_VAR(idev->nd_parms, RETRANS_TIME), HZ/100) / HZ;
+	regen_advance = ipv6_get_regen_advance(idev);
 
 	/* recalculate max_desync_factor each time and update
 	 * idev->desync_factor if it's larger
@@ -4595,9 +4600,7 @@ static void addrconf_verify_rtnl(struct net *net)
 			    !ifp->regen_count && ifp->ifpub) {
 				/* This is a non-regenerated temporary addr. */
 
-				unsigned long regen_advance = ifp->idev->cnf.regen_max_retry *
-					ifp->idev->cnf.dad_transmits *
-					max(NEIGH_VAR(ifp->idev->nd_parms, RETRANS_TIME), HZ/100) / HZ;
+				unsigned long regen_advance = ipv6_get_regen_advance(ifp->idev);
 
 				if (age + regen_advance >= ifp->prefered_lft) {
 					struct inet6_ifaddr *ifpub = ifp->ifpub;