net: add sysctl to disable rfc4862 5.5.3e lifetime handling

This change adds a sysctl to opt-out of RFC4862 section 5.5.3e's valid
lifetime derivation mechanism.

RFC4862 section 5.5.3e prescribes that the valid lifetime in a Router
Advertisement PIO shall be ignored if it less than 2 hours and to reset
the lifetime of the corresponding address to 2 hours. An in-progress
6man draft (see draft-ietf-6man-slaac-renum-07 section 4.2) is currently
looking to remove this mechanism. While this draft has not been moving
particularly quickly for other reasons, there is widespread consensus on
section 4.2 which updates RFC4862 section 5.5.3e.

Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Jen Linkova <furry@google.com>
Signed-off-by: Patrick Rohr <prohr@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230925214711.959704-1-prohr@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/ip-sysctl.rst

@@ -2311,6 +2311,17 @@ accept_ra_pinfo - BOOLEAN
 		- enabled if accept_ra is enabled.
 		- disabled if accept_ra is disabled.
 
+ra_honor_pio_life - BOOLEAN
+	Whether to use RFC4862 Section 5.5.3e to determine the valid
+	lifetime of an address matching a prefix sent in a Router
+	Advertisement Prefix Information Option.
+
+	- If enabled, the PIO valid lifetime will always be honored.
+	- If disabled, RFC4862 section 5.5.3e is used to determine
+	  the valid lifetime of the address.
+
+	Default: 0 (disabled)
+
 accept_ra_rt_info_min_plen - INTEGER
 	Minimum prefix length of Route Information in RA.
 

include/linux/ipv6.h

@@ -82,6 +82,7 @@ struct ipv6_devconf {
 	__u32		ioam6_id_wide;
 	__u8		ioam6_enabled;
 	__u8		ndisc_evict_nocarrier;
+	__u8		ra_honor_pio_life;
 
 	struct ctl_table_header *sysctl_header;
 };

net/ipv6/addrconf.c

@@ -236,6 +236,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
 	.ioam6_id               = IOAM6_DEFAULT_IF_ID,
 	.ioam6_id_wide		= IOAM6_DEFAULT_IF_ID_WIDE,
 	.ndisc_evict_nocarrier	= 1,
+	.ra_honor_pio_life	= 0,
 };
 
 static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
@@ -297,6 +298,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
 	.ioam6_id               = IOAM6_DEFAULT_IF_ID,
 	.ioam6_id_wide		= IOAM6_DEFAULT_IF_ID_WIDE,
 	.ndisc_evict_nocarrier	= 1,
+	.ra_honor_pio_life	= 0,
 };
 
 /* Check if link is ready: is it up and is a valid qdisc available */
@@ -2657,22 +2659,23 @@ int addrconf_prefix_rcv_add_addr(struct net *net, struct net_device *dev,
 			stored_lft = ifp->valid_lft - (now - ifp->tstamp) / HZ;
 		else
 			stored_lft = 0;
-		if (!create && stored_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 = !create && stored_lft;
+
+		if (update_lft && !in6_dev->cnf.ra_honor_pio_life) {
 			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) {
@@ -6846,6 +6849,15 @@ static const struct ctl_table addrconf_sysctl[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname	= "ra_honor_pio_life",
+		.data		= &ipv6_devconf.ra_honor_pio_life,
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	{
 		.procname	= "accept_ra_rtr_pref",