net/ipv6: add sysctl option accept_ra_min_hop_limit

[ Upstream commit 8013d1d7 ]

Commit 6fd99094 ("ipv6: Don't reduce hop limit for an interface")
disabled accept hop limit from RA if it is smaller than the current hop
limit for security stuff. But this behavior kind of break the RFC definition.

RFC 4861, 6.3.4.  Processing Received Router Advertisements
   A Router Advertisement field (e.g., Cur Hop Limit, Reachable Time,
   and Retrans Timer) may contain a value denoting that it is
   unspecified.  In such cases, the parameter should be ignored and the
   host should continue using whatever value it is already using.

   If the received Cur Hop Limit value is non-zero, the host SHOULD set
   its CurHopLimit variable to the received value.

So add sysctl option accept_ra_min_hop_limit to let user choose the minimum
hop limit value they can accept from RA. And set default to 1 to meet RFC
standards.
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2:
 - Adjust filename, context
 - Number DEVCONF enumerators explicitly to match upstream]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

Documentation/networking/ip-sysctl.txt

@@ -1070,6 +1070,14 @@ accept_ra_defrtr - BOOLEAN
 	Functional default: enabled if accept_ra is enabled.
 			    disabled if accept_ra is disabled.
 
+accept_ra_min_hop_limit - INTEGER
+	Minimum hop limit Information in Router Advertisement.
+
+	Hop limit Information in Router Advertisement less than this
+	variable shall be ignored.
+
+	Default: 1
+
 accept_ra_pinfo - BOOLEAN
 	Learn Prefix Information in Router Advertisement.
 

include/linux/ipv6.h

@@ -153,6 +153,7 @@ struct ipv6_devconf {
 #endif
 	__s32		max_addresses;
 	__s32		accept_ra_defrtr;
+	__s32		accept_ra_min_hop_limit;
 	__s32		accept_ra_pinfo;
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	__s32		accept_ra_rtr_pref;
@@ -213,6 +214,8 @@ enum {
 	DEVCONF_DISABLE_IPV6,
 	DEVCONF_ACCEPT_DAD,
 	DEVCONF_FORCE_TLLAO,
+	DEVCONF_USE_OIF_ADDRS_ONLY = 37,
+	DEVCONF_ACCEPT_RA_MIN_HOP_LIMIT,
 	DEVCONF_MAX
 };
 

net/ipv6/addrconf.c

@@ -185,6 +185,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
 #endif
 	.max_addresses		= IPV6_MAX_ADDRESSES,
 	.accept_ra_defrtr	= 1,
+	.accept_ra_min_hop_limit= 1,
 	.accept_ra_pinfo	= 1,
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	.accept_ra_rtr_pref	= 1,
@@ -219,6 +220,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
 #endif
 	.max_addresses		= IPV6_MAX_ADDRESSES,
 	.accept_ra_defrtr	= 1,
+	.accept_ra_min_hop_limit= 1,
 	.accept_ra_pinfo	= 1,
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	.accept_ra_rtr_pref	= 1,
@@ -3943,6 +3945,7 @@ static inline void ipv6_store_devconf(struct ipv6_devconf *cnf,
 #endif
 	array[DEVCONF_MAX_ADDRESSES] = cnf->max_addresses;
 	array[DEVCONF_ACCEPT_RA_DEFRTR] = cnf->accept_ra_defrtr;
+	array[DEVCONF_ACCEPT_RA_MIN_HOP_LIMIT] = cnf->accept_ra_min_hop_limit;
 	array[DEVCONF_ACCEPT_RA_PINFO] = cnf->accept_ra_pinfo;
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	array[DEVCONF_ACCEPT_RA_RTR_PREF] = cnf->accept_ra_rtr_pref;
@@ -4545,6 +4548,13 @@ static struct addrconf_sysctl_table
 			.mode		= 0644,
 			.proc_handler	= proc_dointvec,
 		},
+		{
+			.procname	= "accept_ra_min_hop_limit",
+			.data		= &ipv6_devconf.accept_ra_min_hop_limit,
+			.maxlen		= sizeof(int),
+			.mode		= 0644,
+			.proc_handler	= proc_dointvec,
+		},
 		{
 			.procname	= "accept_ra_pinfo",
 			.data		= &ipv6_devconf.accept_ra_pinfo,

net/ipv6/ndisc.c

@@ -1276,18 +1276,16 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 	if (rt)
 		rt->rt6i_expires = jiffies + (HZ * lifetime);
 
-	if (ra_msg->icmph.icmp6_hop_limit) {
-		/* Only set hop_limit on the interface if it is higher than
-		 * the current hop_limit.
-		 */
-		if (in6_dev->cnf.hop_limit < ra_msg->icmph.icmp6_hop_limit) {
+	if (in6_dev->cnf.accept_ra_min_hop_limit < 256 &&
+	    ra_msg->icmph.icmp6_hop_limit) {
+		if (in6_dev->cnf.accept_ra_min_hop_limit <= ra_msg->icmph.icmp6_hop_limit) {
 			in6_dev->cnf.hop_limit = ra_msg->icmph.icmp6_hop_limit;
+			if (rt)
+				dst_metric_set(&rt->dst, RTAX_HOPLIMIT,
+					       ra_msg->icmph.icmp6_hop_limit);
 		} else {
-			ND_PRINTK2(KERN_WARNING "RA: Got route advertisement with lower hop_limit than current\n");
+			ND_PRINTK2(KERN_WARNING "RA: Got route advertisement with lower hop_limit than minimum\n");
 		}
-		if (rt)
-			dst_metric_set(&rt->dst, RTAX_HOPLIMIT,
-				       ra_msg->icmph.icmp6_hop_limit);
 	}
 
 skip_defrtr: