[PATCHv2] net/ipv6: add sysctl option accept_ra_hop_limit

[Hangbin Liu <liuhangbin@gmail.com>]

Commit 6fd99094de2b ("ipv6: Don't reduce hop limit for an interface")
disabled accept hop limit from RA if it is higher 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
   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_hop_limit to let user choose whether accept
hop limit info in RA.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
---
 Documentation/networking/ip-sysctl.txt | 11 +++++++++++
 include/linux/ipv6.h                   |  1 +
 include/uapi/linux/ipv6.h              |  1 +
 net/ipv6/addrconf.c                    | 10 ++++++++++
 net/ipv6/ndisc.c                       | 17 +++++++++++------
 5 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 5fae770..778c479 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -1346,6 +1346,17 @@ accept_ra_pinfo - BOOLEAN
 	Functional default: enabled if accept_ra is enabled.
 			    disabled if accept_ra is disabled.
 
+accept_ra_hop_limit - INTEGER
+	Learn hop limit in Router Advertisement.
+
+	Possible values are:
+		0 Do not accept hop limit in Router Advertisements.
+		1 Accept hop limit in Router Advertisements if it is higher
+		  than the current hop limit.
+		2 Accept hop limit in Router Advertisements anyway.
+
+	Default: 1
+
 accept_ra_rt_info_max_plen - INTEGER
 	Maximum prefix length of Route Information in RA.
 
diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 82806c6..a21a9c6 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -30,6 +30,7 @@ struct ipv6_devconf {
 	__s32		max_addresses;
 	__s32		accept_ra_defrtr;
 	__s32		accept_ra_pinfo;
+	__s32           accept_ra_hop_limit;
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	__s32		accept_ra_rtr_pref;
 	__s32		rtr_probe_interval;
diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
index 5efa54a..a8c1083 100644
--- a/include/uapi/linux/ipv6.h
+++ b/include/uapi/linux/ipv6.h
@@ -171,6 +171,7 @@ enum {
 	DEVCONF_USE_OPTIMISTIC,
 	DEVCONF_ACCEPT_RA_MTU,
 	DEVCONF_STABLE_SECRET,
+	DEVCONF_ACCEPT_RA_HOP_LIMIT,
 	DEVCONF_MAX
 };
 
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 21c2c81..486a7a5 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -196,6 +196,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
 	.accept_ra_defrtr	= 1,
 	.accept_ra_from_local	= 0,
 	.accept_ra_pinfo	= 1,
+	.accept_ra_hop_limit    = 1,
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	.accept_ra_rtr_pref	= 1,
 	.rtr_probe_interval	= 60 * HZ,
@@ -237,6 +238,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
 	.accept_ra_defrtr	= 1,
 	.accept_ra_from_local	= 0,
 	.accept_ra_pinfo	= 1,
+	.accept_ra_hop_limit    = 1,
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	.accept_ra_rtr_pref	= 1,
 	.rtr_probe_interval	= 60 * HZ,
@@ -4561,6 +4563,7 @@ static inline void ipv6_store_devconf(struct ipv6_devconf *cnf,
 	array[DEVCONF_MAX_ADDRESSES] = cnf->max_addresses;
 	array[DEVCONF_ACCEPT_RA_DEFRTR] = cnf->accept_ra_defrtr;
 	array[DEVCONF_ACCEPT_RA_PINFO] = cnf->accept_ra_pinfo;
+	array[DEVCONF_ACCEPT_RA_HOP_LIMIT] = cnf->accept_ra_hop_limit;
 #ifdef CONFIG_IPV6_ROUTER_PREF
 	array[DEVCONF_ACCEPT_RA_RTR_PREF] = cnf->accept_ra_rtr_pref;
 	array[DEVCONF_RTR_PROBE_INTERVAL] =
@@ -5462,6 +5465,13 @@ static struct addrconf_sysctl_table
 			.mode		= 0644,
 			.proc_handler	= proc_dointvec,
 		},
+		{
+			.procname	= "accept_ra_hop_limit",
+			.data		= &ipv6_devconf.accept_ra_hop_limit,
+			.maxlen		= sizeof(int),
+			.mode		= 0644,
+			.proc_handler	= proc_dointvec,
+		},
 #ifdef CONFIG_IPV6_ROUTER_PREF
 		{
 			.procname	= "accept_ra_rtr_pref",
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 0a05b35..aca67da 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1226,13 +1226,18 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 	if (rt)
 		rt6_set_expires(rt, 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) {
+		switch (in6_dev->cnf.accept_ra_hop_limit) {
+		case 0:
+			break;
+		case 1:
+			if (in6_dev->cnf.hop_limit > ra_msg->icmph.icmp6_hop_limit) {
+				ND_PRINTK(2, warn,
+					  "RA: Got route advertisement with lower hop_limit than current\n");
+				break;
+			}
+		default:
 			in6_dev->cnf.hop_limit = ra_msg->icmph.icmp6_hop_limit;
-		} else {
-			ND_PRINTK(2, warn, "RA: Got route advertisement with lower hop_limit than current\n");
+			break;
 		}
 		if (rt)
 			dst_metric_set(&rt->dst, RTAX_HOPLIMIT,
-- 
1.9.3