Subject: Re: [PATCH] net/ipv6: add sysctl option min_hop_limit

From: YOSHIFUJI Hideaki <[email-masked]>
Date: Wed, 29 Jul 2015 19:32:07 +0900

Hi,

Hangbin Liu wrote:
> 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
>    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 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 <[email-masked]>
> ---
>  Documentation/networking/ip-sysctl.txt |  4 ++++
>  include/linux/ipv6.h                   |  1 +
>  include/uapi/linux/ipv6.h              |  1 +
>  net/ipv6/addrconf.c                    | 10 ++++++++++
>  net/ipv6/ndisc.c                       |  8 +++-----
>  5 files changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index 5fae770..844904b 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -1431,6 +1431,10 @@ hop_limit - INTEGER
>  	Default Hop Limit to set.
>  	Default: 64
>  
> +min_hop_limit - INTEGER
> +	Minimum hop limit value can be accepted from Router Advertisement.
> +	Default: 1
> +

I prefer accept_ra_min_hop_limit, as we have accept_ra_rt_info_max_plen.

>  mtu - INTEGER
>  	Default Maximum Transfer Unit
>  	Default: 1280 (IPv6 required minimum)
> diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
> index 82806c6..cd9e6af 100644
> --- a/include/linux/ipv6.h
> +++ b/include/linux/ipv6.h
> @@ -11,6 +11,7 @@
>  struct ipv6_devconf {
>  	__s32		forwarding;
>  	__s32		hop_limit;
> +	__s32           min_hop_limit;
>  	__s32		mtu6;
>  	__s32		accept_ra;
>  	__s32		accept_redirects;
> diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
> index 5efa54a..c1c09bb 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_MIN_HOP_LIMIT,
>  	DEVCONF_MAX
>  };
>  
> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index 21c2c81..b6ed39e 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -176,6 +176,7 @@ static bool ipv6_chk_same_addr(struct net *net, const struct in6_addr *addr,
>  static struct ipv6_devconf ipv6_devconf __read_mostly = {
>  	.forwarding		= 0,
>  	.hop_limit		= IPV6_DEFAULT_HOPLIMIT,
> +	.min_hop_limit		= 1,
>  	.mtu6			= IPV6_MIN_MTU,
>  	.accept_ra		= 1,
>  	.accept_redirects	= 1,
> @@ -217,6 +218,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
>  static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
>  	.forwarding		= 0,
>  	.hop_limit		= IPV6_DEFAULT_HOPLIMIT,
> +	.min_hop_limit		= 1,
>  	.mtu6			= IPV6_MIN_MTU,
>  	.accept_ra		= 1,
>  	.accept_redirects	= 1,
> @@ -4538,6 +4540,7 @@ static inline void ipv6_store_devconf(struct ipv6_devconf *cnf,
>  	memset(array, 0, bytes);
>  	array[DEVCONF_FORWARDING] = cnf->forwarding;
>  	array[DEVCONF_HOPLIMIT] = cnf->hop_limit;
> +	array[DEVCONF_MIN_HOP_LIMIT] = cnf->min_hop_limit;
>  	array[DEVCONF_MTU6] = cnf->mtu6;
>  	array[DEVCONF_ACCEPT_RA] = cnf->accept_ra;
>  	array[DEVCONF_ACCEPT_REDIRECTS] = cnf->accept_redirects;
> @@ -5328,6 +5331,13 @@ static struct addrconf_sysctl_table
>  			.proc_handler	= proc_dointvec,
>  		},
>  		{
> +			.procname	= "min_hop_limit",
> +			.data		= &ipv6_devconf.min_hop_limit,
> +			.maxlen		= sizeof(int),
> +			.mode		= 0644,
> +			.proc_handler	= proc_dointvec,
> +		},
> +		{
>  			.procname	= "mtu",
>  			.data		= &ipv6_devconf.mtu6,
>  			.maxlen		= sizeof(int),
> diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
> index 0a05b35..c22f3ac 100644
> --- a/net/ipv6/ndisc.c
> +++ b/net/ipv6/ndisc.c
> @@ -1226,13 +1226,11 @@ 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) {
> +		if (in6_dev->cnf.min_hop_limit <= ra_msg->icmph.icmp6_hop_limit &&
> +		    ra_msg->icmph.icmp6_hop_limit != 0) {
>  			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");
> +			ND_PRINTK(2, warn, "RA: Got route advertisement with lower hop_limit than minimum\n");
>  		}

ra_msg->icmph.icmp6_hop_limit != 0 is checkd by outer "if".

You do not need to update cnf.hop_limit if it is already equal to
hop limit received.

How about ignoring hop limit without message is configured value is
larger than 255, BTW?

>  		if (rt)
>  			dst_metric_set(&rt->dst, RTAX_HOPLIMIT,
> 

This can be inside the inner "if".

-- 
Hideaki Yoshifuji <[email-masked]>
Technical Division, MIRACLE LINUX CORPORATION