Re: [[PATCH net-next v5] net/ipv6: add sysctl option accept_ra_min_hop_limit

[YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>]

Hi,

Hangbin Liu wrote:
> Commit 6fd99094de2b ("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>

> ---
>  Documentation/networking/ip-sysctl.txt |  8 ++++++++
>  include/linux/ipv6.h                   |  1 +
>  include/uapi/linux/ipv6.h              |  1 +
>  net/ipv6/addrconf.c                    | 10 ++++++++++
>  net/ipv6/ndisc.c                       | 16 +++++++---------
>  5 files changed, 27 insertions(+), 9 deletions(-)
> 
> diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> index 1a5ab21b..00d26d9 100644
> --- a/Documentation/networking/ip-sysctl.txt
> +++ b/Documentation/networking/ip-sysctl.txt
> @@ -1340,6 +1340,14 @@ accept_ra_from_local - BOOLEAN
>  	   disabled if accept_ra_from_local is disabled
>                 on a specific interface.
>  
> +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.
>  
> diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
> index 06ed637..cb9dcad 100644
> --- a/include/linux/ipv6.h
> +++ b/include/linux/ipv6.h
> @@ -29,6 +29,7 @@ struct ipv6_devconf {
>  	__s32		max_desync_factor;
>  	__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;
> diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
> index 641a146..80f3b74 100644
> --- a/include/uapi/linux/ipv6.h
> +++ b/include/uapi/linux/ipv6.h
> @@ -172,6 +172,7 @@ enum {
>  	DEVCONF_ACCEPT_RA_MTU,
>  	DEVCONF_STABLE_SECRET,
>  	DEVCONF_USE_OIF_ADDRS_ONLY,
> +	DEVCONF_ACCEPT_RA_MIN_HOP_LIMIT,
>  	DEVCONF_MAX
>  };
>  
> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index eb0c6a3..53e3a9d 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -195,6 +195,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
>  	.max_addresses		= IPV6_MAX_ADDRESSES,
>  	.accept_ra_defrtr	= 1,
>  	.accept_ra_from_local	= 0,
> +	.accept_ra_min_hop_limit= 1,
>  	.accept_ra_pinfo	= 1,
>  #ifdef CONFIG_IPV6_ROUTER_PREF
>  	.accept_ra_rtr_pref	= 1,
> @@ -237,6 +238,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
>  	.max_addresses		= IPV6_MAX_ADDRESSES,
>  	.accept_ra_defrtr	= 1,
>  	.accept_ra_from_local	= 0,
> +	.accept_ra_min_hop_limit= 1,
>  	.accept_ra_pinfo	= 1,
>  #ifdef CONFIG_IPV6_ROUTER_PREF
>  	.accept_ra_rtr_pref	= 1,
> @@ -4588,6 +4590,7 @@ static inline void ipv6_store_devconf(struct ipv6_devconf *cnf,
>  	array[DEVCONF_MAX_DESYNC_FACTOR] = cnf->max_desync_factor;
>  	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;
> @@ -5485,6 +5488,13 @@ static struct addrconf_sysctl_table
>  			.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,
>  			.maxlen		= sizeof(int),
> diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
> index 0a05b35..6e184e0 100644
> --- a/net/ipv6/ndisc.c
> +++ b/net/ipv6/ndisc.c
> @@ -1225,18 +1225,16 @@ 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.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_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");
>  		}
> -		if (rt)
> -			dst_metric_set(&rt->dst, RTAX_HOPLIMIT,
> -				       ra_msg->icmph.icmp6_hop_limit);
>  	}
>  
>  skip_defrtr:
> 

-- 
Hideaki Yoshifuji <hideaki.yoshifuji@miraclelinux.com>
Technical Division, MIRACLE LINUX CORPORATION