netdev - Re: [PATCH net-next 6/7] ipv4: nexthop: Correctly update nexthop group when replacing a nexthop

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Date:   Wed, 26 Aug 2020 13:15:54 -0600
From:   David Ahern <dsahern@...il.com>
To:     Ido Schimmel <idosch@...sch.org>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, mlxsw@...dia.com,
        Ido Schimmel <idosch@...dia.com>
Subject: Re: [PATCH net-next 6/7] ipv4: nexthop: Correctly update nexthop
 group when replacing a nexthop

On 8/26/20 10:48 AM, Ido Schimmel wrote:
> From: Ido Schimmel <idosch@...dia.com>
> 
> Each nexthop group contains an indication if it has IPv4 nexthops
> ('has_v4'). Its purpose is to prevent IPv6 routes from using groups with
> IPv4 nexthops.
> 
> However, the indication is not updated when a nexthop is replaced. This
> results in the kernel wrongly rejecting IPv6 routes from pointing to
> groups that only contain IPv6 nexthops. Example:
> 
> # ip nexthop replace id 1 via 192.0.2.2 dev dummy10
> # ip nexthop replace id 10 group 1
> # ip nexthop replace id 1 via 2001:db8:1::2 dev dummy10
> # ip route replace 2001:db8:10::/64 nhid 10
> Error: IPv6 routes can not use an IPv4 nexthop.
> 
> Solve this by iterating over all the nexthop groups that the replaced
> nexthop is a member of and potentially update their IPv4 indication
> according to the new set of member nexthops.
> 
> Avoid wasting cycles by only performing the update in case an IPv4
> nexthop is replaced by an IPv6 nexthop.
> 
> Signed-off-by: Ido Schimmel <idosch@...dia.com>
> ---
>  net/ipv4/nexthop.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
> index 5199a2815df6..bf9d4cd2d6e5 100644
> --- a/net/ipv4/nexthop.c
> +++ b/net/ipv4/nexthop.c
> @@ -964,6 +964,23 @@ static int replace_nexthop_grp(struct net *net, struct nexthop *old,
>  	return 0;
>  }
>  
> +static void nh_group_v4_update(struct nh_group *nhg)
> +{
> +	struct nh_grp_entry *nhges;
> +	bool has_v4 = false;
> +	int i;
> +
> +	nhges = nhg->nh_entries;
> +	for (i = 0; i < nhg->num_nh; i++) {
> +		struct nh_info *nhi;
> +
> +		nhi = rtnl_dereference(nhges[i].nh->nh_info);
> +		if (nhi->family == AF_INET)
> +			has_v4 = true;
> +	}
> +	nhg->has_v4 = has_v4;
> +}
> +
>  static int replace_nexthop_single(struct net *net, struct nexthop *old,
>  				  struct nexthop *new,
>  				  struct netlink_ext_ack *extack)
> @@ -987,6 +1004,21 @@ static int replace_nexthop_single(struct net *net, struct nexthop *old,
>  	rcu_assign_pointer(old->nh_info, newi);
>  	rcu_assign_pointer(new->nh_info, oldi);
>  
> +	/* When replacing an IPv4 nexthop with an IPv6 nexthop, potentially
> +	 * update IPv4 indication in all the groups using the nexthop.
> +	 */
> +	if (oldi->family == AF_INET && newi->family == AF_INET6) {
> +		struct nh_grp_entry *nhge;
> +
> +		list_for_each_entry(nhge, &old->grp_list, nh_list) {
> +			struct nexthop *nhp = nhge->nh_parent;
> +			struct nh_group *nhg;
> +
> +			nhg = rtnl_dereference(nhp->nh_grp);
> +			nh_group_v4_update(nhg);
> +		}
> +	}
> +
>  	return 0;
>  }
>  
> 

Hopefully userspace apps create a new nexthop versus overwriting
existing ones with different address family. Thanks for handling this
case and adding a test case.

Reviewed-by: David Ahern <dsahern@...il.com>