netdev - Re: [PATCH net] net: ipv6: prevent use after free in ip6_route_mpath

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Message-ID: <4b46d531-904b-6e5f-67ce-a275f0826d47@cumulusnetworks.com>
Date:   Sun, 3 Jun 2018 08:01:19 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net] net: ipv6: prevent use after free in
 ip6_route_mpath_notify()

On 6/3/18 7:35 AM, Eric Dumazet wrote:
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index f4d61736c41abe8cd7f439c4a37100e90c1eacca..830eefdbdb6734eb81ea0322fb6077ee20be1889 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -4263,7 +4263,9 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
>  
>  	err_nh = NULL;
>  	list_for_each_entry(nh, &rt6_nh_list, next) {
> +		dst_release(&rt_last->dst);
>  		rt_last = nh->rt6_info;
> +		dst_hold(&rt_last->dst);
>  		err = __ip6_ins_rt(nh->rt6_info, info, &nh->mxc, extack);
>  		/* save reference to first route for notification */
>  		if (!rt_notif && !err)
> @@ -4317,7 +4319,7 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
>  		list_del(&nh->next);
>  		kfree(nh);
>  	}
> -
> +	dst_release(&rt_last->dst);
>  	return err;
>  }

Since the rtnl lock is held, a successfully inserted route can not be
removed until ip6_route_multipath_add finishes. This is a simpler change
that works with net-next as well:

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index f4d61736c41a..1684197c189f 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -4263,11 +4263,12 @@ static int ip6_route_multipath_add(struct
fib6_config *cfg,

        err_nh = NULL;
        list_for_each_entry(nh, &rt6_nh_list, next) {
-               rt_last = nh->rt6_info;
                err = __ip6_ins_rt(nh->rt6_info, info, &nh->mxc, extack);
                /* save reference to first route for notification */
                if (!rt_notif && !err)
                        rt_notif = nh->rt6_info;
+               if (!err)
+                       rt_last = nh->rt6_info;

                /* nh->rt6_info is used or freed at this point, reset to
NULL*/
                nh->rt6_info = NULL;


Is there a reproducer for the syzbot case?