linux-kernel - Re: [PATCH 1/1] ipv6: Check AF_UNSPEC in ip6_route_multipath

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Message-ID: <0da120c0-4b67-4d0e-9a45-2b1a9170e641@kernel.org>
Date: Tue, 5 Aug 2025 10:16:48 -0600
From: David Ahern <dsahern@...nel.org>
To: Maksimilijan Marosevic <maksimilijan.marosevic@...ton.me>,
 davem@...emloft.net
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kernel-mentees@...ts.linux.dev,
 syzbot+a259a17220263c2d73fc@...kaller.appspotmail.com
Subject: Re: [PATCH 1/1] ipv6: Check AF_UNSPEC in ip6_route_multipath_add()

On 8/4/25 2:42 PM, Maksimilijan Marosevic wrote:
> This check was removed in commit e6f497955fb6 ("ipv6: Check GATEWAY
> in rtm_to_fib6_multipath_config().") as part of rt6_qualify_for ecmp().
> The author correctly recognises that rt6_qualify_for_ecmp() returns
> false if fb_nh_gw_family is set to AF_UNSPEC, but then mistakes
> AF_UNSPEC for AF_INET6 when reasoning that the check is unnecessary.
> This means certain malformed entries don't get caught in
> ip6_route_multipath_add().
> 
> This patch reintroduces the AF_UNSPEC check while respecting changes
> of the initial patch.
> 
> Reported-by: syzbot+a259a17220263c2d73fc@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=a259a17220263c2d73fc
> Fixes: e6f497955fb6 ("ipv6: Check GATEWAY in rtm_to_fib6_multipath_config().")
> Signed-off-by: Maksimilijan Marosevic <maksimilijan.marosevic@...ton.me>
> ---
>  net/ipv6/route.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index 3299cfa12e21..d4b988bed920 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -5456,6 +5456,14 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
>  			goto cleanup;
>  		}
>  
> +		if (rt->fib6_nh->fib_nh_gw_family == AF_UNSPEC) {
> +			err = -EINVAL;
> +			NL_SET_ERR_MSG(extack,
> +				       "Device only routes can not be added for IPv6 using the multipath API.");
> +			fib6_info_release(rt);
> +			goto cleanup;
> +		}
> +
>  		rt->fib6_nh->fib_nh_weight = rtnh->rtnh_hops + 1;
>  
>  		err = ip6_route_info_append(&rt6_nh_list, rt, &r_cfg);

can you add another test to the routing selftests to cover this case?