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@syzkaller.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 <redacted>
---
 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);