ipv6: Check GATEWAY in rtm_to_fib6_multipath_config().

In ip6_route_multipath_add(), we call rt6_qualify_for_ecmp() for each
entry.  If it returns false, the request fails.

rt6_qualify_for_ecmp() returns false if either of the conditions below
is true:

  1. f6i->fib6_flags has RTF_ADDRCONF
  2. f6i->nh is not NULL
  3. f6i->fib6_nh->fib_nh_gw_family is AF_UNSPEC

1 is unnecessary because rtm_to_fib6_config() never sets RTF_ADDRCONF
to cfg->fc_flags.

2. is equivalent with cfg->fc_nh_id.

3. can be replaced by checking RTF_GATEWAY in the base and each multipath
entry because AF_INET6 is set to f6i->fib6_nh->fib_nh_gw_family only when
cfg.fc_is_fdb is true or RTF_GATEWAY is set, but the former is always
false.

These checks do not require RCU and can be done earlier.

Let's perform the equivalent checks in rtm_to_fib6_multipath_config().

Signed-off-by: Kuniyuki Iwashima <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: q2ven

net/ipv6/route.c

@@ -5031,6 +5031,7 @@ static int rtm_to_fib6_multipath_config(struct fib6_config *cfg,
 	}
 
 	do {
+		bool has_gateway = cfg->fc_flags & RTF_GATEWAY;
 		int attrlen = rtnh_attrlen(rtnh);
 
 		if (attrlen > 0) {
@@ -5044,9 +5045,17 @@ static int rtm_to_fib6_multipath_config(struct fib6_config *cfg,
 						       "Invalid IPv6 address in RTA_GATEWAY");
 					return -EINVAL;
 				}
+
+				has_gateway = true;
 			}
 		}
 
+		if (newroute && (cfg->fc_nh_id || !has_gateway)) {
+			NL_SET_ERR_MSG(extack,
+				       "Device only routes can not be added for IPv6 using the multipath API.");
+			return -EINVAL;
+		}
+
 		rtnh = rtnh_next(rtnh, &remaining);
 	} while (rtnh_ok(rtnh, remaining));
 
@@ -5388,13 +5397,6 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 			rt = NULL;
 			goto cleanup;
 		}
-		if (!rt6_qualify_for_ecmp(rt)) {
-			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;