ipv6: fix ECMP route replacement

When replacing an IPv6 multipath route with "ip route replace", i.e.
NLM_F_CREATE | NLM_F_REPLACE, fib6_add_rt2node() replaces only first
matching route without fixing its siblings, resulting in corrupted
siblings linked list; removing one of the siblings can then end in an
infinite loop.

IPv6 ECMP implementation is a bit different from IPv4 so that route
replacement cannot work in exactly the same way. This should be a
reasonable approximation:

1. If the new route is ECMP-able and there is a matching ECMP-able one
already, replace it and all its siblings (if any).

2. If the new route is ECMP-able and no matching ECMP-able route exists,
replace first matching non-ECMP-able (if any) or just add the new one.

3. If the new route is not ECMP-able, replace first matching
non-ECMP-able route (if any) or add the new route.

We also need to remove the NLM_F_REPLACE flag after replacing old
route(s) by first nexthop of an ECMP route so that each subsequent
nexthop does not replace previous one.

Fixes: 51ebd3181572 ("ipv6: add support of equal cost multipath (ECMP)")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 4 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 3821a3517478..c73ae5039e46 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2541,11 +2541,14 @@ beginning:
                         }
                 }
                 /* Because each route is added like a single route we remove
-                 * this flag after the first nexthop (if there is a collision,
-                 * we have already fail to add the first nexthop:
-                 * fib6_add_rt2node() has reject it).
+                 * these flags after the first nexthop: if there is a collision,
+                 * we have already failed to add the first nexthop:
+                 * fib6_add_rt2node() has rejected it; when replacing, old
+                 * nexthops have been replaced by first new, the rest should
+                 * be added to it.
                  */
-                cfg->fc_nlinfo.nlh->nlmsg_flags &= ~NLM_F_EXCL;
+                cfg->fc_nlinfo.nlh->nlmsg_flags &= ~(NLM_F_EXCL |
+                                                     NLM_F_REPLACE);
                 rtnh = rtnh_next(rtnh, &remaining);
         }