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>

2 files changed, 44 insertions, 6 deletions

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 96dbffff5a24..bde57b113009 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -693,6 +693,7 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct rt6_info *rt,
 {
         struct rt6_info *iter = NULL;
         struct rt6_info **ins;
+        struct rt6_info **fallback_ins = NULL;
         int replace = (info->nlh &&
                        (info->nlh->nlmsg_flags & NLM_F_REPLACE));
         int add = (!info->nlh ||
@@ -716,8 +717,13 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct rt6_info *rt,
                             (info->nlh->nlmsg_flags & NLM_F_EXCL))
                                 return -EEXIST;
                         if (replace) {
-                                found++;
-                                break;
+                                if (rt_can_ecmp == rt6_qualify_for_ecmp(iter)) {
+                                        found++;
+                                        break;
+                                }
+                                if (rt_can_ecmp)
+                                        fallback_ins = fallback_ins ?: ins;
+                                goto next_iter;
                         }
 
                         if (iter->dst.dev == rt->dst.dev &&
@@ -753,9 +759,17 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct rt6_info *rt,
                 if (iter->rt6i_metric > rt->rt6i_metric)
                         break;
 
+next_iter:
                 ins = &iter->dst.rt6_next;
         }
 
+        if (fallback_ins && !found) {
+                /* No ECMP-able route found, replace first non-ECMP one */
+                ins = fallback_ins;
+                iter = *ins;
+                found++;
+        }
+
         /* Reset round-robin state, if necessary */
         if (ins == &fn->leaf)
                 fn->rr_ptr = NULL;
@@ -815,6 +829,8 @@ add:
                 }
 
         } else {
+                int nsiblings;
+
                 if (!found) {
                         if (add)
                                 goto add;
@@ -835,8 +851,27 @@ add:
                         info->nl_net->ipv6.rt6_stats->fib_route_nodes++;
                         fn->fn_flags |= RTN_RTINFO;
                 }
+                nsiblings = iter->rt6i_nsiblings;
                 fib6_purge_rt(iter, fn, info->nl_net);
                 rt6_release(iter);
+
+                if (nsiblings) {
+                        /* Replacing an ECMP route, remove all siblings */
+                        ins = &rt->dst.rt6_next;
+                        iter = *ins;
+                        while (iter) {
+                                if (rt6_qualify_for_ecmp(iter)) {
+                                        *ins = iter->dst.rt6_next;
+                                        fib6_purge_rt(iter, fn, info->nl_net);
+                                        rt6_release(iter);
+                                        nsiblings--;
+                                } else {
+                                        ins = &iter->dst.rt6_next;
+                                }
+                                iter = *ins;
+                        }
+                        WARN_ON(nsiblings != 0);
+                }
         }
 
         return 0;
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);
         }