Better profylaction recursive route loops

In some specific configurations, it was possible to send BIRD into an
infinite loop of recursive next hop resolution. This was caused by route
priority inversion.

To prevent priority inversions affecting other next hops, we simply
refuse to resolve any next hop if the best route for the matching prefix
is recursive or any other route with the same preference is recursive.

Next hop resolution doesn't change route priority, therefore it is
perfectly OK to resolve BGP next hops e.g. by an OSPF route, yet if the
same (or covering) prefix is also announced by iBGP, by retraction of
the OSPF route we would get a possible priority inversion.

diff --git a/nest/rt-table.c b/nest/rt-table.c
index abb29fe1baceb3c506f6f974d944565dd681fd8a..4127912cc690bd70d4d0c63b74818158ccc3c95c 100644 (file)
--- a/nest/rt-table.c
+++ b/nest/rt-table.c
@@ -3529,9 +3529,10 @@ rt_update_hostentry(rtable *tab, struct hostentry *he)
     {
       rte *e = n->routes;
       rta *a = e->attrs;
-      pxlen = n->n.addr->pxlen;
+      word pref = a->pref;
 
-      if (a->hostentry)
+      for (rte *ee = n->routes; ee; ee = ee->next)
+       if ((ee->attrs->pref >= pref) && ee->attrs->hostentry)
        {
          /* Recursive route should not depend on another recursive route */
          log(L_WARN "Next hop address %I resolvable through recursive route for %N",
@@ -3539,6 +3540,8 @@ rt_update_hostentry(rtable *tab, struct hostentry *he)
          goto done;
        }
 
+      pxlen = n->n.addr->pxlen;
+
       if (a->dest == RTD_UNICAST)
        {
          for (struct nexthop *nh = &(a->nh); nh; nh = nh->next)