bgpd: fix double free after extcommunity set (BZ#799)

The route-map extcommunity set code was incorrectly assuming that it
owns the intern'd struct ecommunity reference.  In reality, the intern'd
reference belongs to bgp_update_receive() and we're not supposed to
touch it in the route-map code.

Instead, like all the other set commands, we use a on-heap but
non-intern'd ecommunity to set the new value.  This is then either
intern'd in bgp_update_main/_rsclient() through bgp_attr_intern(), or
free'd through bgp_attr_flush().

This fixes Bugzilla #799, which is that bgpd otherwise crashes with a
double free.  The ecommunity got unintern'd first in the route-map set
command, then in bgp_update_receive().

Debugged-by: Milan Kocian <milon@wq.cz>
Reported-by: Florian S <florian@herrenlohe.de>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

1 files changed, 10 insertions, 5 deletions

diff --git a/bgpd/bgp_routemap.c b/bgpd/bgp_routemap.c
index eb5e80f83..36d177d2a 100644
--- a/bgpd/bgp_routemap.c
+++ b/bgpd/bgp_routemap.c
@@ -1552,14 +1552,19 @@ route_set_ecommunity (void *rule, struct prefix *prefix,
       old_ecom = (bgp_attr_extra_get (bgp_info->attr))->ecommunity;
 
       if (old_ecom)
-	new_ecom = ecommunity_merge (ecommunity_dup (old_ecom), ecom);
+	{
+	  new_ecom = ecommunity_merge (ecommunity_dup (old_ecom), ecom);
+
+	  /* old_ecom->refcnt = 1 => owned elsewhere, e.g. bgp_update_receive()
+	   *         ->refcnt = 0 => set by a previous route-map statement */
+	  if (!old_ecom->refcnt)
+	    ecommunity_free (&old_ecom);
+	}
       else
 	new_ecom = ecommunity_dup (ecom);
 
-      bgp_info->attr->extra->ecommunity = ecommunity_intern (new_ecom);
-
-      if (old_ecom)
-	ecommunity_unintern (&old_ecom);
+      /* will be intern()'d or attr_flush()'d by bgp_update_main() */
+      bgp_info->attr->extra->ecommunity = new_ecom;
 
       bgp_info->attr->flag |= ATTR_FLAG_BIT (BGP_ATTR_EXT_COMMUNITIES);
     }