bgpd: Fix VU#270232, VPNv4 NLRI parser memcpys to stack on unchecked length

Address CERT vulnerability report VU#270232, memcpy to stack data structure
based on length field from packet data whose length field upper-bound was
not properly checked.

This likely allows BGP peers that are enabled to send Labeled-VPN SAFI
routes to Quagga bgpd to remotely exploit Quagga bgpd.

Mitigation: Do not enable Labeled-VPN SAFI with untrusted neighbours.

Impact: Labeled-VPN SAFI is not enabled by default.

* bgp_mplsvpn.c: (bgp_nlri_parse_vpnv4) The prefixlen is checked for
  lower-bound, but not for upper-bound against received data length.
  The packet data is then memcpy'd to the stack based on the prefixlen.

  Extend the prefixlen check to ensure it is within the bound of the NLRI
  packet data AND the on-stack prefix structure AND the maximum size for the
  address family.

Reported-by: Kostya Kortchinsky <kostyak@google.com>

This commit a joint effort between:

Lou Berger <lberger@labn.net>
Donald Sharp <sharpd@cumulusnetworks.com>
Paul Jakma <paul.jakma@hpe.com> / <paul@jakma.org>

1 files changed, 36 insertions, 16 deletions

diff --git a/bgpd/bgp_mplsvpn.c b/bgpd/bgp_mplsvpn.c
index a72d5ede..75c90cd7 100644
--- a/bgpd/bgp_mplsvpn.c
+++ b/bgpd/bgp_mplsvpn.c
@@ -101,6 +101,7 @@ bgp_nlri_parse_vpnv4 (struct peer *peer, struct attr *attr,
   pnt = packet->nlri;
   lim = pnt + packet->length;
 
+#define VPN_PREFIXLEN_MIN_BYTES (3 + 8) /* label + RD */
   for (; pnt < lim; pnt += psize)
     {
       /* Clear prefix structure. */
@@ -108,17 +109,38 @@ bgp_nlri_parse_vpnv4 (struct peer *peer, struct attr *attr,
 
       /* Fetch prefix length. */
       prefixlen = *pnt++;
-      p.family = AF_INET;
+      p.family = afi2family (packet->afi);
       psize = PSIZE (prefixlen);
-
-      if (prefixlen < 88)
-	{
-	  zlog_err ("prefix length is less than 88: %d", prefixlen);
-	  return -1;
-	}
-
+      
+      /* sanity check against packet data */
+      if (prefixlen < VPN_PREFIXLEN_MIN_BYTES*8 || (pnt + psize) > lim)
+        {
+          zlog_err ("prefix length (%d) is less than 88"
+                    " or larger than received (%u)",
+                    prefixlen, (uint)(lim-pnt));
+          return -1;
+        }
+      
+      /* sanity check against storage for the IP address portion */
+      if ((psize - VPN_PREFIXLEN_MIN_BYTES) > (ssize_t) sizeof(p.u))
+        {
+          zlog_err ("prefix length (%d) exceeds prefix storage (%zu)",
+                    prefixlen - VPN_PREFIXLEN_MIN_BYTES*8, sizeof(p.u));
+          return -1;
+        }
+      
+      /* Sanity check against max bitlen of the address family */
+      if ((psize - VPN_PREFIXLEN_MIN_BYTES) > prefix_blen (&p))
+        {
+          zlog_err ("prefix length (%d) exceeds family (%u) max byte length (%u)",
+                    prefixlen - VPN_PREFIXLEN_MIN_BYTES*8, 
+                    p.family, prefix_blen (&p));
+          return -1;
+                  
+        }
+      
       /* Copyr label to prefix. */
-      tagpnt = pnt;;
+      tagpnt = pnt;
 
       /* Copy routing distinguisher to rd. */
       memcpy (&prd.val, pnt + 3, 8);
@@ -137,8 +159,9 @@ bgp_nlri_parse_vpnv4 (struct peer *peer, struct attr *attr,
 	  return -1;
 	}
 
-      p.prefixlen = prefixlen - 88;
-      memcpy (&p.u.prefix, pnt + 11, psize - 11);
+      p.prefixlen = prefixlen - VPN_PREFIXLEN_MIN_BYTES*8;
+      memcpy (&p.u.prefix, pnt + VPN_PREFIXLEN_MIN_BYTES, 
+              psize - VPN_PREFIXLEN_MIN_BYTES);
 
 #if 0
       if (type == RD_TYPE_AS)
@@ -149,9 +172,6 @@ bgp_nlri_parse_vpnv4 (struct peer *peer, struct attr *attr,
 		   rd_ip.val, inet_ntoa (p.u.prefix4), p.prefixlen);
 #endif /* 0 */
 
-      if (pnt + psize > lim)
-	return -1;
-
       if (attr)
 	bgp_update (peer, &p, attr, AFI_IP, SAFI_MPLS_VPN,
 		    ZEBRA_ROUTE_BGP, BGP_ROUTE_NORMAL, &prd, tagpnt, 0);
@@ -159,12 +179,12 @@ bgp_nlri_parse_vpnv4 (struct peer *peer, struct attr *attr,
 	bgp_withdraw (peer, &p, attr, AFI_IP, SAFI_MPLS_VPN,
 		      ZEBRA_ROUTE_BGP, BGP_ROUTE_NORMAL, &prd, tagpnt);
     }
-
   /* Packet length consistency check. */
   if (pnt != lim)
     return -1;
-
+  
   return 0;
+#undef VPN_PREFIXLEN_MIN_BYTES
 }
 
 int