mpls: Prevent use of implicit NULL label as outgoing label

The reserved implicit-NULL label isn't allowed to appear in the label
stack for packets, so make it an error for the control plane to
specify it as an outgoing label.

Suggested-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Robert Shearman <rshearma@brocade.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 0 deletions

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index 9fdd94cba83e..954810c76a86 100644
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -646,6 +646,15 @@ int nla_get_labels(const struct nlattr *nla,
                 if ((dec.bos != bos) || dec.ttl || dec.tc)
                         return -EINVAL;
 
+                switch (dec.label) {
+                case LABEL_IMPLICIT_NULL:
+                        /* RFC3032: This is a label that an LSR may
+                         * assign and distribute, but which never
+                         * actually appears in the encapsulation.
+                         */
+                        return -EINVAL;
+                }
+
                 label[i] = dec.label;
         }
         *labels = nla_labels;