Security: Add Hook to test if the particular xattr is part of a MAC model.

The interface to request security labels from user space is the xattr
interface. When requesting the security label from an NFS server it is
important to make sure the requested xattr actually is a MAC label. This allows
us to make sure that we get the desired semantics from the attribute instead of
something else such as capabilities or a time based LSM.

Acked-by: Eric Paris <eparis@redhat.com>
Acked-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Matthew N. Dodd <Matthew.Dodd@sparta.com>
Signed-off-by: Miguel Rodel Felipe <Rodel_FM@dsi.a-star.edu.sg>
Signed-off-by: Phua Eu Gene <PHUA_Eu_Gene@dsi.a-star.edu.sg>
Signed-off-by: Khin Mi Mi Aung <Mi_Mi_AUNG@dsi.a-star.edu.sg>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

security/smack/smack_lsm.c

@@ -3328,6 +3328,16 @@ static void smack_audit_rule_free(void *vrule)
 
 #endif /* CONFIG_AUDIT */
 
+/**
+ * smack_ismaclabel - check if xattr @name references a smack MAC label
+ * @name: Full xattr name to check.
+ */
+static int smack_ismaclabel(const char *name)
+{
+	return (strcmp(name, XATTR_SMACK_SUFFIX) == 0);
+}
+
+
 /**
  * smack_secid_to_secctx - return the smack label for a secid
  * @secid: incoming integer
@@ -3524,6 +3534,7 @@ struct security_operations smack_ops = {
 	.audit_rule_free =		smack_audit_rule_free,
 #endif /* CONFIG_AUDIT */
 
+	.ismaclabel =			smack_ismaclabel,
 	.secid_to_secctx = 		smack_secid_to_secctx,
 	.secctx_to_secid = 		smack_secctx_to_secid,
 	.release_secctx = 		smack_release_secctx,