keys+nts: warn if loading world-readable/writable key

Log a warning message if the file specified by the keyfile or
ntsserverkey directive is world-readable or writable, which is likely
an insecure misconfiguration. There is no check of directories
containing the file.

keys.c

@@ -182,6 +182,9 @@ KEY_Reload(void)
   if (!key_file)
     return;
 
+  if (!UTI_CheckFilePermissions(key_file, 0771))
+    ;
+
   in = UTI_OpenFile(NULL, key_file, NULL, 'r', 0);
   if (!in) {
     LOG(LOGS_WARN, "Could not open keyfile %s", key_file);

nts_ke_session.c

@@ -667,6 +667,8 @@ create_credentials(const char **certs, const char **keys, int n_certs_keys,
       assert(0);
 
     for (i = 0; i < n_certs_keys; i++) {
+      if (!UTI_CheckFilePermissions(keys[i], 0771))
+        ;
       r = gnutls_certificate_set_x509_key_file(credentials, certs[i], keys[i],
                                                GNUTLS_X509_FMT_PEM);
       if (r < 0)

util.c

@@ -1248,6 +1248,29 @@ UTI_CheckDirPermissions(const char *path, mode_t perm, uid_t uid, gid_t gid)
 
 /* ================================================== */
 
+int
+UTI_CheckFilePermissions(const char *path, mode_t perm)
+{
+  mode_t extra_perm;
+  struct stat buf;
+
+  if (stat(path, &buf) < 0 || !S_ISREG(buf.st_mode)) {
+    /* Not considered an error */
+    return 1;
+  }
+
+  extra_perm = (buf.st_mode & 0777) & ~perm;
+  if (extra_perm != 0) {
+    LOG(LOGS_WARN, "%s permissions on %s", extra_perm & 0006 ?
+        (extra_perm & 0004 ? "World-readable" : "World-writable") : "Wrong", path);
+    return 0;
+  }
+
+  return 1;
+}
+
+/* ================================================== */
+
 static int
 join_path(const char *basedir, const char *name, const char *suffix,
           char *buffer, size_t length, LOG_Severity severity)

util.h

@@ -196,6 +196,10 @@ extern int UTI_CreateDirAndParents(const char *path, mode_t mode, uid_t uid, gid
    permissions and its uid/gid must match the specified values. */
 extern int UTI_CheckDirPermissions(const char *path, mode_t perm, uid_t uid, gid_t gid);
 
+/* Check and log a warning message if a file has more permissions than
+   specified.  It does not return error if it is not an accessible file. */
+extern int UTI_CheckFilePermissions(const char *path, mode_t perm);
+
 /* Open a file.  The full path of the file is constructed from the basedir
    (may be NULL), '/' (if basedir is not NULL), name, and suffix (may be NULL).
    Created files have specified permissions (umasked).  Returns NULL on error.