util: add base64 decoding function

The standard glib provided g_base64_decode doesn't provide any
kind of sensible error checking on its input. Add a QEMU custom
wrapper qbase64_decode which can be used with untrustworthy
input that can contain invalid base64 characters, embedded
NUL characters, or not be NUL terminated at all.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

util/base64.c

@@ -0,0 +1,60 @@
+/*
+ * QEMU base64 helpers
+ *
+ * Copyright (c) 2015 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <config-host.h>
+
+#include "qemu/base64.h"
+
+static const char *base64_valid_chars =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=\n";
+
+uint8_t *qbase64_decode(const char *input,
+                        size_t in_len,
+                        size_t *out_len,
+                        Error **errp)
+{
+    *out_len = 0;
+
+    if (in_len != -1) {
+        /* Lack of NUL terminator is an error */
+        if (input[in_len] != '\0') {
+            error_setg(errp, "Base64 data is not NUL terminated");
+            return NULL;
+        }
+        /* Check there's no NULs embedded since we expect
+         * this to be valid base64 data */
+        if (memchr(input, '\0', in_len) != NULL) {
+            error_setg(errp, "Base64 data contains embedded NUL characters");
+            return NULL;
+        }
+
+        /* Now we know its a valid nul terminated string
+         * strspn is safe to use... */
+    } else {
+        in_len = strlen(input);
+    }
+
+    if (strspn(input, base64_valid_chars) != in_len) {
+        error_setg(errp, "Base64 data contains invalid characters");
+        return NULL;
+    }
+
+    return g_base64_decode(input, out_len);
+}