usb-mtp: add sanity checks on rootdir

Currently, we don't check if rootdir exists and is accessible.
Furthermore, a trailing slash results in a null "desc" string which
ends up in the share not visible in the guest. Add some simple
sanity checks for appropriate permissions. Also, bail out if the
user does not supply an absolute path.

Signed-off-by: Bandan Das <bsd@redhat.com>
Message-id: jpga7bto3on.fsf@linux.bootlegged.copy
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/dev-mtp.c

@@ -2038,26 +2038,36 @@ static void usb_mtp_realize(USBDevice *dev, Error **errp)
 {
     MTPState *s = USB_MTP(dev);
 
-    usb_desc_create_serial(dev);
-    usb_desc_init(dev);
-    QTAILQ_INIT(&s->objects);
-    if (s->desc == NULL) {
-        if (s->root == NULL) {
-            error_setg(errp, "usb-mtp: rootdir property must be configured");
-            return;
-        }
-        s->desc = strrchr(s->root, '/');
-        if (s->desc && s->desc[0]) {
-            s->desc = g_strdup(s->desc + 1);
-        } else {
-            s->desc = g_strdup("none");
-        }
+    if ((s->root == NULL) || !g_path_is_absolute(s->root)) {
+        error_setg(errp, "usb-mtp: rootdir must be configured and be an absolute path");
+        return;
     }
+
+    if (access(s->root, R_OK) != 0) {
+        error_setg(errp, "usb-mtp: rootdir does not exist/not readable");
+        return;
+    } else if (!s->readonly && access(s->root, W_OK) != 0) {
+        error_setg(errp, "usb-mtp: rootdir does not have write permissions");
+        return;
+    }
+
     /* Mark store as RW */
     if (!s->readonly) {
         s->flags |= (1 << MTP_FLAG_WRITABLE);
     }
 
+    if (s->desc == NULL) {
+        /*
+         * This does not check if path exists
+         * but we have the checks above
+         */
+        s->desc = g_path_get_basename(s->root);
+    }
+
+    usb_desc_create_serial(dev);
+    usb_desc_init(dev);
+    QTAILQ_INIT(&s->objects);
+
 }
 
 static const VMStateDescription vmstate_usb_mtp = {