diff --git a/ChangeLog b/ChangeLog
index ccd8ac1f1..47ff3552c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2014-08-10  Vladimir Serbinenko  <phcoder@gmail.com>
+
+	* grub-core/fs/cbfs.c: Don't probe disks of unknow size.
+
 2014-07-08  Colin Watson  <cjwatson@ubuntu.com>
 
 	* util/grub.d/10_hurd.in: Make kernel list progression not fail on
diff --git a/grub-core/fs/cbfs.c b/grub-core/fs/cbfs.c
index 7ad148361..35750a0e4 100644
--- a/grub-core/fs/cbfs.c
+++ b/grub-core/fs/cbfs.c
@@ -149,6 +149,9 @@ grub_cbfs_mount (grub_disk_t disk)
   grub_off_t header_off;
   struct cbfs_header head;
 
+  if (grub_disk_get_size (disk) == GRUB_DISK_SIZE_UNKNOWN)
+    goto fail;
+
   if (grub_disk_read (disk, grub_disk_get_size (disk) - 1,
 		      GRUB_DISK_SECTOR_SIZE - sizeof (ptr),
 		      sizeof (ptr), &ptr))