Grub2: fs/bfs: integer overflow leads to heap oob read in the bfs parser

An integer overflow flaw was found in the BFS file system driver in grub2. When reading a file with an indirect extent map, grub2 fails to validate the number of extent entries to be read. A crafted or corrupted BFS filesystem may cause an integer overflow during the file reading, leading to a heap of bounds read. As a consequence, sensitive data may be leaked, or grub2 will crash.