fat: don't allow to mount if the FAT length == 0

If FAT length == 0, the image doesn't have any data. And it can be the
cause of overlapping the root dir and FAT entries.

Also Windows treats it as invalid format.

Reported-by: syzbot+6f1624f937d9d6911e2d@syzkaller.appspotmail.com
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Link: http://lkml.kernel.org/r/87r1wz8mrd.fsf@mail.parknet.co.jp
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index e6e68b2274a5cc5fbd3d4954cab5ee9f7f5d97fd..a0cf99debb1ecd02244dfaeb14a7e087b7efa1eb 100644 (file)
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -1519,6 +1519,12 @@ static int fat_read_bpb(struct super_block *sb, struct fat_boot_sector *b,
                goto out;
        }
 
+       if (bpb->fat_fat_length == 0 && bpb->fat32_length == 0) {
+               if (!silent)
+                       fat_msg(sb, KERN_ERR, "bogus number of FAT sectors");
+               goto out;
+       }
+
        error = 0;
 
 out: