[PATCH AUTOSEL 4.4 18/60] fat: don't allow to mount if the FAT length == 0

[Sasha Levin]

From: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>

[ Upstream commit b1b65750b8db67834482f758fc385bfa7560d228 ]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/87r1wz8mrd.fsf@xxxxxxxxxxxxxxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/fat/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 5e87b9aa7ba6..944fff1ef536 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -1422,6 +1422,12 @@ 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:
-- 
2.25.1