squashfs: be more careful about metadata corruption

Anatoly Trosinenko reports that a corrupted squashfs image can cause a
kernel oops.  It turns out that squashfs can end up being confused about
negative fragment lengths.

The regular squashfs_read_data() does check for negative lengths, but
squashfs_read_metadata() did not, and the fragment size code just
blindly trusted the on-disk value.  Fix both the fragment parsing and
the metadata reading code.

Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Phillip Lougher <phillip@squashfs.org.uk>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/squashfs/fragment.c

@@ -61,9 +61,7 @@ int squashfs_frag_lookup(struct super_block *sb, unsigned int fragment,
 		return size;
 
 	*fragment_block = le64_to_cpu(fragment_entry.start_block);
-	size = le32_to_cpu(fragment_entry.size);
-
-	return size;
+	return squashfs_block_size(fragment_entry.size);
 }