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/squashfs_fs.h

@@ -129,6 +129,12 @@
 
 #define SQUASHFS_COMPRESSED_BLOCK(B)	(!((B) & SQUASHFS_COMPRESSED_BIT_BLOCK))
 
+static inline int squashfs_block_size(__le32 raw)
+{
+	u32 size = le32_to_cpu(raw);
+	return (size >> 25) ? -EIO : size;
+}
+
 /*
  * Inode number ops.  Inodes consist of a compressed block number, and an
  * uncompressed offset within that block