corrupted cramfs filesystems cause kernel oops (CVE-2006-5823)

Steve Grubb's fzfuzzer tool (http://people.redhat.com/sgrubb/files/
fsfuzzer-0.6.tar.gz) generates corrupt Cramfs filesystems which cause
Cramfs to kernel oops in cramfs_uncompress_block().  The cause of the oops
is an unchecked corrupted block length field read by cramfs_readpage().

This patch adds a sanity check to cramfs_readpage() which checks that the
block length field is sensible.  The (PAGE_CACHE_SIZE << 1) size check is
intentional, even though the uncompressed data is not going to be larger
than PAGE_CACHE_SIZE, gzip sometimes generates compressed data larger than
the original source data.  Mkcramfs checks that the compressed size is
always less than or equal to PAGE_CACHE_SIZE << 1.  Of course Cramfs could
use the original uncompressed data in this case, but it doesn't.
Signed-off-by: Phillip Lougher <phillip@lougher.org.uk>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

fs/cramfs/inode.c

@@ -482,6 +482,8 @@ static int cramfs_readpage(struct file *file, struct page * page)
 		pgdata = kmap(page);
 		if (compr_len == 0)
 			; /* hole */
+		else if (compr_len > (PAGE_CACHE_SIZE << 1))
+			printk(KERN_ERR "cramfs: bad compressed blocksize %u\n", compr_len);
 		else {
 			down(&read_mutex);
 			bytes_filled = cramfs_uncompress_block(pgdata,