grow_buffers() infinite loop fix (CVE-2006-5757/CVE-2006-6060)

If grow_buffers() is for some reason passed a block number which wants to li
outside the maximum-addressable pagecache range (PAGE_SIZE * 4G bytes) then
will accidentally truncate `index' and will then instnatiate a page at the
wrong pagecache offset.  This causes __getblk_slow() to go into an infinite
loop.

This can happen with corrupted disks, or with software errors elsewhere.

Detect that, and handle it.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

fs/buffer.c

@@ -1179,8 +1179,21 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
 	} while ((size << sizebits) < PAGE_SIZE);
 
 	index = block >> sizebits;
-	block = index << sizebits;
 
+	/*
+	 * Check for a block which wants to lie outside our maximum possible
+	 * pagecache index.  (this comparison is done using sector_t types).
+	 */
+	if (unlikely(index != block >> sizebits)) {
+		char b[BDEVNAME_SIZE];
+
+		printk(KERN_ERR "%s: requested out-of-range block %llu for "
+			"device %s\n",
+			__FUNCTION__, (unsigned long long)block,
+			bdevname(bdev, b));
+		return -EIO;
+	}
+	block = index << sizebits;
 	/* Create a page with the proper size buffers.. */
 	page = grow_dev_page(bdev, block, index, size);
 	if (!page)
@@ -1207,12 +1220,16 @@ __getblk_slow(struct block_device *bdev, sector_t block, int size)
 
 	for (;;) {
 		struct buffer_head * bh;
+		int ret;
 
 		bh = __find_get_block(bdev, block, size);
 		if (bh)
 			return bh;
 
-		if (!grow_buffers(bdev, block, size))
+		ret = grow_buffers(bdev, block, size);
+		if (ret < 0)
+			return NULL;
+		if (ret == 0)
 			free_more_memory();
 	}
 }