zram: Return zero'd pages on new reads

Currently zram will do nothing to the page in the bvec when that page
has not been previously written.  This allows random data to leak to
user space.  That can be seen by doing the following:

 ## Load the module and create a 256Mb zram device called /dev/zram0
 # modprobe zram
 # echo $((256*1024*1024)) > /sys/class/block/zram0/disksize

 ## Initialize the device by writing zero to the first block
 # dd if=/dev/zero of=/dev/zram0 bs=512 count=1

 ## Read ~256Mb of memory into a file and hope for something interesting
 # dd if=/dev/zram0 of=file

This patch will treat an unwritten page as a zero-filled page.  If a
page is read before a write has occurred the data returned is all 0's.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
index 7d11c595c7d24a8014fc14dc64d1b6d06cb0c057..1017d6df17d1e0da8c849455ac545753e7cb5812 100644 (file)
--- a/drivers/staging/zram/zram_drv.c
+++ b/drivers/staging/zram/zram_drv.c
@@ -237,7 +237,7 @@ static void zram_read(struct zram *zram, struct bio *bio)
                if (unlikely(!zram->table[index].page)) {
                        pr_debug("Read before write: sector=%lu, size=%u",
                                (ulong)(bio->bi_sector), bio->bi_size);
-                       /* Do nothing */
+                       handle_zero_page(page);
                        continue;
                }