zram: clear trailing bytes of compressed writeback pages

zram: clear trailing bytes of compressed writeback pages

Patch series "zram: writeback fixes", v2.

Brian (privately) reported a "leak" of writeback bitmap in certain cases,
so that backing device can store less pages; and a theoretical data leak
in the trailing bytes of compressed writeback pages.  Both issues are low
risk.

This patch (of 2):

When compressed writeback is available writtenback pages contain "garbage"
in PAGE_SIZE - obj_size trailing bytes.  That "garbage" is, basically,
whatever data that page held before we got it for writeback.  To get
advantage of it an attacker needs to be able to read from active backing
swap device, which is already catastrophic.  Still, just in case, zero out
those trailing bytes before writeback to a backing device so that we only
store swap-ed out data there.

Link: https://lore.kernel.org/20260526022754.2377730-1-senozhatsky@chromium.org
Link: https://lore.kernel.org/20260526022754.2377730-3-senozhatsky@chromium.org
Fixes: d38fab605c66 ("zram: introduce compressed data writeback")
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Brian Geffon <bgeffon@google.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Richard Chang <richardycc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>