zram: pass down the bvec we need to read into in the work struct

When scheduling work item to read page we need to pass down the proper
bvec struct which points to the page to read into.  Before this patch it
uses a randomly initialized bvec (only if PAGE_SIZE != 4096) which is
wrong.

Note that without this patch on arch/kernel where PAGE_SIZE != 4096
userspace could read random memory through a zram block device (thought
userspace probably would have no control on the address being read).

Link: http://lkml.kernel.org/r/20190408183219.26377-1-jglisse@redhat.comSigned-off-by: NJérôme Glisse <jglisse@redhat.com>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: NMinchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -774,18 +774,18 @@ struct zram_work {
 	struct zram *zram;
 	unsigned long entry;
 	struct bio *bio;
+	struct bio_vec bvec;
 };
 
 #if PAGE_SIZE != 4096
 static void zram_sync_read(struct work_struct *work)
 {
-	struct bio_vec bvec;
 	struct zram_work *zw = container_of(work, struct zram_work, work);
 	struct zram *zram = zw->zram;
 	unsigned long entry = zw->entry;
 	struct bio *bio = zw->bio;
 
-	read_from_bdev_async(zram, &bvec, entry, bio);
+	read_from_bdev_async(zram, &zw->bvec, entry, bio);
 }
 
 /*
@@ -798,6 +798,7 @@ static int read_from_bdev_sync(struct zram *zram, struct bio_vec *bvec,
 {
 	struct zram_work work;
 
+	work.bvec = *bvec;
 	work.zram = zram;
 	work.entry = entry;
 	work.bio = bio;