zram: avoid invalid memory access in zram_exit()

commit 6030ea9b35971a4200062f010341ab832e878ac9 upstream.

Memory for zram->disk object may have already been freed after returning
from destroy_device(zram), then it's unsafe for zram_reset_device(zram)
to access zram->disk again.

We can't solve this bug by flipping the order of destroy_device(zram)
and zram_reset_device(zram), that will cause deadlock issues to the
zram sysfs handler.

So fix it by holding an extra reference to zram->disk before calling
destroy_device(zram).

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
index 2594a31..e87af29 100644 (file)
--- a/drivers/staging/zram/zram_drv.c
+++ b/drivers/staging/zram/zram_drv.c
@@ -846,9 +846,11 @@ static void __exit zram_exit(void)
        for (i = 0; i < zram_num_devices; i++) {
                zram = &zram_devices[i];
 
+               get_disk(zram->disk);
                destroy_device(zram);
                if (zram->init_done)
                        zram_reset_device(zram);
+               put_disk(zram->disk);
        }
 
        unregister_blkdev(zram_major, "zram");