[PATCH 3.4 103/214] zram: avoid invalid memory access in zram_exit()

[Greg Kroah-Hartman]

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jiang Liu <liuj97@xxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Cc: Jianguo Wu <wujianguo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/staging/zram/zram_drv.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/staging/zram/zram_drv.c
+++ b/drivers/staging/zram/zram_drv.c
@@ -841,9 +841,11 @@ static void __exit zram_exit(void)
 	for (i = 0; i < 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");


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