zram: protect sysfs handler from invalid memory access

commit 5863e10b upstream.

Use zram->init_lock to protect access to zram->meta, otherwise it
may cause invalid memory access if zram->meta has been freed by
zram_reset_device().

This issue may be triggered by:
Thread 1:
while true; do cat mem_used_total; done
Thread 2:
while true; do echo 8M > disksize; echo 1 > reset; done
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Acked-by: Minchan Kim <minchan@kernel.org>
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>

drivers/staging/zram/zram_sysfs.c

@@ -186,10 +186,12 @@ static ssize_t mem_used_total_show(struct device *dev,
 	u64 val = 0;
 	struct zram *zram = dev_to_zram(dev);
 
+	down_read(&zram->init_lock);
 	if (zram->init_done) {
 		val = xv_get_total_size_bytes(zram->mem_pool) +
 			((u64)(zram->stats.pages_expand) << PAGE_SHIFT);
 	}
+	up_read(&zram->init_lock);
 
 	return sprintf(buf, "%llu\n", val);
 }