zram: don't call idr_remove() from zram_remove()

The use of idr_remove() is forbidden in the callback functions of
idr_for_each().  It is therefore unsafe to call idr_remove in
zram_remove().

This patch moves the call to idr_remove() from zram_remove() to
hot_remove_store().  In the detroy_devices() path, idrs are removed by
idr_destroy().  This solves an use-after-free detected by KASan.

[[email protected]: fix coding stype, per Sergey]
Signed-off-by: Jerome Marchand <[email protected]>
Acked-by: Sergey Senozhatsky <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: <[email protected]>	[4.2+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/block/zram/zram_drv.c

@@ -1325,7 +1325,6 @@ static int zram_remove(struct zram *zram)
 
 	pr_info("Removed device: %s\n", zram->disk->disk_name);
 
-	idr_remove(&zram_index_idr, zram->disk->first_minor);
 	blk_cleanup_queue(zram->disk->queue);
 	del_gendisk(zram->disk);
 	put_disk(zram->disk);
@@ -1367,10 +1366,12 @@ static ssize_t hot_remove_store(struct class *class,
 	mutex_lock(&zram_index_mutex);
 
 	zram = idr_find(&zram_index_idr, dev_id);
-	if (zram)
+	if (zram) {
 		ret = zram_remove(zram);
-	else
+		idr_remove(&zram_index_idr, dev_id);
+	} else {
 		ret = -ENODEV;
+	}
 
 	mutex_unlock(&zram_index_mutex);
 	return ret ? ret : count;