[PATCH 4.4 05/16] zram: restrict add/remove attributes to root only

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

commit 5c7e9ccd91b90d87029261f8856294ee51934cab upstream.

zram hot_add sysfs attribute is a very 'special' attribute - reading
from it creates a new uninitialized zram device.  This file, by a
mistake, can be read by a 'normal' user at the moment, while only root
must be able to create a new zram device, therefore hot_add attribute
must have S_IRUSR mode, not S_IRUGO.

[akpm@linux-foundation.org: s/sence/sense/, reflow comment to use 80 cols]
Fixes: 6566d1a32bf72 ("zram: add dynamic device add/remove functionality")
Link: http://lkml.kernel.org/r/20161205155845.20129-1-sergey.senozhatsky@gmail.com
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reported-by: Steven Allen <steven@stebalien.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/block/zram/zram_drv.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1378,8 +1378,14 @@ static ssize_t hot_remove_store(struct c
 	return ret ? ret : count;
 }
 
+/*
+ * NOTE: hot_add attribute is not the usual read-only sysfs attribute. In a
+ * sense that reading from this file does alter the state of your system -- it
+ * creates a new un-initialized zram device and returns back this device's
+ * device_id (or an error code if it fails to create a new device).
+ */
 static struct class_attribute zram_control_class_attrs[] = {
-	__ATTR_RO(hot_add),
+	__ATTR(hot_add, 0400, hot_add_show, NULL),
 	__ATTR_WO(hot_remove),
 	__ATTR_NULL,
 };