linux-kernel - Re: [PATCH] zram: restrict add/remove attributes to root only

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Date:   Sun, 4 Dec 2016 11:28:07 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Steven Allen <steven@...balien.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH] zram: restrict add/remove attributes to root only

On Sun, Dec 04, 2016 at 11:35:15AM +0900, Sergey Senozhatsky wrote:
> Only root must be able to create a new zram device, therefore
> hot_add attribute must have S_IRUSR mode, not S_IRUGO. Otherwise,
> anyone can create a new zram device (device initialization with
> the disksize attr requires root permission).
> 
> Fixes: 6566d1a32bf72 ("zram: add dynamic device add/remove functionality")
> Reported-by: Steven Allen <steven@...balien.com>
> Cc: <stable@...r.kernel.org>    [4.2+]
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
>  drivers/block/zram/zram_drv.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 5163c8f..ee03464 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -1414,8 +1414,8 @@ static ssize_t hot_remove_store(struct class *class,
>  }
>  
>  static struct class_attribute zram_control_class_attrs[] = {
> -	__ATTR_RO(hot_add),
> -	__ATTR_WO(hot_remove),
> +	__ATTR(hot_add, 0400, hot_add_show, NULL),
> +	__ATTR(hot_remove, 0200, NULL, hot_remove_store),

Huh?  The only change you are making here is that a "normal" user can
not read the value, ATTR_WO only allows root to write to the file.

This change doesn't match up with your changelog, what really is the
problem here with the _RO and _WO values that you can not use them?

Why can't a normal user read the attribute?  Does a read actually modify
something?  If so, that's really not a good idea.

confused,

greg k-h