Re: [PATCH 2/2] zram: avoid NULL pointer access when reading mem_used_total

[Sergey Senozhatsky]

On (10/25/14 17:26), Weijie Yang wrote:
> Date: Sat, 25 Oct 2014 17:26:31 +0800
> From: Weijie Yang <weijie.yang@xxxxxxxxxxx>
> To: 'Minchan Kim' <minchan@xxxxxxxxxx>
> Cc: 'Andrew Morton' <akpm@xxxxxxxxxxxxxxxxxxxx>, 'Dan Streetman'
>  <ddstreet@xxxxxxxx>, 'Sergey Senozhatsky' <sergey.senozhatsky@xxxxxxxxx>,
>  'Nitin Gupta' <ngupta@xxxxxxxxxx>, 'Linux-MM' <linux-mm@xxxxxxxxx>,
>  'linux-kernel' <linux-kernel@xxxxxxxxxxxxxxx>, 'Weijie Yang'
>  <weijie.yang.kh@xxxxxxxxx>
> Subject: [PATCH 2/2] zram: avoid NULL pointer access when reading
>  mem_used_total
> X-Mailer: Microsoft Office Outlook 12.0
> 
> There is a rare NULL pointer bug in mem_used_total_show() in concurrent
> situation, like this:
> zram is not initialized, process A is a mem_used_total reader which runs
> periodicity, while process B try to init zram.
> 
> 	process A 				process B
> access meta, get a NULL value
> 						init zram, done
> init_done() is true
> access meta->mem_pool, get a NULL pointer BUG
> 
> This patch fixes this issue.
> 	
> Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>

Acked-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>

	-ss

> ---
>  drivers/block/zram/zram_drv.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 64dd79a..2ffd7d8 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -99,11 +99,12 @@ static ssize_t mem_used_total_show(struct device *dev,
>  {
>  	u64 val = 0;
>  	struct zram *zram = dev_to_zram(dev);
> -	struct zram_meta *meta = zram->meta;
>  
>  	down_read(&zram->init_lock);
> -	if (init_done(zram))
> +	if (init_done(zram)) {
> +		struct zram_meta *meta = zram->meta;
>  		val = zs_get_total_pages(meta->mem_pool);
> +	}
>  	up_read(&zram->init_lock);
>  
>  	return scnprintf(buf, PAGE_SIZE, "%llu\n", val << PAGE_SHIFT);
> -- 
> 1.7.0.4
> 
> 
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