linux-kernel - Re: [PATCH] mm/memcg: return value of the function mem_cgroup_from

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Date:   Thu, 22 Aug 2019 09:05:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yizhuo <yzhai003@....edu>
Cc:     csong@...ucr.edu, zhiyunq@...ucr.edu,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/memcg: return value of the function
 mem_cgroup_from_css() is not checked

On Wed 21-08-19 23:22:09, Yizhuo wrote:
> Inside function mem_cgroup_wb_domain(), the pointer memcg
> could be NULL via mem_cgroup_from_css(). However, this pointer is
> not checked and directly dereferenced in the if statement,
> which is potentially unsafe.

Could you describe circumstances when this would happen? The code is
this way for 5 years without any issues. Are we just lucky or something
has changed recently to make this happen?
 
> Signed-off-by: Yizhuo <yzhai003@....edu>
> ---
>  mm/memcontrol.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 661f046ad318..bd84bdaed3b0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3665,7 +3665,7 @@ struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
>  
> -	if (!memcg->css.parent)
> +	if (!memcg || !memcg->css.parent)
>  		return NULL;
>  
>  	return &memcg->cgwb_domain;
> -- 
> 2.17.1
> 

-- 
Michal Hocko
SUSE Labs