Re: [PATCH] mm/memcg: return value of the function mem_cgroup_from_css() is not checked

[Michal Hocko]

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@xxxxxxx>
> ---
>  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