Re: [PATCH] mm: kmem: properly initialize local objcg variable in current_obj_cgroup()

[Roman Gushchin]

On Thu, Nov 16, 2023 at 08:04:18AM +0100, Vlastimil Babka wrote:
> On 11/16/23 03:51, Roman Gushchin wrote:
> > Actually the problem is caused by uninitialized local variable in
> > current_obj_cgroup(). If the root memory cgroup is set as an active
> > memory cgroup for a charging scope (as in the trace, where systemd
> > tries to create the first non-root cgroup, so the parent cgroup is
> > the root cgroup), the "for" loop is skipped and uninitialized objcg is
> > returned, causing a panic down the accounting stack.
> > 
> > The fix is trivial: initialize the objcg variable to NULL
> > unconditionally before the "for" loop.
> > 
> > Fixes: e86828e5446d ("mm: kmem: scoped objcg protection")
> > Reported-by: Erhard Furtner <erhard_f@xxxxxxxxxxx>
> > Closes: https://github.com/ClangBuiltLinux/linux/issues/1959
> > Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@xxxxxxxxx>
> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Dennis Zhou <dennis@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Cc: Muchun Song <muchun.song@xxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Thanks!
> 
> We could also do this to make it less confusing?
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 774bd6e21e27..a08bcec661b6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3175,7 +3175,6 @@ __always_inline struct obj_cgroup *current_obj_cgroup(void)
>  		objcg = rcu_dereference_check(memcg->objcg, 1);
>  		if (likely(objcg))
>  			break;
> -		objcg = NULL;
>  	}
>  
>  	return objcg;

Yeah, good catch, it's a remaining of the code with try_get() inside
the loop.

Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>

Thank you!