Re: [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing in mem_cgroup_init()

[Michal Hocko]

On Mon 02-08-21 18:00:10, Miaohe Lin wrote:
> On 2021/8/2 14:43, Michal Hocko wrote:
> > On Sat 31-07-21 10:05:51, Miaohe Lin wrote:
> >> On 2021/7/30 14:44, Michal Hocko wrote:
> >>> On Thu 29-07-21 20:12:43, Roman Gushchin wrote:
> >>>> On Thu, Jul 29, 2021 at 08:57:54PM +0800, Miaohe Lin wrote:
> >>>>> rtpn might be NULL in very rare case. We have better to check it before
> >>>>> dereferencing it. Since memcg can live with NULL rb_tree_per_node in
> >>>>> soft_limit_tree, warn this case and continue.
> >>>>>
> >>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> >>>>> ---
> >>>>>  mm/memcontrol.c | 2 ++
> >>>>>  1 file changed, 2 insertions(+)
> >>>>>
> >>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >>>>> index 5b4592d1e0f2..70a32174e7c4 100644
> >>>>> --- a/mm/memcontrol.c
> >>>>> +++ b/mm/memcontrol.c
> >>>>> @@ -7109,6 +7109,8 @@ static int __init mem_cgroup_init(void)
> >>>>>  		rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL,
> >>>>>  				    node_online(node) ? node : NUMA_NO_NODE);
> >>>>>  
> >>>>> +		if (WARN_ON_ONCE(!rtpn))
> >>>>> +			continue;
> >>>>
> >>>> I also really doubt that it makes any sense to continue in this case.
> >>>> If this allocations fails (at the very beginning of the system's life, it's an __init function),
> >>>> something is terribly wrong and panic'ing on a NULL-pointer dereference sounds like
> >>>> a perfect choice.
> >>>
> >>> Moreover this is 24B allocation during early boot. Kernel will OOM and
> >>> panic when not being able to find any victim. I do not think we need to
> >>
> >> Agree with you. But IMO it may not be a good idea to leave the rtpn without NULL check. We should defend
> >> it though it could hardly happen. But I'm not insist on this check. I will drop this patch if you insist.
> > 
> > It is not that I would insist. I just do not see any point in the code
> > churn. This check is not going to ever trigger and there is nothing you
> > can do to recover anyway so crashing the kernel is likely the only
> > choice left.
> > 
> 
> I hope I get the point now. What you mean is nothing we can do to recover and panic'ing on a
> NULL-pointer dereference is a perfect choice ? Should we declare that we leave the rtpn without
> NULL check on purpose like below ?
> 
> Many thanks.
> 
> @@ -7109,8 +7109,12 @@ static int __init mem_cgroup_init(void)
>                 rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL,
>                                     node_online(node) ? node : NUMA_NO_NODE);
> 
> -               if (WARN_ON_ONCE(!rtpn))
> -                       continue;
> +               /*
> +                * If this allocation fails (at the very beginning of the
> +                * system's life, it's an __init function), something is
> +                * terribly wrong and panic'ing on a NULL-pointer
> +                * dereference sounds like a perfect choice.
> +                */

I am not really sure this is really worth it. Really we do not really
want to have similar comments all over the early init code, do we?

>                 rtpn->rb_root = RB_ROOT;
>                 rtpn->rb_rightmost = NULL;
>                 spin_lock_init(&rtpn->lock);

-- 
Michal Hocko
SUSE Labs