Re: [External] Re: [PATCH] vmscan: fix potential arbitrary pointer passed to kfree in unregister_shrinker

[锦江屠 <tujinjiang@bytedance.com>]

On Wed, Jul 27, 2022 at 11:50 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Wed, Jul 27, 2022 at 7:43 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > [Cc Yang Shi]
>
> Thanks, Michal.
>
> > On Wed 27-07-22 17:07:00, tujinjiang@bytedance.com wrote:
> > > From: Jinjiang Tu <tujinjiang@bytedance.com>
> > >
> > > when shrinker is registered with SHRINKER_MEMCG_AWARE flag,
> > > register_shrinker will not initialize shrinker->nr_deferred,
> > > but the pointer will be passed to kfree in unregister_shrinker
> > > when the shrinker is unregistered. This leads to kernel crash
> > > when the shrinker object is dynamically allocated.
> >
> > Is this a real life problem? I thought shrinkers were pre-zeroed
> > already. Not that we should be relying on that but it would be good to
> > mention whether this is a code fortification or something that we should
> > be really worried about.
>
> Yes, all memcg aware shrinkers are actually pre-zeroed. The fs
> shrinkers (embedded in super_block) are allocated by kzalloc, all
> other shrinkers are static declared. So I don't think it will cause
> any crash in real life.
>

Yes, the shrinkers in the current kernel will not cause crash, but a new
memcg aware shrinker may be added in the future,  and I think we
should not assume the shrinker is pre-zeroed.

Function free_prealloced_shrinker does not assume the shrinker is pre-zeroed,
and does not call kfree(shrinker->nr_deferred) if the shrinker is memcg aware.
So I think it is better for unregister_shrinker to call kfree only
when the shrinker
is not memcg aware.

> >
> > > To fix it, this patch initialize shrinker->nr_deferred at the
> > > beginning of prealloc_shrinker.
> >
> > It would be great to add
> > Fixes: 476b30a0949a ("mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers")
> >
> > > Signed-off-by: Jinjiang Tu <tujinjiang@bytedance.com>
> > > ---
> > >  mm/vmscan.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index f7d9a683e3a7..06ab5a398971 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -613,6 +613,7 @@ int prealloc_shrinker(struct shrinker *shrinker)
> > >       unsigned int size;
> > >       int err;
> > >
> > > +     shrinker->nr_deferred = NULL;
> > >       if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> > >               err = prealloc_memcg_shrinker(shrinker);
> > >               if (err != -ENOSYS)
> >
> > You should be able to move it under SHRINKER_MEMCG_AWARE branch, no?
> >
> > --
> > Michal Hocko
> > SUSE Labs