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Date:   Thu, 28 Jul 2022 09:34:36 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     =?utf-8?B?6ZSm5rGf5bGg?= <tujinjiang@bytedance.com>
Cc:     Yang Shi <shy828301@gmail.com>, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [External] Re: [PATCH] vmscan: fix potential arbitrary pointer
 passed to kfree in unregister_shrinker
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On Thu 28-07-22 10:37:26, 锦江屠 wrote:
> 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.

Agreed. Especially when that is not documented anywhere.

> 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.

It would be really great to mention this intention in the changelog.
Your initial wording might make an impression this is a fix for an
existing problem.

> > > > 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")

Do not use Fixes tag as this is not a real problem currently.
-- 
Michal Hocko
SUSE Labs