linux-kernel - Re: [PATCH v6 7/8] fs/proc/task_mmu: read proc/pid/maps under per-vma lock

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Message-ID: <CAEf4BzaEouFx8EuZF_PUKdc5wsq-5FYNyAE19VRxV7_YJkrfww@mail.gmail.com>
Date: Tue, 15 Jul 2025 10:05:49 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, David Hildenbrand <david@...hat.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, akpm@...ux-foundation.org, 
	peterx@...hat.com, jannh@...gle.com, hannes@...xchg.org, mhocko@...nel.org, 
	paulmck@...nel.org, shuah@...nel.org, adobriyan@...il.com, brauner@...nel.org, 
	josef@...icpanda.com, yebin10@...wei.com, linux@...ssschuh.net, 
	willy@...radead.org, osalvador@...e.de, andrii@...nel.org, 
	ryan.roberts@....com, christophe.leroy@...roup.eu, tjmercier@...gle.com, 
	kaleshsingh@...gle.com, aha310510@...il.com, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 7/8] fs/proc/task_mmu: read proc/pid/maps under per-vma lock

On Tue, Jul 15, 2025 at 3:31 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Tue, Jul 15, 2025 at 12:23:31PM +0200, Vlastimil Babka wrote:
> > On 7/15/25 11:52, David Hildenbrand wrote:
> > > On 15.07.25 11:40, Lorenzo Stoakes wrote:
> > >> On Tue, Jul 15, 2025 at 10:16:41AM +0200, Vlastimil Babka wrote:
> > >>>> Andrew, could you please remove this patchset from mm-unstable for now
> > >>>> until I fix the issue and re-post the new version?
> > >>>
> > >>> Andrew can you do that please? We keep getting new syzbot reports.
> > >>
> > >> I also pinged up top :P just to be extra specially clear...
> > >>
> > >>>
> > >>>> The error I got after these fixes is:
> > >>>
> > >>> I suspect the root cause is the ioctls are not serialized against each other
> > >>> (probably not even against read()) and yet we treat m->private as safe to
> > >>> work on. Now we have various fields that are dangerous to race on - for
> > >>> example locked_vma and iter races would explain a lot of this.
> > >>>
> > >>> I suspect as long as we used purely seq_file workflow, it did the right
> > >>> thing for us wrt serialization, but the ioctl addition violates that. We
> > >>> should rather recheck even the code before this series, if dangerous ioctl
> > >>> vs read() races are possible. And the ioctl implementation should be
> > >>> refactored to use an own per-ioctl-call private context, not the seq_file's
> > >>> per-file-open context.
> > >>
> > >> Entirely agree with this analysis. I had a look at most recent report, see:
> > >>
> > >> https://lore.kernel.org/linux-mm/f13cda37-06a0-4281-87d1-042678a38a6b@lucifer.local/
> > >>
> > >> AFAICT we either have to lock around the ioctl or find a new way of storing
> > >> per-ioctl state.
> > >>
> > >> We'd probably need to separate out the procmap query stuff to do that
> > >> though. Probably.
> > >
> > > When I skimmed that series the first time, I was wondering "why are we
> > > even caring about PROCMAP_QUERY that in the context of this patch series".
> > >
> > > Maybe that helps :)
> >
> > Yeah seems like before patch 8/8 the ioctl handling, specifically
> > do_procmap_query() only looks at priv->mm and nothing else so it should be
> > safe as that's a stable value.
> >
> > So it should be also enough to drop the last patch from mm for now, not
> > whole series.
>
> Yeah to save the mothership we can ditch the landing craft :P
>
> Maybe worth doing that, and figure out in a follow up how to fix this.

For PROCMAP_QUERY, we need priv->mm, but the newly added locked_vma
and locked_vma don't need to be persisted between ioctl calls. So we
can just add those two fields into a small struct, and for seq_file
case have it in priv, but for PROCMAP_QUERY just have it on the stack.
The code can be written to accept this struct to maintain the state,
which for PROCMAP_QUERY ioctl will be very short-lived on the stack
one.

Would that work?

>
> Or we could just sling in a cheeky spinlock