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: <10eb7d95-4a02-4d5b-95ef-495bec7226d5@lucifer.local>
Date: Tue, 15 Jul 2025 11:16:43 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, 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 11:52:49AM +0200, 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 :)

Haha well I think it's _still useful_ for avoid contention of the mmap lock. But
we probably just need to bite bullet and lock per-fd for this

>
> --
> Cheers,
>
> David / dhildenb
>