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: <7568edfa-6992-452d-9eb2-2497221cb22a@lucifer.local>
Date: Tue, 15 Jul 2025 18:10:16 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.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 10:05:49AM -0700, Andrii Nakryiko wrote:
> 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?

Yeah that's a great idea actually, the stack would obviously give us the
per-query invocation thing. Nice!

I am kicking myself because I jokingly suggested (off-list) that a helper
struct would be the answer to everything (I do love them) and of
course... here we are :P