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: <5f8d3100-a0dd-4da3-8797-f097e063ca97@lucifer.local>
Date: Tue, 15 Jul 2025 11:31:23 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: 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 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.

Or we could just sling in a cheeky spinlock