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: <0b8617c1-a150-426f-8fa6-9ab3b5bcfa1e@redhat.com>
Date: Tue, 15 Jul 2025 11:52:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>
Cc: 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 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 :)

-- 
Cheers,

David / dhildenb