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

[David Hildenbrand <david@redhat.com>]

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