linux-kernel - Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

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Date:   Mon, 15 May 2023 14:07:57 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Kirill A . Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Matthew Wilcox <willy@...radead.org>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Leon Romanovsky <leon@...nel.org>,
        Christian Benvenuti <benve@...co.com>,
        Nelson Escobar <neescoba@...co.com>,
        Bernard Metzler <bmt@...ich.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
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        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
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        John Fastabend <john.fastabend@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
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        Matthew Rosato <mjrosato@...ux.ibm.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed
 mappings by default

On Mon, May 15, 2023 at 09:12:49AM -0300, Jason Gunthorpe wrote:
> On Mon, May 15, 2023 at 12:16:21PM +0100, Lorenzo Stoakes wrote:
> > > One thing that came to mind is KVM with "qemu -object memory-backend-file,share=on..."
> > > It is mostly used for pmem emulation.
> > >
> > > Do we have plan B?
> >
> > Yes, we can make it opt-in or opt-out via a FOLL_FLAG. This would be easy
> > to implement in the event of any issues arising.
>
> I'm becoming less keen on the idea of a per-subsystem opt out. I think
> we should make a kernel wide opt out. I like the idea of using lower
> lockdown levels. Lots of things become unavaiable in the uAPI when the
> lockdown level increases already.

This would be the 'safest' in the sense that a user can't be surprised by
higher lockdown = access modes disallowed, however we'd _definitely_ need
to have an opt-in in that instance so io_uring can make use of this
regardless. That's easy to add however.

If we do go down that road, we can be even stricter/vary what we do at
different levels right?

>
> > Jason will have some thoughts on this I'm sure. I guess the key question
> > here is - is it actually feasible for this to work at all? Once we
> > establish that, the rest are details :)
>
> Surely it is, but like Ted said, the FS folks are not interested and
> they are at least half the solution..

:'(

>
> The FS also has to actively not write out the page while it cannot be
> write protected unless it copies the data to a stable page. The block
> stack needs the source data to be stable to do checksum/parity/etc
> stuff. It is a complicated subject.

Yes my sense was that being able to write arbitrarily to these pages _at
all_ was a big issue, not only the dirty tracking aspect.

I guess at some level letting filesystems have such total flexibility as to
how they implement things leaves us in a difficult position.

>
> Jason