netdev - Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default

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Message-ID: <ZEZ4VCyfRNCZOO8/@infradead.org>
Date:   Mon, 24 Apr 2023 05:38:44 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>, 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>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bjorn Topel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        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>, Jan Kara <jack@...e.cz>,
        Chris Mason <clm@...com>, John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings
 by default

On Mon, Apr 24, 2023 at 09:28:07AM -0300, Jason Gunthorpe wrote:
> On Mon, Apr 24, 2023 at 11:17:55AM +0100, Lorenzo Stoakes wrote:
> > On Mon, Apr 24, 2023 at 02:43:56AM -0700, Christoph Hellwig wrote:
> > > I'm pretty sure DIRECT I/O reads that write into file backed mappings
> > > are out there in the wild.
> 
> I wonder if that is really the case? I know people tried this with
> RDMA and it didn't get very far before testing uncovered data
> corruption and kernel crashes.. Maybe O_DIRECT has a much smaller race
> window so people can get away with it?

It absolutely causes all kinds of issues even with O_DIRECT.  I'd be
all for trying to disallow it as it simplies a lot of things, but I fear
it's not going to stick.

> So, my suggestion was to mark the places where we want to allow this,
> eg O_DIRECT, and block everwhere else. Lorenzo, I would significantly
> par back the list you have.

I think an opt-in is a good idea no matter how many places end up
needing it.  I'd prefer a less dramatic name and a better explanation
on why it should only be set when needed.

> I also suggest we force block it at some kernel lockdown level..
> 
> Alternatively, perhaps we abuse FOLL_LONGTERM and prevent it from
> working with filebacked pages since, I think, the ease of triggering a
> bug goes up the longer the pages are pinned.

FOLL_LONGTERM on file backed pages is a nightmare.  If you think you
can get away with prohibiting it for RDMA, and KVM doesn't need it
I'd be all for not allowing that at all.