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

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Message-ID: <ZEbBLzy4SU8IZR68@ziepe.ca>
Date:   Mon, 24 Apr 2023 14:49:35 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Dave Chinner <david@...morbit.com>, 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
Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by
 default

On Sun, Apr 23, 2023 at 11:56:48PM +0100, Lorenzo Stoakes wrote:

> This warned upon check should in reality not occur, because it implies the
> GUP user is trying to do something broken and is _not_ explicitly telling
> GUP that it knows it's doing it and can live with the consequences. And on
> that basis, is worthy of a warning so we know we have to go put this flag
> in that place (and know it is a source of problematic GUP usage), or fix
> the caller.

It is fine for debugging, but we can't merge user triggerable
WARN_ONs..

Since the GUP caller has no idea if userspace might be maliciously
passing in a file VMA we can't throw warnings.

Jason