netdev - Re: [PATCH v3] mm/gup: disallow GUP writing to file-backed mappings by default

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Date:   Tue, 25 Apr 2023 13:11:53 +0300
From:   "Kirill A . Shutemov" <kirill@...temov.name>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, 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>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [PATCH v3] mm/gup: disallow GUP writing to file-backed mappings
 by default

On Tue, Apr 25, 2023 at 08:14:14AM +0100, Lorenzo Stoakes wrote:
> GUP does not correctly implement write-notify semantics, nor does it
> guarantee that the underlying pages are correctly dirtied, which could lead
> to a kernel oops or data corruption when writing to file-backed mappings.
> 
> This is only relevant when the mappings are file-backed and the underlying
> file system requires folio dirty tracking. File systems which do not, such
> as shmem or hugetlb, are not at risk and therefore can be written to
> without issue.
> 
> Unfortunately this limitation of GUP has been present for some time and
> requires future rework of the GUP API in order to provide correct write
> access to such mappings.
> 
> In the meantime, we add a check for the most broken GUP case -
> FOLL_LONGTERM - which really under no circumstances can safely access
> dirty-tracked file mappings.
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
> v3:
> - Rebased on latest mm-unstable as of 24th April 2023.
> - Explicitly check whether file system requires folio dirtying. Note that
>   vma_wants_writenotify() could not be used directly as it is very much focused
>   on determining if the PTE r/w should be set (e.g. assuming private mapping
>   does not require it as already set, soft dirty considerations).

Hm. Okay. Have you considered having a common base for your case and
vma_wants_writenotify()? Code duplication doesn't look good.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov