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

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Date:   Sun, 23 Apr 2023 20:40:53 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Simon Horman <simon.horman@...igine.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
Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by
 default

On Sun, Apr 23, 2023 at 09:32:01PM +0200, Simon Horman wrote:
> On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote:
> > It isn't safe to write to file-backed mappings as GUP does not ensure that
> > the semantics associated with such a write are performed correctly, for
> > instance filesystems which rely upon write-notify will not be correctly
> > notified.
> >
> > There are exceptions to this - shmem and hugetlb mappings are (in effect)
> > anonymous mappings by other names so we do permit this operation in these
> > cases.
> >
> > In addition, if no pinning takes place (neither FOLL_GET nor FOLL_PIN is
> > specified and neither flags gets implicitly set) then no writing can occur
> > so we do not perform the check in this instance.
> >
> > This is an important exception, as populate_vma_page_range() invokes
> > __get_user_pages() in this way (and thus so does __mm_populate(), used by
> > MAP_POPULATE mmap() and mlock() invocations).
> >
> > There are GUP users within the kernel that do nevertheless rely upon this
> > behaviour, so we introduce the FOLL_ALLOW_BROKEN_FILE_MAPPING flag to
> > explicitly permit this kind of GUP access.
> >
> > This is required in order to not break userspace in instances where the
> > uAPI might permit file-mapped addresses - a number of RDMA users require
> > this for instance, as do the process_vm_[read/write]v() system calls,
> > /proc/$pid/mem, ptrace and SDT uprobes. Each of these callers have been
> > updated to use this flag.
> >
> > Making this change is an important step towards a more reliable GUP, and
> > explicitly indicates which callers might encouter issues moving forward.
>
> nit: s/encouter/encounter/
>

Ack, I always seem to leave at least one or two easter egg spelling
mistakes in :)

Will fix up on next respin (in unlikely event of no further review,
hopefully Andrew would pick up!)