netdev - Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

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Message-ID: <077c4b21-8806-455f-be98-d7052a584259@lucifer.local>
Date:   Fri, 28 Apr 2023 16:33:06 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Jason Gunthorpe <jgg@...dia.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,
        Oleg Nesterov <oleg@...hat.com>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Pavel Begunkov <asml.silence@...il.com>,
        Mika Penttila <mpenttil@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default

On Fri, Apr 28, 2023 at 05:23:29PM +0200, David Hildenbrand wrote:
> > >
> > > Security is the primary case where we have historically closed uAPI
> > > items.
> >
> > As this patch
> >
> > 1) Does not tackle GUP-fast
> > 2) Does not take care of !FOLL_LONGTERM
> >
> > I am not convinced by the security argument in regard to this patch.
> >
> >
> > If we want to sells this as a security thing, we have to block it
> > *completely* and then CC stable.
>
> Regarding GUP-fast, to fix the issue there as well, I guess we could do
> something similar as I did in gup_must_unshare():
>
> If we're in GUP-fast (no VMA), and want to pin a !anon page writable,
> fallback to ordinary GUP. IOW, if we don't know, better be safe.

How do we determine it's non-anon in the first place? The check is on the
VMA. We could do it by following page tables down to folio and checking
folio->mapping for PAGE_MAPPING_ANON I suppose?

>
> Of course, this would prevent hugetlb/shmem from getting pinned writable
> during gup-fast. Unless we're able to whitelist them somehow in there.

We could degrade those to non-fast assuming not FOLL_FAST_ONLY. But it'd be
a pity.

>
>
> For FOLL_LONGTERM it might fairly uncontroversial. For everything else I'm
> not sure if there could be undesired side-effects.

Yeah this is why I pared the patch down to this alone :) there are
definitely concerns and issues with other cases, notably ptrace + friends
but obviously not only.

FOLL_LONGTERM is just the most egregious case.

>
> --
> Thanks,
>
> David / dhildenb
>