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

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Date:   Fri, 28 Apr 2023 18:00:22 +0200
From:   David Hildenbrand <david@...hat.com>
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>,
        "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

[...]

>>>
>>> Personally I come at this from the 'I just want my vmas patch series' unblocked
>>> perspective :) and feel there's a functional aspect here too.
>>
>> I know, it always gets messy when touching such sensible topics :P
> 
> I feel that several people owe me drinks at LSF/MM :P
> 
> To cut a long story short to your other points, I'm _really_ leaning
> towards an opt-in variant of this change that we just hand to io_uring to
> make everything simple with minimum risk (if Jens was also open to this
> idea, it'd simply be deleting the open coded vma checks there and adding
> FOLL_SAFE_FILE_WRITE).
> 
> That way we can save the delightful back and forth for another time while
> adding a useful feature and documenting the issue.

Just for the records: I'm not opposed to disabling it system-wide, 
especially once this is an actual security issue and can bring down the 
machine easily (thanks to Jason for raising the security aspect). I just 
wanted to raise awareness that there might be users affected ...

Sure, we could glue this to some system knob like Jason said, if we want 
to play safe.

> 
> Altneratively I could try to adapt this to also do the GUP-fast check,
> hoping that no FOLL_FAST_ONLY users would get nixed (I'd have to check who
> uses that). The others should just get degraded to a standard GUP right?

Yes. When you need the VMA to make a decision, fallback to standard GUP.

The only problematic part is something like get_user_pages_fast_only(), 
that would observe a change. But KVM never passes FOLL_LONGTERM, so at 
least in that context the change should be fine I guess.

The performance concern is the most problematic thing (how to identify 
shmem pages).

> 
> I feel these various series have really helped beat out some details about
> GUP, so as to your point on another thread (trying to reduce noise here
> :P), I think discussion at LSF/MM is also a sensible idea, also you know,
> if beers were bought too it could all work out nicely :]

The issue is, that GUP is so complicated, that each and every MM 
developer familiar with GUP has something to add :P

What stood out to me is that we disallow something for ordinary GUP but 
disallow it for GUP-fast, which looks very odd.

So sorry again for jumping in late ...

-- 
Thanks,

David / dhildenb