linux-kernel - Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user

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Message-ID: <34959b70-6270-46cf-94c5-d6da12b0c62d@lucifer.local>
Date:   Mon, 17 Apr 2023 15:00:16 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org
Subject: Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on
 pin_user_pages()

On Mon, Apr 17, 2023 at 10:26:09AM -0300, Jason Gunthorpe wrote:
> On Mon, Apr 17, 2023 at 02:19:16PM +0100, Lorenzo Stoakes wrote:
>
> > > I'd rather see something like FOLL_ALLOW_BROKEN_FILE_MAPPINGS than
> > > io_uring open coding this kind of stuff.
> > >
> >
> > How would the semantics of this work? What is broken? It is a little
> > frustrating that we have FOLL_ANON but hugetlb as an outlying case, adding
> > FOLL_ANON_OR_HUGETLB was another consideration...
>
> It says "historically this user has accepted file backed pages and we
> we think there may actually be users doing that, so don't break the
> uABI"

Having written a bunch here I suddenly realised that you probably mean for
this flag to NOT be applied to the io_uring code and thus have it enforce
the 'anonymous or hugetlb' check by default?

>
> Without the flag GUP would refuse to return file backed pages that can
> trigger kernel crashes or data corruption.
>
> Eg we'd want most places to not specify the flag and the few that do
> to have some justification.
>

So you mean to disallow file-backed page pinning as a whole unless this
flag is specified? For FOLL_GET I can see that access to the underlying
data is dangerous as the memory may get reclaimed or migrated, but surely
DMA-pinned memory (as is the case here) is safe?

Or is this a product more so of some kernel process accessing file-backed
pages for a file system which expects write-notify semantics and doesn't
get them in this case, which could indeed be horribly broken.

In which case yes this seems sensible.

> We should consdier removing FOLL_ANON, I'm not sure it really makes
> sense these days for what proc is doing with it. All that proc stuff
> could likely be turned into a kthread_use_mm() and a simple
> copy_to/from user?
>
> I suspect that eliminates the need to check for FOLL_ANON?
>
> Jason

I am definitely in favour of cutting things down if possible, and very much
prefer the use of uaccess if we are able to do so rather than GUP.

I do feel that GUP should be focused purely on pinning memory rather than
manipulating it (whether read or write) so I agree with this sentiment.