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Date:   Tue, 2 May 2023 20:31:45 -0400
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Subject: Re: [PATCH v8 0/3] mm/gup: disallow GUP writing to file-backed
 mappings by default
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To:     Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>,
        Leon Romanovsky <leon@kernel.org>,
        Christian Benvenuti <benve@cisco.com>,
        Nelson Escobar <neescoba@cisco.com>,
        Bernard Metzler <bmt@zurich.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Bjorn Topel <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        Richard Cochran <richardcochran@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        Oleg Nesterov <oleg@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Mika Penttila <mpenttil@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Dave Chinner <david@fromorbit.com>,
        "Theodore Ts'o" <tytso@mit.edu>, Peter Xu <peterx@redhat.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Christian Borntraeger <borntraeger@linux.ibm.com>
References: <cover.1683067198.git.lstoakes@gmail.com>
From:   Matthew Rosato <mjrosato@linux.ibm.com>
In-Reply-To: <cover.1683067198.git.lstoakes@gmail.com>
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On 5/2/23 6:51 PM, Lorenzo Stoakes wrote:
> Writing to file-backed mappings which require folio dirty tracking using
> GUP is a fundamentally broken operation, as kernel write access to GUP
> mappings do not adhere to the semantics expected by a file system.
> 
> A GUP caller uses the direct mapping to access the folio, which does not
> cause write notify to trigger, nor does it enforce that the caller marks
> the folio dirty.
> 
> The problem arises when, after an initial write to the folio, writeback
> results in the folio being cleaned and then the caller, via the GUP
> interface, writes to the folio again.
> 
> As a result of the use of this secondary, direct, mapping to the folio no
> write notify will occur, and if the caller does mark the folio dirty, this
> will be done so unexpectedly.
> 
> For example, consider the following scenario:-
> 
> 1. A folio is written to via GUP which write-faults the memory, notifying
>    the file system and dirtying the folio.
> 2. Later, writeback is triggered, resulting in the folio being cleaned and
>    the PTE being marked read-only.
> 3. The GUP caller writes to the folio, as it is mapped read/write via the
>    direct mapping.
> 4. The GUP caller, now done with the page, unpins it and sets it dirty
>    (though it does not have to).
> 
> This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As
> pin_user_pages_fast_only() does not exist, we can rely on a slightly
> imperfect whitelisting in the PUP-fast case and fall back to the slow case
> should this fail.
> 
> v8:
> - Fixed typo writeable -> writable.
> - Fixed bug in writable_file_mapping_allowed() - must check combination of
>   FOLL_PIN AND FOLL_LONGTERM not either/or.
> - Updated vma_needs_dirty_tracking() to include write/shared to account for
>   MAP_PRIVATE mappings.
> - Move to open-coding the checks in folio_pin_allowed() so we can
>   READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to
>   account for fact we now check flags here.
> - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than
>   anon. Defer to slow path.
> - Perform GUP-fast check _after_ the lowest page table level is confirmed to
>   be stable.
> - Updated comments and commit message for final patch as per Jason's
>   suggestions.

Tested again on s390 using QEMU with a memory backend file (on ext4) and vfio-pci -- This time both vfio_pin_pages_remote (which will call pin_user_pages_remote(flags | FOLL_LONGTERM)) and the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable are being allowed (e.g. returning positive pin count)