linux-kernel - Re: [PATCH v7 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

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Message-ID: <ce86e956-173f-848a-a1f3-f102134ccd94@linux.ibm.com>
Date:   Tue, 2 May 2023 14:45:01 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>,
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        Namhyung Kim <namhyung@...nel.org>,
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        Bjorn Topel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
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        Paolo Abeni <pabeni@...hat.com>,
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        Oleg Nesterov <oleg@...hat.com>,
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        "Theodore Ts'o" <tytso@....edu>, Peter Xu <peterx@...hat.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v7 0/3] mm/gup: disallow GUP writing to file-backed
 mappings by default

On 5/2/23 12:34 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.
> 
> v7:
> - Fixed very silly bug in writeable_file_mapping_allowed() inverting the
>   logic.
> - Removed unnecessary RCU lock code and replaced with adaptation of Peter's
>   idea.
> - Removed unnecessary open-coded folio_test_anon() in
>   folio_longterm_write_pin_allowed() and restructured to generally permit
>   NULL folio_mapping().
> 

FWIW, I realize you are planning another respin, but I went and tried this version out on s390 -- Now when using a memory backend file and vfio-pci on s390 I see vfio_pin_pages_remote failing consistently.  However, the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable will still return positive.