Message-ID: <d2ca4a03-dd7e-29d8-d932-4ee5a31e1ab2@redhat.com>
Date:   Tue, 2 May 2023 18:42:42 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
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Subject: Re: [PATCH v7 2/3] mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing
 to file-backed mappings

On 02.05.23 18:34, 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 results in both data being written to a folio without writenotify, and
> the folio being dirtied unexpectedly (if the caller decides to do so).
> 
> This issue was first reported by Jan Kara [1] in 2018, where the problem
> resulted in file system crashes.
> 
> This is only relevant when the mappings are file-backed and the underlying
> file system requires folio dirty tracking. File systems which do not, such
> as shmem or hugetlb, are not at risk and therefore can be written to
> without issue.
> 
> Unfortunately this limitation of GUP has been present for some time and
> requires future rework of the GUP API in order to provide correct write
> access to such mappings.
> 
> However, for the time being we introduce this check to prevent the most
> egregious case of this occurring, use of the FOLL_LONGTERM pin.
> 
> These mappings are considerably more likely to be written to after
> folios are cleaned and thus simply must not be permitted to do so.
> 
> This patch changes only the slow-path GUP functions, a following patch
> adapts the GUP-fast path along similar lines.
> 
> [1]:https://lore.kernel.org/linux-mm/20180103100430.GE4911@quack2.suse.cz/
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> Reviewed-by: John Hubbard <jhubbard@...dia.com>
> Reviewed-by: Mika Penttilä <mpenttil@...hat.com>
> Reviewed-by: Jan Kara <jack@...e.cz>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> ---
>   mm/gup.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 42 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index ff689c88a357..6e209ca10967 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -959,16 +959,53 @@ static int faultin_page(struct vm_area_struct *vma,
>   	return 0;
>   }
>   
> +/*
> + * 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.
> + *
> + * 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 results in both data being written to a folio without writenotify, and
> + * the folio being dirtied unexpectedly (if the caller decides to do so).
> + */
> +static bool writeable_file_mapping_allowed(struct vm_area_struct *vma,
> +					   unsigned long gup_flags)
> +{
> +	/*
> +	 * If we aren't pinning then no problematic write can occur. A long term
> +	 * pin is the most egregious case so this is the case we disallow.
> +	 */
> +	if (!(gup_flags & (FOLL_PIN | FOLL_LONGTERM)))
> +		return true;

If you really want to keep FOLL_PIN here ... this has to be

if ((gup_flags & (FOLL_PIN | FOLL_LONGTERM)) != (FOLL_PIN | FOLL_LONGTERM))

or two separate checks.

Otherwise you'd also proceed if only FOLL_PIN is set.

Unless my tired eyes betrayed me.


-- 
Thanks,

David / dhildenb

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