Re: [PATCH v6 2/3] mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings

[David Hildenbrand]

On 02.05.23 01:11, 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@xxxxxxxxxxxxxx/
>
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Reviewed-by: Mika Penttilä <mpenttil@xxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> ---
>   mm/gup.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 40 insertions(+), 1 deletion(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index ff689c88a357..0f09dec0906c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -959,16 +959,51 @@ 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. */
> +	if (!(gup_flags & (FOLL_GET | FOLL_PIN)))
> +		return true;

I think we should really not look at FOLL_GET here. Just check for 
FOLL_PIN (as said, even FOLL_LONGTERM would be sufficient, but I 
understand the reasoning to keep it, although I would drop it :P ). It 
also better matches your comment regarding pinning ...

See the comment in is_valid_gup_args() regarding "LONGTERM can only be 
specified when pinning". (well, there we also check that FOLL_PIN has to 
be set ... ;) )

> +
> +	/* We limit this check to the most egregious case - a long term pin. */
> +	if (!(gup_flags & FOLL_LONGTERM))
> +		return true;
> +
> +	/* If the VMA requires dirty tracking then GUP will be problematic. */
> +	return vma_needs_dirty_tracking(vma);


... should that be "!vma_needs_dirty_tracking(vma)" ?

If the fs needs dirty tracking, it should be disallowed.

Maybe that explains why it's still working for Matthew in his s390x 
test. ... or I am too tired and messed up :)

--
Thanks,

David / dhildenb