Date:   Tue, 2 May 2023 16:17:19 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
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Subject: Re: [PATCH v6 2/3] mm/gup: disallow FOLL_LONGTERM GUP-nonfast
 writing to file-backed mappings

On Tue, May 02, 2023 at 05:04:02PM +0200, David Hildenbrand wrote:
> 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@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 | 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 ... ;) )

I think I will finally give in, in penance for the very silly mistake I made
below...

>
> > +
> > +	/* 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 :)
>

No, no it was I who was too tired it seems! You're correct, this is wrong,
will respin with fix :))

> --
> Thanks,
>
> David / dhildenb
>

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