Re: [PATCH v4] mm/gup: disallow GUP writing to file-backed mappings by default

[John Hubbard <jhubbard@nvidia.com>]

On 4/26/23 00:20, Lorenzo Stoakes wrote:
...
>> Could you elaborate? GUP calls handle_mm_fault() that invokes the write
>> notify the pte is made first writable. Of course, virt(pinned_page)[0] = 'x'
>> is not supposed to fault while using the kernel mapping.
>>
> 
> The issue is how dirtying works. Typically for a dirty-tracking mapping the
> kernel makes the mapping read-only, then when a write fault occurs,
> writenotify is called and the folio is marked dirty. This way the file
> system knows which files to writeback, then after writeback it 'cleans'
> them, restoring the read-only mapping and relying on the NEXT write marking
> write notifying and marking the folio dirty again.
> 
> If we GUP, _especially_ if it's long term, we run the risk of a write to
> the folio _after_ it has been cleaned and if the caller tries to do the
> 'right thing' and mark the folio dirty, it being marked dirty at an
> unexpected time which might race with other things and thus oops.
> 
> The issue is that this dirtying mechanism implicitly relies on the memory
> _only_ being accessed via user mappings, but we're providing another way to
> access that memory bypassing all that.
> 
> It's been a fundamental flaw in GUP for some time. This change tries to
> make things a little better by precluding the riskiest version of this
> which is the caller indicating that the pin is longterm (via
> FOLL_LONGTERM).
> 
> For an example of a user trying to sensibly avoid this, see io_pin_pages()
> in io_uring/rsrc.c. This is where io_uring tries to explicitly avoid this
> themselves, something that GUP should clearly be doing.
> 
> After this change, that code can be removed and we will live in a world
> where linux has a saner GUP :)

Hi Lorenzo,

This is the "missing writeup" that really helps people visualize the
problem, very nice. I think if you include the above, plus maybe a link
to Jan Kara's original report [1], in the commit description, that will
help a lot.


> 
> Of course we need to make more fundamental changes moving forward, the idea
> is this improves the situation and eliminates the need for the open-coded
> solution for io_uring which unblocks my other patch series which is also
> trying to make GUP more sane.

It is true that solving the problem has taken "a few"...years, and is still
not done, yes. :)

I'll respond to the patch with a review, as well.


[1] https://lore.kernel.org/linux-mm/20180103100430.GE4911@quack2.suse.cz/
 thanks,
-- 
John Hubbard
NVIDIA