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

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Tue, May 02, 2023 at 02:45:01PM -0400, Matthew Rosato wrote:
> 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.
>

Hey thanks very much for checking that :)

This version will unconditionally apply the retriction to non-FOLL_LONGTERM
by mistake (ugh) but vfio_pin_pages_remote() does seem to be setting
FOLL_LONGTERM anyway so this seems a legitimate test.

Interesting the _fast() variant succeeds...

David, Jason et al. can speak more to the ins and outs of these
virtualisation cases which I am not so familiar with, but I wonder if we do
need a flag to provide an exception for VFIO.