Re: [PATCH v8 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

[David Hildenbrand]

On 03.05.23 02:31, Matthew Rosato wrote:
> On 5/2/23 6:51 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.
> >
> > v8:
> > - Fixed typo writeable -> writable.
> > - Fixed bug in writable_file_mapping_allowed() - must check combination of
> >    FOLL_PIN AND FOLL_LONGTERM not either/or.
> > - Updated vma_needs_dirty_tracking() to include write/shared to account for
> >    MAP_PRIVATE mappings.
> > - Move to open-coding the checks in folio_pin_allowed() so we can
> >    READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to
> >    account for fact we now check flags here.
> > - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than
> >    anon. Defer to slow path.
> > - Perform GUP-fast check _after_ the lowest page table level is confirmed to
> >    be stable.
> > - Updated comments and commit message for final patch as per Jason's
> >    suggestions.
>
> Tested again on s390 using QEMU with a memory backend file (on ext4) and vfio-pci -- This time both vfio_pin_pages_remote (which will call pin_user_pages_remote(flags | FOLL_LONGTERM)) and the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable are being allowed (e.g. returning positive pin count)

At least it's consistent now ;) And it might be working as expected ...

In v7:
* pin_user_pages_fast() succeeded
* vfio_pin_pages_remote() failed

But also in v7:
* GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file
  mappings
* Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings

In v8:
* pin_user_pages_fast() succeeds
* vfio_pin_pages_remote() succeeds

But also in v8:
* GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file
  mappings
* Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings


I have to speculate, but ... could it be that you are using a private 
mapping?

In QEMU, unfortunately, the default for memory-backend-file is 
"share=off" (private) ... for memory-backend-memfd it is "share=on" 
(shared). The default is stupid ...

If you invoke QEMU manually, can you specify "share=on" for the 
memory-backend-file? I thought libvirt would always default to 
"share=on" for file mappings (everything else doesn't make much sense) 
... but you might have to specify
	<access mode="shared"/>
in addition to
	<source type="file"/>

--
Thanks,

David / dhildenb