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

[David Hildenbrand]

On 03.05.23 13:25, Matthew Rosato wrote:
> On 5/3/23 3:08 AM, David Hildenbrand wrote:
> > 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"/>
>
> Ah, there we go.  Yes, I was using the default of share=off.  When I instead specify share=on, now the pins will fail in both cases.

Out of curiosity, how does that manifest?

I assume the VM is successfully created and as Linux tries initializing 
and using the device, we get a bunch of errors inside the VM, correct?

--
Thanks,

David / dhildenb