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

[Matthew Rosato]

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)