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

[Matthew Rosato <mjrosato@linux.ibm.com>]

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.