Re: [PATCH v3] mm/gup: disallow GUP writing to file-backed mappings by default

[Kirill A . Shutemov]

On Tue, Apr 25, 2023 at 08:14:14AM +0100, Lorenzo Stoakes wrote:
> GUP does not correctly implement write-notify semantics, nor does it
> guarantee that the underlying pages are correctly dirtied, which could lead
> to a kernel oops or data corruption when writing to file-backed mappings.
> 
> This is only relevant when the mappings are file-backed and the underlying
> file system requires folio dirty tracking. File systems which do not, such
> as shmem or hugetlb, are not at risk and therefore can be written to
> without issue.
> 
> Unfortunately this limitation of GUP has been present for some time and
> requires future rework of the GUP API in order to provide correct write
> access to such mappings.
> 
> In the meantime, we add a check for the most broken GUP case -
> FOLL_LONGTERM - which really under no circumstances can safely access
> dirty-tracked file mappings.
> 
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> ---
> v3:
> - Rebased on latest mm-unstable as of 24th April 2023.
> - Explicitly check whether file system requires folio dirtying. Note that
>   vma_wants_writenotify() could not be used directly as it is very much focused
>   on determining if the PTE r/w should be set (e.g. assuming private mapping
>   does not require it as already set, soft dirty considerations).

Hm. Okay. Have you considered having a common base for your case and
vma_wants_writenotify()? Code duplication doesn't look good.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov