Re: [PATCH v2 2/4] hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled

[Mike Kravetz]

On 2/17/21 12:46 PM, Peter Xu wrote:
> Huge pmd sharing could bring problem to userfaultfd.  The thing is that
> userfaultfd is running its logic based on the special bits on page table
> entries, however the huge pmd sharing could potentially share page table
> entries for different address ranges.  That could cause issues on either:
> 
>   - When sharing huge pmd page tables for an uffd write protected range, the
>     newly mapped huge pmd range will also be write protected unexpectedly, or,
> 
>   - When we try to write protect a range of huge pmd shared range, we'll first
>     do huge_pmd_unshare() in hugetlb_change_protection(), however that also
>     means the UFFDIO_WRITEPROTECT could be silently skipped for the shared
>     region, which could lead to data loss.
> 
> Since at it, a few other things are done altogether:
> 
>   - Move want_pmd_share() from mm/hugetlb.c into linux/hugetlb.h, because
>     that's definitely something that arch code would like to use too
> 
>   - ARM64 currently directly check against CONFIG_ARCH_WANT_HUGE_PMD_SHARE when
>     trying to share huge pmd.  Switch to the want_pmd_share() helper.
> 
> Since at it, move vma_shareable() from huge_pmd_share() into want_pmd_share().
> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  arch/arm64/mm/hugetlbpage.c   |  3 +--
>  include/linux/hugetlb.h       |  2 ++
>  include/linux/userfaultfd_k.h |  9 +++++++++
>  mm/hugetlb.c                  | 20 ++++++++++++++------
>  4 files changed, 26 insertions(+), 8 deletions(-)

Thanks,
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
-- 
Mike Kravetz