Re: [PATCH v5 02/10] hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled

[Mike Kravetz]

On 2/10/21 1:21 PM, Axel Rasmussen wrote:
> From: Peter Xu <peterx@xxxxxxxxxx>
> 
> 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>
> Signed-off-by: Axel Rasmussen <axelrasmussen@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(-)
> 
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 6e3bcffe2837..58987a98e179 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -284,8 +284,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>  		 */
>  		ptep = pte_alloc_map(mm, pmdp, addr);
>  	} else if (sz == PMD_SIZE) {
> -		if (IS_ENABLED(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) &&
> -		    pud_none(READ_ONCE(*pudp)))
> +		if (want_pmd_share(vma, addr) && pud_none(READ_ONCE(*pudp)))
>  			ptep = huge_pmd_share(mm, vma, addr, pudp);
>  		else
>  			ptep = (pte_t *)pmd_alloc(mm, pudp, addr);
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index ca6e5ba56f73..d971e7efd17d 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1030,4 +1030,6 @@ static inline __init void hugetlb_cma_check(void)
>  }
>  #endif
>  
> +bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr);
> +
>  #endif /* _LINUX_HUGETLB_H */
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index a8e5f3ea9bb2..c63ccdae3eab 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -52,6 +52,15 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
>  	return vma->vm_userfaultfd_ctx.ctx == vm_ctx.ctx;
>  }
>  
> +/*
> + * Never enable huge pmd sharing on uffd-wp registered vmas, because uffd-wp
> + * protect information is per pgtable entry.
> + */
> +static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
> +{
> +	return vma->vm_flags & VM_UFFD_WP;
> +}
> +
>  static inline bool userfaultfd_missing(struct vm_area_struct *vma)
>  {
>  	return vma->vm_flags & VM_UFFD_MISSING;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 32d4d2e277ad..5710286e1984 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5245,6 +5245,18 @@ static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
>  	return false;
>  }
>  
> +bool want_pmd_share(struct vm_area_struct *vma, unsigned long addr)
> +{
> +#ifndef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
> +	return false;
> +#endif
> +#ifdef CONFIG_USERFAULTFD
> +	if (uffd_disable_huge_pmd_share(vma))
> +		return false;
> +#endif
> +	return vma_shareable(vma, addr);
> +}
> +

This code certainly does the right thing, however I wonder if it should
be structured a little differently.

want_pmd_share() is currently just a check for CONFIG_ARCH_WANT_HUGE_PMD_SHARE.
How about leaving that mostly as is, and adding the new vma checks to
vma_shareable().  vma_shareable() would then be something like:

	if (!(vma->vm_flags & VM_MAYSHARE))
		return false;
#ifdef CONFIG_USERFAULTFD
	if (uffd_disable_huge_pmd_share(vma)
		return false;
#endif
#ifdef /* XXX */
	/* add other checks for things like uffd wp and soft dirty here */
#endif /* XXX */

	if (range_in_vma(vma, base, end)
		return true;
	return false;

Of course, this would require we leave the call to vma_shareable() at the
beginning of huge_pmd_share.  It also means that we are always making a
function call into huge_pmd_share to determine if sharing is possible.
That is not any different than today.  If we do not want to make that extra
function call, then I would suggest putting all that code in want_pmd_share.
It just seems that all the vma checks for sharing should be in one place
if possible.
-- 
Mike Kravetz

>  /*
>   * Determine if start,end range within vma could be mapped by shared pmd.
>   * If yes, adjust start and end to cover range associated with possible
> @@ -5301,9 +5313,6 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
>  	pte_t *pte;
>  	spinlock_t *ptl;
>  
> -	if (!vma_shareable(vma, addr))
> -		return (pte_t *)pmd_alloc(mm, pud, addr);
> -
>  	i_mmap_assert_locked(mapping);
>  	vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
>  		if (svma == vma)
> @@ -5367,7 +5376,7 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
>  	*addr = ALIGN(*addr, HPAGE_SIZE * PTRS_PER_PTE) - HPAGE_SIZE;
>  	return 1;
>  }
> -#define want_pmd_share()	(1)
> +
>  #else /* !CONFIG_ARCH_WANT_HUGE_PMD_SHARE */
>  pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct vma,
>  		      unsigned long addr, pud_t *pud)
> @@ -5385,7 +5394,6 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
>  				unsigned long *start, unsigned long *end)
>  {
>  }
> -#define want_pmd_share()	(0)
>  #endif /* CONFIG_ARCH_WANT_HUGE_PMD_SHARE */
>  
>  #ifdef CONFIG_ARCH_WANT_GENERAL_HUGETLB
> @@ -5407,7 +5415,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>  			pte = (pte_t *)pud;
>  		} else {
>  			BUG_ON(sz != PMD_SIZE);
> -			if (want_pmd_share() && pud_none(*pud))
> +			if (want_pmd_share(vma, addr) && pud_none(*pud))
>  				pte = huge_pmd_share(mm, vma, addr, pud);
>  			else
>  				pte = (pte_t *)pmd_alloc(mm, pud, addr);
>