Re: [PATCH v3] mm/hugetlb: Fix uffd wr-protection for CoW optimization path

[Mike Kravetz]

On 03/24/23 10:26, Peter Xu wrote:
> This patch fixes an issue that a hugetlb uffd-wr-protected mapping can be
> writable even with uffd-wp bit set.  It only happens with hugetlb private
> mappings, when someone firstly wr-protects a missing pte (which will
> install a pte marker), then a write to the same page without any prior
> access to the page.
> 
> Userfaultfd-wp trap for hugetlb was implemented in hugetlb_fault() before
> reaching hugetlb_wp() to avoid taking more locks that userfault won't need.
> However there's one CoW optimization path that can trigger hugetlb_wp()
> inside hugetlb_no_page(), which will bypass the trap.
> 
> This patch skips hugetlb_wp() for CoW and retries the fault if uffd-wp bit
> is detected.  The new path will only trigger in the CoW optimization path
> because generic hugetlb_fault() (e.g. when a present pte was wr-protected)
> will resolve the uffd-wp bit already.  Also make sure anonymous UNSHARE
> won't be affected and can still be resolved, IOW only skip CoW not CoR.
> 
> This patch will be needed for v5.19+ hence copy stable.
> 
> Reported-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
> Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
> 
> Notes:
> 
> v2 is not on the list but in an attachment in the reply; this v3 is mostly
> to make sure it's not the same as the patch used to be attached.  Sorry
> Andrew, we need to drop the queued one as I rewrote the commit message.

My appologies!  I saw the code path missed in v2 and assumed you did not
think it applied.  So, I said nothing.  My bad!

> Muhammad, I didn't attach your T-b because of the slight functional change.
> Please feel free to re-attach if it still works for you (which I believe
> should).
> 
> thanks,
> ---
>  mm/hugetlb.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 8bfd07f4c143..a58b3739ed4b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5478,7 +5478,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>  		       struct folio *pagecache_folio, spinlock_t *ptl)
>  {
>  	const bool unshare = flags & FAULT_FLAG_UNSHARE;
> -	pte_t pte;
> +	pte_t pte = huge_ptep_get(ptep);
>  	struct hstate *h = hstate_vma(vma);
>  	struct page *old_page;
>  	struct folio *new_folio;
> @@ -5487,6 +5487,17 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>  	unsigned long haddr = address & huge_page_mask(h);
>  	struct mmu_notifier_range range;
>  
> +	/*
> +	 * Never handle CoW for uffd-wp protected pages.  It should be only
> +	 * handled when the uffd-wp protection is removed.
> +	 *
> +	 * Note that only the CoW optimization path (in hugetlb_no_page())
> +	 * can trigger this, because hugetlb_fault() will always resolve
> +	 * uffd-wp bit first.
> +	 */
> +	if (!unshare && huge_pte_uffd_wp(pte))
> +		return 0;

This looks correct.  However, since the previous version looked correct I must
ask.  Can we have unshare set and huge_pte_uffd_wp true?  If so, then it seems
we would need to possibly propogate that uffd_wp to the new pte as in v2

> +
>  	/*
>  	 * hugetlb does not support FOLL_FORCE-style write faults that keep the
>  	 * PTE mapped R/O such as maybe_mkwrite() would do.
> @@ -5500,7 +5511,6 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>  		return 0;
>  	}
>  
> -	pte = huge_ptep_get(ptep);
>  	old_page = pte_page(pte);
>  
>  	delayacct_wpcopy_start();
> -- 
> 2.39.1
> 

-- 
Mike Kravetz