Re: [PATCH] mm/ksm: fix userfaultfd protection during merge and unmerge

[Kiryl Shutsemau]

On Mon, Oct 26, 2026 at 09:00:54PM +0800, Lance Yang wrote:
> KSM loses userfaultfd protection when replace_page() builds a new PTE
> from vma->vm_page_prot. Dropping its uffd bit lets a subsequent write go
> through ordinary COW instead of userfaultfd write protection handling. For
> RWP, a merged page also becomes readable without faulting.
> 
> Keeping the uffd bit and restoring PAGE_NONE fixes merging, but we also
> have to handle unmerging. An existing KSM page can be protected with RWP
> before MADV_UNMERGEABLE, so unmerging already needs fixing independently
> of preserving RWP during merging.
> 
> Unsharing replaces a shared page with a private one and should leave
> userfaultfd protection in place, as do_wp_page() already does for write
> protection. Currently, an unshare fault reaches do_uffd_rwp() before it
> can proceed to do_wp_page().
> 
> - With synchronous RWP, handle_userfault() returns VM_FAULT_SIGBUS.
>   break_ksm() then reports success with a KSM page still merged.
> 
> - With asynchronous RWP, protection is cleared before unsharing.
> 
> Preserve the uffd bit when merging, and restore PAGE_NONE for RWP. Let
> FAULT_FLAG_UNSHARE skip RWP fault handling and keep PAGE_NONE on each
> private copy as well. Unsharing can then complete with RWP still armed
> for a subsequent access.
> 
> Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
> Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP")
> Link: https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@xxxxxxxxx/
> Link: https://lore.kernel.org/all/20261020055518.36087-1-lance.yang@xxxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Co-developed-by: Samarth Narang <samarth.colleges@xxxxxxxxx>
> Signed-off-by: Samarth Narang <samarth.colleges@xxxxxxxxx>
> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>

Thanks for fixing this:

Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>

> @@ -1463,9 +1464,14 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
>  		dec_mm_counter(mm, MM_ANONPAGES);
>  	}
>  
> -	/* Preserve soft-dirty when replacing the page. */
> +	/* Preserve soft-dirty and uffd protection when replacing the page. */

Small nit: the period at the end of a single-line comment looks odd to
me. Am I the only one?

If you are going to respin the patch for some reason, maybe drop it?

>  	if (pte_soft_dirty(orig_pte))
>  		newpte = pte_mksoft_dirty(newpte);
> +	if (pte_uffd(orig_pte)) {
> +		newpte = pte_mkuffd(newpte);
> +		if (userfaultfd_rwp(vma))
> +			newpte = pte_modify(newpte, PAGE_NONE);
> +	}
>  
>  	flush_cache_page(vma, addr, pte_pfn(ptep_get(ptep)));
>  	/*

-- 
  Kiryl Shutsemau / Kirill A. Shutemov