Re: [PATCH v5 14/18] mm/pagemap: add PAGE_IS_ACCESSED for RWP tracking

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Bypass of explicit PM_SCAN_CHECK_WPASYNC abort request for RWP VMAs.
--

commit ad3030249d2003121c666af3030236ae064a1680
Author: Kiryl Shutsemau (Meta) <kas@kernel.org>

mm/pagemap: add PAGE_IS_ACCESSED for RWP tracking

This commit introduces PAGE_IS_ACCESSED to the PAGEMAP_SCAN UAPI for
differentiating read-write protection (RWP) from write protection (WP) in
userfaultfd. It narrows PAGE_IS_WRITTEN to VM_UFFD_WP VMAs and restricts
the new flag to VM_UFFD_RWP VMAs, providing distinct semantics.

> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 974c5f4aa5331..0db29c3a8639f 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c

[ ... ]

> @@ -2677,6 +2702,18 @@ static int pagemap_scan_test_walk(unsigned long start, unsigned long end,
>  	bool wp_allowed = userfaultfd_wp_async(vma) &&
>  	    userfaultfd_wp_use_markers(vma);
>  
> +	/*
> +	 * PM_SCAN_WP_MATCHING is the atomic read-and-reset flavour of the
> +	 * scan and is implemented for the WP marker only. Silently skip
> +	 * VM_UFFD_RWP VMAs, matching the convention used below for VMAs
> +	 * that lack the WP-async capability. Returning -EINVAL here would
> +	 * abort the walk after preceding VMAs had already been mutated,
> +	 * destroying the atomic read-and-reset guarantee. Re-arming RWP
> +	 * is done with UFFDIO_RWPROTECT(MODE_RWP).
> +	 */
> +	if (userfaultfd_rwp(vma) && (p->arg.flags & PM_SCAN_WP_MATCHING))
> +		return 1;
> +

[Severity: Medium]
Does this early return bypass the user's explicit request to abort via
PM_SCAN_CHECK_WPASYNC?

If a VMA is registered for RWP, it lacks WP-async capability, meaning
wp_allowed is false. By returning 1 here, we skip the VMA instead of reaching
the PM_SCAN_CHECK_WPASYNC check immediately below, which would normally return
-EPERM to honor the user's UAPI request to abort the scan.

>  	if (!wp_allowed) {
>  		/* User requested explicit failure over wp-async capability */
>  		if (p->arg.flags & PM_SCAN_CHECK_WPASYNC)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260526130509.2748441-1-kirill@shutemov.name?part=14