Re: [PATCH] mm/userfaultfd: Allow hugetlb change protection upon poison entry

[David Hildenbrand <david@redhat.com>]

On 06.04.24 01:19, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> After UFFDIO_POISON, there can be two kinds of hugetlb pte markers, either
> the POISON one or UFFD_WP one.
> 
> Allow change protection to run on a poisoned marker just like !hugetlb
> cases, ignoring the marker irrelevant of the permission.
> 
> Here the two bits are mutual exclusive. For example, when install a
> poisoned entry it must not be UFFD_WP already (by checking pte_none()
> before such install).  And it also means if UFFD_WP is set there must have
> no POISON bit set.  It makes sense because UFFD_WP is a bit to reflect
> permission, and permissions do not apply if the pte is poisoned and
> destined to sigbus.
> 
> So here we simply check uffd_wp bit set first, do nothing otherwise.
> 
> Attach the Fixes to UFFDIO_POISON work, as before that it should not be
> possible to have poison entry for hugetlb (e.g., hugetlb doesn't do swap,
> so no chance of swapin errors).
> 
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: linux-stable <stable@vger.kernel.org> # 6.6+
> Link: https://lore.kernel.org/r/000000000000920d5e0615602dd1@google.com
> Reported-by: syzbot+b07c8ac8eee3d4d8440f@syzkaller.appspotmail.com
> Fixes: fc71884a5f59 ("mm: userfaultfd: add new UFFDIO_POISON ioctl")
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   mm/hugetlb.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 8267e221ca5d..ba7162441adf 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6960,9 +6960,13 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
>   			if (!pte_same(pte, newpte))
>   				set_huge_pte_at(mm, address, ptep, newpte, psize);
>   		} else if (unlikely(is_pte_marker(pte))) {
> -			/* No other markers apply for now. */
> -			WARN_ON_ONCE(!pte_marker_uffd_wp(pte));
> -			if (uffd_wp_resolve)
> +			/*
> +			 * Do nothing on a poison marker; page is
> +			 * corrupted, permissons do not apply.  Here
> +			 * pte_marker_uffd_wp()==true implies !poison
> +			 * because they're mutual exclusive.
> +			 */
> +			if (pte_marker_uffd_wp(pte) && uffd_wp_resolve)
>   				/* Safe to modify directly (non-present->none). */
>   				huge_pte_clear(mm, address, ptep, psize);
>   		} else if (!huge_pte_none(pte)) {

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb