[PATCH 09/10] mm/hugetlb: don't leave a uffd-wp marker when zapping an RWP entry

["David Hildenbrand (Arm)" <david@kernel.org>]

From: Josef Bacik <josef@toxicpanda.com>

An entry that is protected with UFFDIO_RWPROTECT is PAGE_NONE with the
uffd bit set, the bit that userfaultfd write protection uses too.  When
__unmap_hugepage_range() zaps an entry with that bit it leaves a
PTE_MARKER_UFFD_WP behind, whatever the VMA is registered for.

In a VMA registered with UFFDIO_REGISTER_MODE_RWP that marker means
nothing.  The next fault has hugetlb_no_page() turn it into a present,
read-only entry with the uffd bit.  That is not an RWP entry, since it is
not PAGE_NONE, and the VMA has no VM_UFFD_WP (the two modes can not be
registered together).  An earlier fix solved some side effects, now just
stop the zap from wrongly installing a marker.

Leave a marker only if the VMA is registered for write protection, both
for present and for non-present entries.  The zap of ordinary page
tables has the same test, in cond_install_uffd_wp_ptes(), and
hugetlb_change_protection() says that "uffd_rwp does not install markers".

After a zap the range of an RWP VMA is simply empty, as it is for
anonymous memory and shmem.  Nothing changes for VMAs with VM_UFFD_WP.  In
a VMA that is not registered at all a leftover uffd bit or marker is now
dropped by the zap and no longer kept alive.

Original fix generated by an LLM and sent in private by Josef.

Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP")
Cc: stable@vger.kernel.org
Reported-by: Chris Mason <mason@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[ david: a bit of description overhaul+simplification ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/hugetlb.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1548a6156206b..fb51f7d02cebf 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5267,7 +5267,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 			 * drop the uffd-wp bit in this zap, then replace the
 			 * pte with a marker.
 			 */
-			if (pte_swp_uffd_any(pte) &&
+			if (userfaultfd_wp(vma) && pte_swp_uffd_any(pte) &&
 			    !(zap_flags & ZAP_FLAG_DROP_MARKER))
 				set_huge_pte_at(mm, address, ptep,
 						make_pte_marker(PTE_MARKER_UFFD_WP),
@@ -5303,7 +5303,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		if (huge_pte_dirty(pte))
 			folio_mark_dirty(folio);
 		/* Leave a uffd-wp pte marker if needed */
-		if (huge_pte_uffd(pte) &&
+		if (userfaultfd_wp(vma) && huge_pte_uffd(pte) &&
 		    !(zap_flags & ZAP_FLAG_DROP_MARKER))
 			set_huge_pte_at(mm, address, ptep,
 					make_pte_marker(PTE_MARKER_UFFD_WP),

-- 
2.43.0