[PATCH v1 1/2] mm/userfaultfd: rely on vma->vm_page_prot in uffd_wp_range()

[David Hildenbrand]

uffd_wp_range() currently calculates page protection manually using
vm_get_page_prot(). This will ignore any other reason for active
writenotify: one mechanism applicable to shmem is softdirty tracking.

For example, the following sequence

1) Write to mapped shmem page
2) Clear softdirty
3) Register uffd-wp covering the mapped page
4) Unregister uffd-wp covering the mapped page
5) Write to page again

will not set the modified page softdirty, because uffd_wp_range() will
ignore that writenotify is required for softdirty tracking and simply map
the page writable again using change_protection(). Similarly, instead of
unregistering, protecting followed by un-protecting the page using
uffd-wp would result in the same situation.

Now that we enable writenotify whenever enabling uffd-wp on a VMA,
vma->vm_page_prot will already properly reflect our requirements: the
default is to write-protect all PTEs. However, for shared mappings we
would now not remap the PTEs writable if possible when unprotecting, just
like for private mappings (COW). To compensate, set
MM_CP_TRY_CHANGE_WRITABLE just like mprotect() does to try mapping
individual PTEs writable.

For private mappings, this change implies that we will now always try
setting PTEs writable when un-protecting, just like when upgrading write
permissions using mprotect(), which is an improvement.

For shared mappings, we will only set PTEs writable if
can_change_pte_writable()/can_change_pmd_writable() indicates that it's
ok. For ordinary shmem, this will be the case when PTEs are dirty, which
should usually be the case -- otherwise we could special-case shmem in
can_change_pte_writable()/can_change_pmd_writable() easily, because
shmem itself doesn't require writenotify.

Note that hugetlb does not yet implement MM_CP_TRY_CHANGE_WRITABLE, so we
won't try setting PTEs writable when unprotecting or when unregistering
uffd-wp. This can be added later on top by implementing
MM_CP_TRY_CHANGE_WRITABLE.

While commit ffd05793963a ("userfaultfd: wp: support write protection for
userfault vma range") introduced that code, it should only be applicable
to uffd-wp on shared mappings -- shmem (hugetlb does not support softdirty
tracking). I don't think this corner cases justifies to cc stable. Let's
just handle it correctly and prepare for change_protection() cleanups.

Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 mm/userfaultfd.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 0499907b6f1a..351e8d6b398b 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -727,17 +727,25 @@ ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start,
 void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma,
 		   unsigned long start, unsigned long len, bool enable_wp)
 {
+	unsigned int mm_cp_flags;
 	struct mmu_gather tlb;
-	pgprot_t newprot;
 
 	if (enable_wp)
-		newprot = vm_get_page_prot(dst_vma->vm_flags & ~(VM_WRITE));
+		mm_cp_flags = MM_CP_UFFD_WP;
 	else
-		newprot = vm_get_page_prot(dst_vma->vm_flags);
+		mm_cp_flags = MM_CP_UFFD_WP_RESOLVE;
 
+	/*
+	 * vma->vm_page_prot already reflects that uffd-wp is enabled for this
+	 * VMA (see userfaultfd_set_vm_flags()) and that all PTEs are supposed
+	 * to be write-protected as default whenever protection changes.
+	 * Try upgrading write permissions manually.
+	 */
+	if (vma_wants_manual_pte_write_upgrade(dst_vma))
+		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
 	tlb_gather_mmu(&tlb, dst_mm);
-	change_protection(&tlb, dst_vma, start, start + len, newprot,
-			  enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE);
+	change_protection(&tlb, dst_vma, start, start + len, vma->vm_page_prot,
+			  mm_cp_flags);
 	tlb_finish_mmu(&tlb);
 }
 
-- 
2.38.1