[PATCH v4 31/38] mm/uffd: use predicates for userfaultfd checks

[Lorenzo Stoakes (ARM)]

Rather than directly checking VMA flags, use the newly introduced
vma_is_mm_managed() and vma_is_mm_backed() helpers in userfaultfd
when assessing VMA suitability for userfaultfd and UFFDIO_MOVE.

Update vma_move_compatible() so it's expressed in terms of VMA
characteristics rather than arbitrary flags.

Additionally, update the use of the deprecated VMA flag API when checking
VMA_SHADOW_STACK_BIT.

A VMA_IO_BIT check is no longer required but that is fine as a hard
invariant has been established that mm-managed mappings may not set
VMA_IO_BIT so the check is now redundant.

Acked-by: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 mm/userfaultfd.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 949017e60608..f33b04700838 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1754,10 +1754,16 @@ static inline bool move_splits_huge_pmd(unsigned long dst_addr,
 }
 #endif
 
-static inline bool vma_move_compatible(struct vm_area_struct *vma)
+static inline bool vma_move_compatible(const struct vm_area_struct *vma)
 {
-	return !(vma->vm_flags & (VM_PFNMAP | VM_IO |  VM_HUGETLB |
-				  VM_MIXEDMAP | VM_SHADOW_STACK));
+	/* uffd is generally incompatible with mappings not managed by core mm. */
+	if (!vma_is_mm_managed(vma))
+		return false;
+	/* The shadow stack should not be written to by userspace. */
+	if (vma_test_single_mask(vma, VMA_SHADOW_STACK))
+		return false;
+	/* hugetlb mappings cannot be safely moved. */
+	return !vma_is_hugetlb(vma);
 }
 
 static int validate_move_areas(struct userfaultfd_ctx *ctx,
@@ -2146,10 +2152,11 @@ static bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
 {
 	const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
 
-	if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK))
+	/* Only mm-backed memory can have its faults handled by userspace. */
+	if (!vma_is_mm_backed(vma))
 		return false;
-
-	if (!vma_is_hugetlb(vma) && (vma->vm_flags & VM_SPECIAL))
+	/* The shadow stack should not be written to by userspace. */
+	if (vma_test_single_mask(vma, VMA_SHADOW_STACK))
 		return false;
 
 	vm_flags &= __VM_UFFD_FLAGS;

-- 
2.55.0