[PATCH v3 5/9] mm/mprotect: add MM_CP_PERM_UPGRADE

["Carlos López" <clopez@suse.de>]

Add a change_protection() flag for changes that only add permissions,
so that MMU notifier users can keep their mappings and fault in the new
permissions later. mprotect() sets it when the new VMA flags only add
read, write or execute permissions, or VM_ACCOUNT, which does not affect
the page tables. Any other change, such as a different protection key,
is not an upgrade.

Signed-off-by: Carlos López <clopez@suse.de>
---
 include/linux/mm.h |  2 ++
 mm/mprotect.c      | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6af3b7008f1a..001829a61405 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3585,6 +3585,8 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen);
 #define  MM_CP_UFFD_RWP_RESOLVE            (1UL << 5) /* resolve rwp */
 #define  MM_CP_UFFD_RWP_ALL                (MM_CP_UFFD_RWP | \
 					    MM_CP_UFFD_RWP_RESOLVE)
+/* Whether this change only upgrades/adds permissions */
+#define  MM_CP_PERM_UPGRADE                (1UL << 6)
 
 bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
 			     pte_t pte);
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 84bcfe0281d3..8c0fcd10f5a3 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -767,6 +767,17 @@ static const struct mm_walk_ops prot_none_walk_ops = {
 	.walk_lock		= PGWALK_WRLOCK,
 };
 
+static bool vma_flags_is_perm_upgrade(vma_flags_t old_vma_flags,
+				      vma_flags_t new_vma_flags)
+{
+	vm_flags_t old = vma_flags_to_legacy(old_vma_flags);
+	vm_flags_t new = vma_flags_to_legacy(new_vma_flags);
+	/* VM_ACCOUNT does not affect page tables */
+	vm_flags_t mask = VM_READ | VM_WRITE | VM_EXEC | VM_ACCOUNT;
+
+	return new == (old | (new & mask));
+}
+
 int
 mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
 	       struct vm_area_struct *vma, struct vm_area_struct **pprev,
@@ -847,6 +858,8 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
 	vma_overwrite_flags(vma, new_vma_flags);
 	if (vma_wants_manual_pte_write_upgrade(vma))
 		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
+	if (vma_flags_is_perm_upgrade(old_vma_flags, new_vma_flags))
+		mm_cp_flags |= MM_CP_PERM_UPGRADE;
 	vma_set_page_prot(vma);
 
 	change_protection(tlb, vma, start, end, mm_cp_flags);
-- 
2.51.0