[PATCH v2] userfaultfd: clear uffd state in child PTEs if uffd is released during fork

[PATCH v2] userfaultfd: clear uffd state in child PTEs if uffd is released during fork

[Mike Rapoport (Microsoft)]

With UFFD_FEATURE_EVENT_FORK the userfaultfd context is duplicated for the
child process, the child VMAs are registered with the new context, and uffd
protection state from the parent PTEs is copied into the child.

If the parent's userfaultfd is released before the fork event is read,
userfaultfd_release_new() resets the VMA flags but keeps uffd state in the
child PTEs.

This leaves the child's VMAs in an inconsistent state: they are not
registered with userfaultfd but their mappings carry uffd protection state.

If the child registers the range with userfaultfd again, the stale state
turns into protection the child never asked for.

When a write fault makes such a PTE writable without copying the page, the
uffd bit remains set and page_table_check_pte_flags() warns about it.

Split resolving of the uffd PTE state out of userfaultfd_clear_vma() and
call it from userfaultfd_release_new() before resetting the VMA.

Fixes: f369b07c8614 ("mm/uffd: reset write protection when unregister with wp-mode")
Cc: stable@vger.kernel.org
Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
Tested-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
Reviewed-by: Tal Zussman <tz2294@columbia.edu>
Tested-by: Tal Zussman <tz2294@columbia.edu>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
Changes in v2:
- make comment about what userfaultfd_clear_ptes() does more elaborate
- Link to v1: https://patch.msgid.link/20261031-uffd-prot-bits-fork-event-v1-1-3572921c2111@kernel.org
---
 mm/userfaultfd.c | 47 ++++++++++++++++++++++++++++++-----------------
 1 file changed, 30 insertions(+), 17 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 3b71712edab4c..087b66260d04c 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2220,6 +2220,32 @@ static void userfaultfd_reset_ctx(struct vm_area_struct *vma)
 	userfaultfd_set_ctx(vma, NULL, 0);
 }
 
+/*
+ * Clear uffd state from a WP or RWP range: the uffd bit, WP markers and
+ * RWP PAGE_NONE entries. Call this before dropping the registration, as it
+ * needs the VMA flags to know what to resolve.
+ */
+static void userfaultfd_clear_ptes(struct vm_area_struct *vma,
+		unsigned long start, unsigned long end)
+{
+	unsigned int mm_cp_flags = 0;
+	struct mmu_gather tlb;
+
+	if (!userfaultfd_protected(vma))
+		return;
+
+	if (userfaultfd_wp(vma))
+		mm_cp_flags |= MM_CP_UFFD_WP_RESOLVE;
+	if (userfaultfd_rwp(vma))
+		mm_cp_flags |= MM_CP_UFFD_RWP_RESOLVE;
+	if (vma_wants_manual_pte_write_upgrade(vma))
+		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
+
+	tlb_gather_mmu(&tlb, vma->vm_mm);
+	change_protection(&tlb, vma, start, end, mm_cp_flags);
+	tlb_finish_mmu(&tlb);
+}
+
 static struct vm_area_struct *userfaultfd_clear_vma(struct vma_iterator *vmi,
 					     struct vm_area_struct *prev,
 					     struct vm_area_struct *vma,
@@ -2239,22 +2265,7 @@ static struct vm_area_struct *userfaultfd_clear_vma(struct vma_iterator *vmi,
 	if (start == vma->vm_start && end == vma->vm_end)
 		give_up_on_oom = true;
 
-	/* Clear the uffd bit and/or restore protnone PTEs */
-	if (userfaultfd_protected(vma)) {
-		unsigned int mm_cp_flags = 0;
-		struct mmu_gather tlb;
-
-		if (userfaultfd_wp(vma))
-			mm_cp_flags |= MM_CP_UFFD_WP_RESOLVE;
-		if (userfaultfd_rwp(vma))
-			mm_cp_flags |= MM_CP_UFFD_RWP_RESOLVE;
-		if (vma_wants_manual_pte_write_upgrade(vma))
-			mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
-
-		tlb_gather_mmu(&tlb, vma->vm_mm);
-		change_protection(&tlb, vma, start, end, mm_cp_flags);
-		tlb_finish_mmu(&tlb);
-	}
+	userfaultfd_clear_ptes(vma, start, end);
 
 	ret = vma_modify_flags_uffd(vmi, prev, vma, start, end,
 				    &new_vma_flags, NULL_VM_UFFD_CTX,
@@ -2353,8 +2364,10 @@ static void userfaultfd_release_new(struct userfaultfd_ctx *ctx)
 	/* the various vma->vm_userfaultfd_ctx still points to it */
 	mmap_write_lock(mm);
 	for_each_vma(vmi, vma) {
-		if (vma->vm_userfaultfd_ctx.ctx == ctx)
+		if (vma->vm_userfaultfd_ctx.ctx == ctx) {
+			userfaultfd_clear_ptes(vma, vma->vm_start, vma->vm_end);
 			userfaultfd_reset_ctx(vma);
+		}
 	}
 	mmap_write_unlock(mm);
 }

---
base-commit: b61fa14ce6bcbba2cb9a96feea31b760226d6b43
change-id: 20261031-uffd-prot-bits-fork-event-de821d651ab6

--
Sincerely yours,
Mike.