[PATCH v3 0/2] userfaultfd: clear the inherited uffd bit in move_swap_pte()

[PATCH v3 0/2] userfaultfd: clear the inherited uffd bit in move_swap_pte()

[Donggeun Yoo]

UFFDIO_MOVE on a swapped-out page installs the source PTE at the
destination unchanged, so a uffd bit set on a write-protected or
RWP-protected source lands in a destination VMA that was never
registered for either, and nothing clears it afterwards.  Patch 1 clears the bit, then re-arms it if
the destination is RWP-registered, which is what the present-page and
zeropage move paths already do.  Patch 2 adds the tests that catch it.

v1: https://lore.kernel.org/all/20260919004630.1159895-1-donggeunyoo.kernel@gmail.com/
v2: https://lore.kernel.org/all/20260925042907.2330519-1-donggeunyoo.kernel@gmail.com/

Changes in v3:
 - patch 1: describe the userspace-visible effects and the backport
   (Andrew Morton, David Hildenbrand)
 - patch 2: drop the MADV_PAGEOUT retry loop (David Hildenbrand)
 - patch 2: add an RWP case (David Hildenbrand)

Changes in v2:
 - patch 1: clear the bit unconditionally (Kiryl Shutsemau)
 - patch 1: rewrite the changelog for readability (Mike Rapoport)
 - add Assisted-by: LLM (Mike Rapoport)

x86_64 defconfig plus USERFAULTFD, TRANSPARENT_HUGEPAGE,
PAGE_TABLE_CHECK_ENFORCED and a swap device, under QEMU, on
6812ce4e4379:

  uffd-unit-tests                    before   after
  move-swap-wp on anon               not ok   ok
  move-swap-rwp on anon              not ok   ok
  the other 103 unit tests           ok       ok
  uffd-wp-mremap, 38 tests           ok       ok

  11 unit tests skip on both, as CONFIG_GUP_TEST is not set.

  pagemap bit 57 at the destination  before   after
  swapped page, dst not armed        set      clear
  swapped page, dst WP-armed         set      clear
  swapped page, dst RWP-armed        set      set
  resident page, dst WP-armed        clear    clear

  MADV_COLLAPSE over 2 MB at dst     EINVAL   0
  fault on the moved page at dst     WARNING  none

Donggeun Yoo (2):
  userfaultfd: clear the inherited uffd bit in move_swap_pte()
  selftests/mm: add tests for UFFDIO_MOVE of a uffd-protected swap entry

 mm/userfaultfd.c                             |  1 +
 tools/testing/selftests/mm/uffd-unit-tests.c | 84 ++++++++++++++++++++
 2 files changed, 85 insertions(+)

-- 
2.53.0

[PATCH v3 1/2] userfaultfd: clear the inherited uffd bit in move_swap_pte()

[Donggeun Yoo]

UFFDIO_MOVE on a swapped-out page installs the source PTE at the
destination unchanged, so a uffd bit set on a write-protected or
RWP-protected source lands in a destination VMA that was never
registered for either.  Nothing clears it there, and userspace sees:

 - /proc/<pid>/pagemap reports the page as uffd-tracked (bit 57), both
   while it is swapped out and after it is faulted back in.
 - MADV_COLLAPSE fails with EINVAL on a range containing it, because the
   collapse scan bails on a swap entry with the uffd bit set.
 - With CONFIG_PAGE_TABLE_CHECK, faulting the page in warns.
   do_swap_page() carries the bit into the present PTE and, since the
   destination isn't WP-registered, also makes it writable:

   WARNING: mm/page_table_check.c:202 at __page_table_check_ptes_set+0x185/0x1e0
   Call Trace:
    set_ptes+0x67/0xc0
    do_swap_page+0x990/0xfe0
    __handle_mm_fault+0x7d0/0xeb0
    handle_mm_fault+0x9c/0x250
    do_user_addr_fault+0x207/0x650
    exc_page_fault+0x65/0x150
    asm_exc_page_fault+0x26/0x30

Reaching it takes UFFDIO_MOVE out of a WP- or RWP-protected area into
one that isn't, on a page that is swapped out at the time.  It hasn't
been seen in practice.

A resident page doesn't carry the bit, because move_present_ptes() builds
the destination PTE from dst_vma->vm_page_prot.  Clear it on the moved
swap entry as well, then re-arm it if the destination is RWP-registered.

The WP case has been there since v6.8, where UFFDIO_MOVE was added.

Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Cc: <stable@vger.kernel.org>
Assisted-by: LLM
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
---
 mm/userfaultfd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 74f04c323c50f..f39f109f17989 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1449,6 +1449,7 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
 	orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
 	if (pgtable_supports_soft_dirty())
 		orig_src_pte = pte_swp_mksoft_dirty(orig_src_pte);
+	orig_src_pte = pte_swp_clear_uffd(orig_src_pte);
 	/* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */
 	if (userfaultfd_rwp(dst_vma))
 		orig_src_pte = pte_swp_mkuffd(orig_src_pte);
-- 
2.53.0

[PATCH v3 2/2] selftests/mm: add tests for UFFDIO_MOVE of a uffd-protected swap entry

[Donggeun Yoo]

Move a swapped-out page out of a write-protected or RWP-protected area into
a destination registered for missing faults only, and read pagemap bit 57
at the destination.  The destination was never protected, so the bit must
be clear.

Assisted-by: LLM
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
---
 tools/testing/selftests/mm/uffd-unit-tests.c | 84 ++++++++++++++++++++
 1 file changed, 84 insertions(+)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index ef9b3956bdcfd..580178630eded 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -2037,6 +2037,75 @@ static void uffd_move_pmd_split_test(uffd_global_test_opts_t *gopts, uffd_test_a
 			      uffd_move_pmd_handle_fault);
 }
 
+/*
+ * Moving a swapped-out page out of a write-protected or RWP-protected area
+ * must not carry the uffd bit into a destination registered for missing
+ * faults only: such a bit is never cleared afterwards, so pagemap keeps
+ * reporting the page as uffd-tracked.
+ *
+ * Needs a swap device; skipped if MADV_PAGEOUT cannot evict the page.
+ */
+static void uffd_move_swap_test_common(uffd_global_test_opts_t *gopts,
+				       bool rwp)
+{
+	unsigned long page_size = gopts->page_size;
+	struct uffdio_move move = { };
+	int pagemap_fd;
+
+	if (rwp) {
+		if (uffd_register_rwp(gopts->uffd, gopts->area_src, page_size))
+			err("register src failure");
+	} else if (uffd_register(gopts->uffd, gopts->area_src, page_size,
+				 false, true, false)) {
+		err("register src failure");
+	}
+	if (uffd_register(gopts->uffd, gopts->area_dst, page_size,
+			  true, false, false))
+		err("register dst failure");
+
+	if (rwp)
+		rwprotect_range(gopts->uffd, (unsigned long)gopts->area_src,
+				page_size, true);
+	else
+		wp_range(gopts->uffd, (unsigned long)gopts->area_src,
+			 page_size, true);
+
+	pagemap_fd = pagemap_open();
+	if (madvise(gopts->area_src, page_size, MADV_PAGEOUT))
+		err("MADV_PAGEOUT");
+	if (!pagemap_is_swapped(pagemap_fd, gopts->area_src)) {
+		uffd_test_skip("MADV_PAGEOUT did not swap the page; is swap enabled?");
+		goto out;
+	}
+
+	move.dst = (unsigned long)gopts->area_dst;
+	move.src = (unsigned long)gopts->area_src;
+	move.len = page_size;
+	if (ioctl(gopts->uffd, UFFDIO_MOVE, &move))
+		err("UFFDIO_MOVE");
+
+	if (pagemap_get_entry(pagemap_fd, gopts->area_dst) & PM_UFFD_WP)
+		uffd_test_fail("uffd bit moved into an area registered for missing faults only");
+	else
+		uffd_test_pass();
+out:
+	close(pagemap_fd);
+	uffd_unregister(gopts->uffd, gopts->area_src, page_size);
+	uffd_unregister(gopts->uffd, gopts->area_dst, page_size);
+}
+
+static void uffd_move_swap_wp_test(uffd_global_test_opts_t *gopts,
+				   uffd_test_args_t *targs)
+{
+	uffd_move_swap_test_common(gopts, false);
+}
+
+static void uffd_move_swap_rwp_test(uffd_global_test_opts_t *gopts,
+				    uffd_test_args_t *targs)
+{
+	uffd_move_swap_test_common(gopts, true);
+}
+
 static bool
 uffdio_verify_results(const char *name, int ret, int error, long result)
 {
@@ -2359,6 +2428,21 @@ uffd_test_case_t uffd_tests[] = {
 		.uffd_feature_required = UFFD_FEATURE_MOVE,
 		.test_case_ops = &uffd_move_test_pmd_case_ops,
 	},
+	{
+		.name = "move-swap-wp",
+		.uffd_fn = uffd_move_swap_wp_test,
+		.mem_targets = MEM_ANON,
+		.uffd_feature_required = UFFD_FEATURE_MOVE |
+		UFFD_FEATURE_PAGEFAULT_FLAG_WP,
+		.test_case_ops = &uffd_move_test_case_ops,
+	},
+	{
+		.name = "move-swap-rwp",
+		.uffd_fn = uffd_move_swap_rwp_test,
+		.mem_targets = MEM_ANON,
+		.uffd_feature_required = UFFD_FEATURE_MOVE | UFFD_FEATURE_RWP,
+		.test_case_ops = &uffd_move_test_case_ops,
+	},
 	{
 		.name = "wp-fork",
 		.uffd_fn = uffd_wp_fork_test,
-- 
2.53.0