* [PATCH v3 1/2] userfaultfd: clear the inherited uffd bit in move_swap_pte()
2026-09-26 12:41 [PATCH v3 0/2] userfaultfd: clear the inherited uffd bit in move_swap_pte() Donggeun Yoo
@ 2026-09-26 12:41 ` Donggeun Yoo
2026-09-26 12:41 ` [PATCH v3 2/2] selftests/mm: add tests for UFFDIO_MOVE of a uffd-protected swap entry Donggeun Yoo
1 sibling, 0 replies; 3+ messages in thread
From: Donggeun Yoo @ 2026-09-26 12:41 UTC (permalink / raw)
To: akpm, rppt
Cc: peterx, surenb, aarcange, david, ljs, liam, vbabka, mhocko,
shuah, kirill, linux-mm, linux-kselftest, linux-kernel,
donggeunyoo.kernel, stable
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
^ permalink raw reply [flat|nested] 3+ messages in thread* [PATCH v3 2/2] selftests/mm: add tests for UFFDIO_MOVE of a uffd-protected swap entry
2026-09-26 12:41 [PATCH v3 0/2] userfaultfd: clear the inherited uffd bit in move_swap_pte() Donggeun Yoo
2026-09-26 12:41 ` [PATCH v3 1/2] " Donggeun Yoo
@ 2026-09-26 12:41 ` Donggeun Yoo
1 sibling, 0 replies; 3+ messages in thread
From: Donggeun Yoo @ 2026-09-26 12:41 UTC (permalink / raw)
To: akpm, rppt
Cc: peterx, surenb, aarcange, david, ljs, liam, vbabka, mhocko,
shuah, kirill, linux-mm, linux-kselftest, linux-kernel,
donggeunyoo.kernel
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
^ permalink raw reply [flat|nested] 3+ messages in thread