[PATCH 0/2] userfaultfd: validate the whole range before changing write protection

[PATCH 0/2] userfaultfd: validate the whole range before changing write protection

[Umang Pokhriyal]

A UFFDIO_WRITEPROTECT range that spans an unregistered VMA fails, but
the VMAs before it are already (un)protected, and on unprotect no waiter
is woken.  Patch 1 validates the whole range first, as
mrwprotect_range() does; it is open-coded rather than shared with it so
that it applies to stable kernels without RWP.  Patch 2 adds a uffd
selftest.

The problem was found while reviewing the userfaultfd range ioctls with
an LLM, which also helped write the patches.  Tested on mm-new in a VM:
the new selftest and a standalone reproducer (both directions, and a
misaligned hugetlb range) fail without patch 1 and pass with it, and the
rest of uffd-unit-tests gives the same results with and without it.

Umang Pokhriyal (2):
  userfaultfd: validate the whole range before changing write protection
  selftests/mm: test a uffd write-protect range with an unregistered
    page

 mm/userfaultfd.c                             | 20 +++++--
 tools/testing/selftests/mm/uffd-unit-tests.c | 56 ++++++++++++++++++++
 2 files changed, 71 insertions(+), 5 deletions(-)


base-commit: d42dfde37af766d1c272467352c0a56d3169ee58
-- 
2.53.0

[PATCH 1/2] userfaultfd: validate the whole range before changing write protection

[Umang Pokhriyal]

mwriteprotect_range() checks each VMA as it walks the range.  When a later
VMA is not registered for write protection, or is a hugetlb VMA the range
is not aligned to, the call fails after the earlier VMAs were already
changed.  When unprotecting, userfaultfd_writeprotect() then skips
wake_userfault(), so threads blocked on those pages are not woken although
the pages are writable again.

An unprivileged process can trigger this with a range that spans an
unregistered VMA.  It hasn't been seen in practice; it was found while
reviewing the userfaultfd range ioctls with an LLM.

Validate the whole range first, as mrwprotect_range() already does.

Fixes: a1b92a3f1498 ("mm/userfaultfd: support WP on multiple VMAs")
Cc: stable@vger.kernel.org
Signed-off-by: Umang Pokhriyal <umangpokhriyall@gmail.com>
---
 mm/userfaultfd.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 87da92526..b8a9d9574 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1105,21 +1105,31 @@ static int mwriteprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
 	if (atomic_read(&ctx->mmap_changing))
 		goto out_unlock;
 
+	/*
+	 * Validate every spanned VMA before changing any of them, so an
+	 * invalid range is rejected without changing part of it.
+	 */
 	err = -ENOENT;
 	for_each_vma_range(vmi, dst_vma, end) {
-
 		if (!userfaultfd_wp(dst_vma)) {
 			err = -ENOENT;
-			break;
+			goto out_unlock;
 		}
 
 		if (vma_is_hugetlb(dst_vma)) {
-			err = -EINVAL;
 			page_mask = vma_kernel_pagesize(dst_vma) - 1;
-			if ((start & page_mask) || (len & page_mask))
-				break;
+			if ((start & page_mask) || (len & page_mask)) {
+				err = -EINVAL;
+				goto out_unlock;
+			}
 		}
+		err = 0;
+	}
+	if (err)
+		goto out_unlock;
 
+	vma_iter_set(&vmi, start);
+	for_each_vma_range(vmi, dst_vma, end) {
 		_start = max(dst_vma->vm_start, start);
 		_end = min(dst_vma->vm_end, end);
 
-- 
2.53.0

[PATCH 2/2] selftests/mm: test a uffd write-protect range with an unregistered page

[Umang Pokhriyal]

Register only the first of two pages and check that UFFDIO_WRITEPROTECT
over both pages fails with ENOENT without changing the first page, when
protecting and when unprotecting.

Signed-off-by: Umang Pokhriyal <umangpokhriyall@gmail.com>
---
 tools/testing/selftests/mm/uffd-unit-tests.c | 56 ++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index 298609544..57e442fd2 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -375,6 +375,56 @@ static void uffd_wp_unpopulated_test(uffd_global_test_opts_t *gopts, uffd_test_a
 	uffd_test_pass();
 }
 
+/*
+ * UFFDIO_WRITEPROTECT over a range whose second page is not registered must
+ * fail without changing the first page, in both directions.
+ */
+static void uffd_wp_partial_range_test(uffd_global_test_opts_t *gopts,
+				       uffd_test_args_t *args)
+{
+	struct uffdio_writeprotect prms = {
+		.range.start = (uint64_t)gopts->area_dst,
+		.range.len = 2 * gopts->page_size,
+	};
+	uint64_t value;
+	int pagemap_fd;
+
+	/* Only the first page is registered; the second is a separate VMA */
+	if (uffd_register(gopts->uffd, gopts->area_dst, gopts->page_size,
+			  false, true, false))
+		err("register failed");
+
+	pagemap_fd = pagemap_open();
+	*gopts->area_dst = 1;
+
+	prms.mode = UFFDIO_WRITEPROTECT_MODE_WP;
+	if (!ioctl(gopts->uffd, UFFDIO_WRITEPROTECT, &prms) || errno != ENOENT) {
+		uffd_test_fail("protect over an unregistered page did not fail with ENOENT");
+		goto out;
+	}
+	value = pagemap_get_entry(pagemap_fd, gopts->area_dst);
+	if (value & PM_UFFD_WP) {
+		uffd_test_fail("failed protect changed the first page");
+		goto out;
+	}
+
+	wp_range(gopts->uffd, (uint64_t)gopts->area_dst, gopts->page_size, true);
+	prms.mode = 0;
+	if (!ioctl(gopts->uffd, UFFDIO_WRITEPROTECT, &prms) || errno != ENOENT) {
+		uffd_test_fail("unprotect over an unregistered page did not fail with ENOENT");
+		goto out;
+	}
+	value = pagemap_get_entry(pagemap_fd, gopts->area_dst);
+	if (!(value & PM_UFFD_WP)) {
+		uffd_test_fail("failed unprotect changed the first page");
+		goto out;
+	}
+
+	uffd_test_pass();
+out:
+	close(pagemap_fd);
+}
+
 static void uffd_wp_fork_test_common(uffd_global_test_opts_t *gopts, uffd_test_args_t *args,
 				     bool with_event)
 {
@@ -2403,6 +2453,12 @@ uffd_test_case_t uffd_tests[] = {
 		.uffd_feature_required =
 		UFFD_FEATURE_PAGEFAULT_FLAG_WP | UFFD_FEATURE_WP_UNPOPULATED,
 	},
+	{
+		.name = "wp-partial-range",
+		.uffd_fn = uffd_wp_partial_range_test,
+		.mem_targets = MEM_ANON,
+		.uffd_feature_required = UFFD_FEATURE_PAGEFAULT_FLAG_WP,
+	},
 	{
 		.name = "minor",
 		.uffd_fn = uffd_minor_test,
-- 
2.53.0