[PATCH] mm/madvise: disallow guard region installation in uffd-armed VMAs

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

In order to establish guard regions, guard markers are installed in the
page tables of a mapped VMA.

However, problems arise when one tries to combine this with registering a
VMA for userfaultfd, as such operations may themselves need to install page
table markers such as uffd-wp.

When this installation is attempted the guard marker prevents these markers
being applied, which can lead to unexpected results (especially if the
guard regions are later removed.)

This is especially problematic because guard region installation causes the
regions to be zapped first, meaning that the removal of userfaultfd-tracked
ranges will not be logged in this case.

Therefore, disallow this.

This makes sense as conceptually, guard regions mark a part of a VMA as
effectively unmapped at the page table level, and removing the markers is
like mapping the VMA again, over which operation uffd markers would in any
case be lost.

Only check for this on installation, as a VMA can be registered for
userfaultfd after guard regions are established.

In this case, the guard regions cannot have new markers installed, and no
faults can occur in the guard regions, until they are removed, at which
point everything should function normally.

A survey of open source users indicates nobody is relying on installing
guard regions in uffd-armed VMAs. Software including glibc, android,
tcmalloc and CRIU were checked.

Fix the selftest to reflect the change.

Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
Reported-by: Tal Zussman <tz2294@columbia.edu>
Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-9-00713d368976@columbia.edu/
Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-10-00713d368976@columbia.edu/
Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-11-00713d368976@columbia.edu/
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/madvise.c                               |  4 ++++
 tools/testing/selftests/mm/guard-regions.c | 12 ++++++++----
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 0214efd2a014..268731e90f12 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1156,6 +1156,10 @@ static long madvise_guard_install(struct madvise_behavior *madv_behavior)
 	if (!is_valid_guard_vma(vma, /* allow_locked = */false))
 		return -EINVAL;
 
+	/* Guard markers are incompatible with UFFD markers. */
+	if (userfaultfd_armed(vma))
+		return -EINVAL;
+
 	/*
 	 * Set atomically under read lock. All pertinent readers will need to
 	 * acquire an mmap/VMA write lock to read it. All remaining readers may
diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
index 02f69244f9a4..97da54dcf3a4 100644
--- a/tools/testing/selftests/mm/guard-regions.c
+++ b/tools/testing/selftests/mm/guard-regions.c
@@ -1476,7 +1476,7 @@ TEST_F(guard_regions, cold_pageout)
 	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
 }
 
-/* Ensure that guard pages do not break userfaultd. */
+/* Ensure that guard regions cannot be installed in uffd-armed VMAs. */
 TEST_F(guard_regions, uffd)
 {
 	const unsigned long page_size = self->page_size;
@@ -1526,10 +1526,15 @@ TEST_F(guard_regions, uffd)
 	reg.mode = UFFDIO_REGISTER_MODE_MISSING;
 	ASSERT_EQ(ioctl(uffd, UFFDIO_REGISTER, &reg), 0);
 
-	/* Guard the range. This should not trigger the uffd. */
+	/* Now try to guard, should fail with EINVAL. */
+	ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), -1);
+	ASSERT_EQ(errno, EINVAL);
+
+	/* Once unregistered, we should be able to guard the range. */
+	ASSERT_EQ(ioctl(uffd, UFFDIO_UNREGISTER, &range), 0);
 	ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), 0);
 
-	/* The guarding should behave as usual with no uffd intervention. */
+	/* The guarding should behave as usual. */
 	for (i = 0; i < 10; i++) {
 		char *curr = &ptr[i * page_size];
 
@@ -1537,7 +1542,6 @@ TEST_F(guard_regions, uffd)
 	}
 
 	/* Cleanup. */
-	ASSERT_EQ(ioctl(uffd, UFFDIO_UNREGISTER, &range), 0);
 	close(uffd);
 	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
 }

---
base-commit: 4318887a3efa4c53d6f083ccfd7105b809a3ac65
change-id: 20261112-disallow-uffd-guard-install-a5294129ad7b

Cheers,
-- 
Lorenzo Stoakes (ARM) <ljs@kernel.org>