[RFC PATCH 16/16] mm: Add basic tests for kpkeys_hardened_pgtables

Kevin Brodsky
2024-12-06T10:14:04Z

Add basic tests for the kpkeys_hardened_pgtables feature: try to
perform a direct write to some kernel and user page table entry and
ensure it fails.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
mm/Makefile | 1 +
mm/kpkeys_hardened_pgtables_test.c | 71 ++++++++++++++++++++++++++++++
security/Kconfig.hardening | 12 +++++
3 files changed, 84 insertions(+)
create mode 100644 mm/kpkeys_hardened_pgtables_test.c

diff --git a/mm/Makefile b/mm/Makefile
index ffe799c1c897..49ac16ae6875 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -147,3 +147,4 @@ obj-$(CONFIG_SHRINKER_DEBUG) += shrinker_debug.o
obj-$(CONFIG_EXECMEM) += execmem.o
obj-$(CONFIG_TMPFS_QUOTA) += shmem_quota.o
obj-$(CONFIG_KPKEYS_HARDENED_PGTABLES) += kpkeys_hardened_pgtables.o
+obj-$(CONFIG_KPKEYS_HARDENED_PGTABLES_TEST) += kpkeys_hardened_pgtables_test.o
diff --git a/mm/kpkeys_hardened_pgtables_test.c b/mm/kpkeys_hardened_pgtables_test.c
new file mode 100644
index 000000000000..37b6ffaa55e6
--- /dev/null
+++ b/mm/kpkeys_hardened_pgtables_test.c
@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <kunit/test.h>
+#include <linux/pgtable.h>
+#include <linux/mman.h>
+
+static void write_kernel_pte(struct kunit *test)
+{
+ pte_t *ptep;
+ pte_t pte;
+ int ret;
+
+ /*
+ * The choice of address is mostly arbitrary - we just need a page
+ * that is definitely mapped, such as the current function.
+ */
+ ptep = virt_to_kpte((unsigned long)&write_kernel_pte);
+ KUNIT_ASSERT_NOT_NULL_MSG(test, ptep, "Failed to get PTE");
+
+ pte = ptep_get(ptep);
+ pte = set_pte_bit(pte, __pgprot(PTE_WRITE));
+ ret = copy_to_kernel_nofault(ptep, &pte, sizeof(pte));
+ KUNIT_EXPECT_EQ_MSG(test, ret, -EFAULT,
+ "Direct PTE write wasn't prevented");
+}
+
+static void write_user_pmd(struct kunit *test)
+{
+ pmd_t *pmdp;
+ pmd_t pmd;
+ unsigned long uaddr;
+ int ret;
+
+ uaddr = kunit_vm_mmap(test, NULL, 0, PAGE_SIZE, PROT_READ,
+ MAP_ANONYMOUS | MAP_PRIVATE | MAP_POPULATE, 0);
+ KUNIT_ASSERT_NE_MSG(test, uaddr, 0, "Could not create userspace mm");
+
+ /* We passed MAP_POPULATE so a PMD should already be allocated */
+ pmdp = pmd_off(current->mm, uaddr);
+ KUNIT_ASSERT_NOT_NULL_MSG(test, pmdp, "Failed to get PMD");
+
+ pmd = pmdp_get(pmdp);
+ pmd = set_pmd_bit(pmd, __pgprot(PROT_SECT_NORMAL));
+ ret = copy_to_kernel_nofault(pmdp, &pmd, sizeof(pmd));
+ KUNIT_EXPECT_EQ_MSG(test, ret, -EFAULT,
+ "Direct PMD write wasn't prevented");
+}
+
+static int kpkeys_hardened_pgtables_suite_init(struct kunit_suite *suite)
+{
+ if (!arch_kpkeys_enabled()) {
+ pr_err("Cannot run kpkeys_hardened_pgtables tests: kpkeys are not supported\n");
+ return 1;
+ }
+
+ return 0;
+}
+
+static struct kunit_case kpkeys_hardened_pgtables_test_cases[] = {
+ KUNIT_CASE(write_kernel_pte),
+ KUNIT_CASE(write_user_pmd),
+ {}
+};
+
+static struct kunit_suite kpkeys_hardened_pgtables_test_suite = {
+ .name = "Hardened pgtables using kpkeys",
+ .test_cases = kpkeys_hardened_pgtables_test_cases,
+ .suite_init = kpkeys_hardened_pgtables_suite_init,
+};
+kunit_test_suite(kpkeys_hardened_pgtables_test_suite);
+
+MODULE_LICENSE("GPL");
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 95f93f1d4055..8bc5d7235f6d 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -312,6 +312,18 @@ config KPKEYS_HARDENED_PGTABLES
This option has no effect if the system does not support
kernel pkeys.

+config KPKEYS_HARDENED_PGTABLES_TEST
+ tristate "KUnit tests for kpkeys_hardened_pgtables" if !KUNIT_ALL_TESTS
+ depends on KPKEYS_HARDENED_PGTABLES
+ depends on KUNIT
+ default KUNIT_ALL_TESTS
+ help
+ Enable this option to check that the kpkeys_hardened_pgtables feature
+ functions as intended, i.e. prevents arbitrary writes to user and
+ kernel page tables.
+
+ If unsure, say N.
+
endmenu

config CC_HAS_RANDSTRUCT
--
2.47.0