Add basic tests for the kpkeys_hardened_pgtables feature: try to
perform a direct write to current->{cred,real_cred} and ensure it
fails. Also check that prepare_creds, protect_creds,
prepare_protected_creds behave as expected.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
---
 mm/Makefile                           |  1 +
 mm/tests/kpkeys_hardened_cred_kunit.c | 79 +++++++++++++++++++++++++++
 security/Kconfig.hardening            | 11 ++++
 3 files changed, 91 insertions(+)
 create mode 100644 mm/tests/kpkeys_hardened_cred_kunit.c

diff --git a/mm/Makefile b/mm/Makefile
index b1e6cf7f753c..c79af57c0aa5 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -149,3 +149,4 @@ obj-$(CONFIG_TMPFS_QUOTA) += shmem_quota.o
 obj-$(CONFIG_PT_RECLAIM) += pt_reclaim.o
 obj-$(CONFIG_KPKEYS_HARDENED_PGTABLES) += kpkeys_hardened_pgtables.o
 obj-$(CONFIG_KPKEYS_HARDENED_PGTABLES_KUNIT_TEST) += tests/kpkeys_hardened_pgtables_kunit.o
+obj-$(CONFIG_KPKEYS_HARDENED_CRED_KUNIT_TEST) += tests/kpkeys_hardened_cred_kunit.o
diff --git a/mm/tests/kpkeys_hardened_cred_kunit.c b/mm/tests/kpkeys_hardened_cred_kunit.c
new file mode 100644
index 000000000000..ed07469b504c
--- /dev/null
+++ b/mm/tests/kpkeys_hardened_cred_kunit.c
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <kunit/test.h>
+#include <linux/cred.h>
+#include <linux/sched.h>
+
+static int increment_cred_uid_nofault(struct cred *cred)
+{
+	uid_t val = __kuid_val(cred->uid) + 1;
+
+	return copy_to_kernel_nofault(&cred->uid, &val, sizeof(cred->uid));
+}
+
+static void write_current_creds(struct kunit *test)
+{
+	int ret;
+
+	if (!arch_kpkeys_enabled())
+		kunit_skip(test, "kpkeys are not supported");
+
+	ret = increment_cred_uid_nofault((struct cred *)current->cred);
+	KUNIT_EXPECT_EQ_MSG(test, ret, -EFAULT,
+			    "Write to current->cred wasn't prevented");
+
+	ret = increment_cred_uid_nofault((struct cred *)current->real_cred);
+	KUNIT_EXPECT_EQ_MSG(test, ret, -EFAULT,
+			    "Write to current->real_cred wasn't prevented");
+}
+
+static void write_new_creds(struct kunit *test)
+{
+	struct cred *cred, *protected_cred;
+	int ret;
+
+	if (!arch_kpkeys_enabled())
+		kunit_skip(test, "kpkeys are not supported");
+
+	/* prepare_creds() + protect_creds() */
+	cred = prepare_creds();
+	KUNIT_ASSERT_NOT_NULL(test, cred);
+
+	ret = increment_cred_uid_nofault(cred);
+	KUNIT_EXPECT_EQ_MSG(test, ret, 0,
+			    "Failed to write to unprotected creds");
+
+	protected_cred = protect_creds(cred);
+	KUNIT_EXPECT_PTR_NE_MSG(test, cred, protected_cred,
+				"protect_creds() failed to move creds to protected memory");
+
+	ret = increment_cred_uid_nofault(protected_cred);
+	KUNIT_EXPECT_EQ_MSG(test, ret, -EFAULT,
+			    "Write to protected_cred wasn't prevented");
+
+	put_cred(protected_cred);
+
+	/* prepare_protected_creds() */
+	protected_cred = prepare_protected_creds();
+
+	ret = increment_cred_uid_nofault(protected_cred);
+	KUNIT_EXPECT_EQ_MSG(test, ret, -EFAULT,
+			    "Write to protected_cred wasn't prevented");
+
+	put_cred(protected_cred);
+
+}
+
+static struct kunit_case kpkeys_hardened_cred_test_cases[] = {
+	KUNIT_CASE(write_current_creds),
+	KUNIT_CASE(write_new_creds),
+	{}
+};
+
+static struct kunit_suite kpkeys_hardened_cred_test_suite = {
+	.name = "Hardened credentials using kpkeys",
+	.test_cases = kpkeys_hardened_cred_test_cases,
+};
+kunit_test_suite(kpkeys_hardened_cred_test_suite);
+
+MODULE_DESCRIPTION("Tests for the kpkeys_hardened_cred feature");
+MODULE_LICENSE("GPL");
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index cb494448c7ae..7ceb1e6846f2 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -302,6 +302,17 @@ config KPKEYS_HARDENED_CRED
 	  This option has no effect if the system does not support
 	  kernel pkeys.
 
+config KPKEYS_HARDENED_CRED_KUNIT_TEST
+	tristate "KUnit tests for kpkeys_hardened_cred" if !KUNIT_ALL_TESTS
+	depends on KPKEYS_HARDENED_CRED
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  Enable this option to check that the kpkeys_hardened_cred feature
+	  functions as intended, i.e. prevents arbitrary writes to live credentials.
+
+	  If unsure, say N.
+
 endmenu
 
 config CC_HAS_RANDSTRUCT
-- 
2.47.0