[PATCH bpf-next 2/2] selftests/bpf: Test bpf_security_locked_down kfunc

[Justin Suess <utilityemal77@gmail.com>]

Test the bpf_security_locked_down() kfunc. An LSM program attached to
the locked_down hook denies LOCKDOWN_HIBERNATION, so a syscall program
querying the kfunc observes both verdicts deterministically without
touching real lockdown state: 0 for LOCKDOWN_KEXEC and -EPERM for
LOCKDOWN_HIBERNATION. Out-of-range reasons must return -EINVAL.

Programs in denied calling contexts (a tracing program, and an LSM
program attached to the locked_down hook itself) must be rejected at
load time by the kfunc filter.

The selftest config guarantees the verdicts are stable: the bpf LSM is
in CONFIG_LSM and the lockdown LSM is not, so the kernel cannot already
be locked down.

Signed-off-by: Justin Suess <utilityemal77@gmail.com>
---
 .../selftests/bpf/prog_tests/lsm_kfuncs.c     | 28 +++++++++++++++
 .../testing/selftests/bpf/progs/lsm_kfuncs.c  | 34 +++++++++++++++++++
 .../selftests/bpf/progs/lsm_kfuncs_fail.c     | 26 ++++++++++++++
 3 files changed, 88 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/lsm_kfuncs.c
 create mode 100644 tools/testing/selftests/bpf/progs/lsm_kfuncs.c
 create mode 100644 tools/testing/selftests/bpf/progs/lsm_kfuncs_fail.c

diff --git a/tools/testing/selftests/bpf/prog_tests/lsm_kfuncs.c b/tools/testing/selftests/bpf/prog_tests/lsm_kfuncs.c
new file mode 100644
index 000000000000..c836851d8397
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/lsm_kfuncs.c
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "lsm_kfuncs.skel.h"
+#include "lsm_kfuncs_fail.skel.h"
+
+void test_lsm_kfuncs(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct lsm_kfuncs *skel;
+
+	RUN_TESTS(lsm_kfuncs_fail);
+
+	skel = lsm_kfuncs__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+	if (!ASSERT_OK(lsm_kfuncs__attach(skel), "attach"))
+		goto out;
+
+	if (!ASSERT_OK(bpf_prog_test_run_opts(bpf_program__fd(skel->progs.query),
+					      &opts), "test_run"))
+		goto out;
+	ASSERT_EQ(skel->data->ret_clear, 0, "not locked down");
+	ASSERT_EQ(skel->data->ret_denied, -EPERM, "locked down");
+	ASSERT_EQ(skel->data->ret_invalid_low, -EINVAL, "LOCKDOWN_NONE invalid");
+	ASSERT_EQ(skel->data->ret_invalid_high, -EINVAL, "CONFIDENTIALITY_MAX invalid");
+out:
+	lsm_kfuncs__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/lsm_kfuncs.c b/tools/testing/selftests/bpf/progs/lsm_kfuncs.c
new file mode 100644
index 000000000000..2637b9bc9025
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/lsm_kfuncs.c
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "vmlinux.h"
+#include <errno.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+extern int bpf_security_locked_down(enum lockdown_reason what) __ksym;
+
+/* Reason nothing in the test environment genuinely queries or locks. */
+#define DENY_REASON LOCKDOWN_HIBERNATION
+#define ALLOW_REASON LOCKDOWN_KEXEC
+
+int ret_clear = 1;
+int ret_denied = 1;
+int ret_invalid_low = 1;
+int ret_invalid_high = 1;
+
+SEC("lsm/locked_down")
+int BPF_PROG(lockdown_hook, enum lockdown_reason what)
+{
+	return what == DENY_REASON ? -EPERM : 0;
+}
+
+SEC("syscall")
+int query(void *ctx)
+{
+	ret_clear = bpf_security_locked_down(ALLOW_REASON);
+	ret_denied = bpf_security_locked_down(DENY_REASON);
+	ret_invalid_low = bpf_security_locked_down(LOCKDOWN_NONE);
+	ret_invalid_high = bpf_security_locked_down(LOCKDOWN_CONFIDENTIALITY_MAX);
+	return 0;
+}
diff --git a/tools/testing/selftests/bpf/progs/lsm_kfuncs_fail.c b/tools/testing/selftests/bpf/progs/lsm_kfuncs_fail.c
new file mode 100644
index 000000000000..b9861d9c4f16
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/lsm_kfuncs_fail.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+extern int bpf_security_locked_down(enum lockdown_reason what) __ksym;
+
+/* Tracing programs must be rejected by the kfunc filter. */
+SEC("fentry/bpf_fentry_test1")
+__failure __msg("calling kernel function bpf_security_locked_down is not allowed")
+int BPF_PROG(tracing_caller, int a)
+{
+	bpf_security_locked_down(LOCKDOWN_KEXEC);
+	return 0;
+}
+
+/* As must locked_down programs, which would recurse into the dispatch. */
+SEC("lsm/locked_down")
+__failure __msg("calling kernel function bpf_security_locked_down is not allowed")
+int BPF_PROG(recursive_caller, enum lockdown_reason what)
+{
+	return bpf_security_locked_down(what);
+}
-- 
2.54.0