lsm: give BPF programs a way to query locked_down state

security/Makefile

@@ -12,6 +12,7 @@ obj-$(CONFIG_MMU)			+= min_addr.o
 
 # Object file lists
 obj-$(CONFIG_SECURITY)			+= security.o lsm_notifier.o lsm_init.o
+obj-$(CONFIG_BPF_SYSCALL)		+= lsm_kfuncs.o
 obj-$(CONFIG_SECURITYFS)		+= inode.o
 obj-$(CONFIG_SECURITY_SELINUX)		+= selinux/
 obj-$(CONFIG_SECURITY_SMACK)		+= smack/

security/lsm_kfuncs.c

@@ -0,0 +1,84 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * kfuncs exposing LSM interfaces to BPF programs.
+ *
+ * Copyright (C) 2026 Justin Suess
+ */
+#include <linux/bpf.h>
+#include <linux/btf.h>
+#include <linux/btf_ids.h>
+#include <linux/init.h>
+#include <linux/security.h>
+
+__bpf_kfunc_start_defs();
+
+/**
+ * bpf_security_locked_down - Call the security_locked_down() LSM hook
+ * @what: lockdown reason to query
+ *
+ * Return: 0 if @what is not locked down, -EPERM if it is, or -EINVAL if
+ * @what is outside (LOCKDOWN_NONE, LOCKDOWN_CONFIDENTIALITY_MAX).
+ */
+__bpf_kfunc int bpf_security_locked_down(enum lockdown_reason what)
+{
+	if (what <= LOCKDOWN_NONE || what >= LOCKDOWN_CONFIDENTIALITY_MAX)
+		return -EINVAL;
+	return security_locked_down(what);
+}
+
+__bpf_kfunc_end_defs();
+
+BTF_KFUNCS_START(lsm_kfunc_ids)
+BTF_ID_FLAGS(func, bpf_security_locked_down)
+BTF_KFUNCS_END(lsm_kfunc_ids)
+
+#ifdef CONFIG_BPF_LSM
+BTF_ID_LIST_SINGLE(lsm_locked_down_hook_id, func, bpf_lsm_locked_down)
+#endif
+
+static int lsm_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id)
+{
+	/* Filters run for every kfunc resolved through the hook. */
+	if (!btf_id_set8_contains(&lsm_kfunc_ids, kfunc_id))
+		return 0;
+
+	/*
+	 * Raw prog->type: keep out the rest of the shared tracing kfunc
+	 * set (incl. perf/NMI) and extension programs.
+	 */
+	switch (prog->type) {
+	case BPF_PROG_TYPE_SYSCALL:
+		return 0;
+#ifdef CONFIG_BPF_LSM
+	case BPF_PROG_TYPE_LSM:
+		/*
+		 * A locked_down program calling this kfunc would recurse.
+		 * Match on attach_btf_id: attach_func_name is not yet set
+		 * when the filter runs from check_cfg.
+		 */
+		if (prog->aux->attach_btf_id == lsm_locked_down_hook_id[0])
+			return -EACCES;
+		return 0;
+#endif
+	default:
+		return -EACCES;
+	}
+}
+
+static const struct btf_kfunc_id_set lsm_kfunc_set = {
+	.owner  = THIS_MODULE,
+	.set    = &lsm_kfunc_ids,
+	.filter = lsm_kfunc_filter,
+};
+
+static int __init lsm_kfuncs_init(void)
+{
+	int err;
+
+	err = register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &lsm_kfunc_set);
+	err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &lsm_kfunc_set);
+	if (err)
+		pr_warn("lsm_kfuncs: kfunc registration failed: %d\n", err);
+	return err;
+}
+late_initcall(lsm_kfuncs_init);

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);
+}

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;
+}

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);
+}