bpf: lsm: Add selftests for BPF_PROG_TYPE_LSM

* Load/attach a BPF program that hooks to file_mprotect (int)
and bprm_committed_creds (void).
* Perform an action that triggers the hook.
* Verify if the audit event was received using the shared global
variables for the process executed.
* Verify if the mprotect returns a -EPERM.

Signed-off-by: KP Singh <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Brendan Jackman <[e-mail]>
Reviewed-by: Florent Revest <[e-mail]>
Reviewed-by: Thomas Garnier <[e-mail]>
Reviewed-by: James Morris <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/config

···
 CONFIG_MPLS_IPTUNNEL=m
 CONFIG_IPV6_SIT=m
 CONFIG_BPF_JIT=y
+CONFIG_BPF_LSM=y
+CONFIG_SECURITY=y

tools/testing/selftests/bpf/prog_tests/test_lsm.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (C) 2020 Google LLC.
+ */
+
+#include <test_progs.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <malloc.h>
+#include <stdlib.h>
+
+#include "lsm.skel.h"
+
+char *CMD_ARGS[] = {"true", NULL};
+
+int heap_mprotect(void)
+{
+	void *buf;
+	long sz;
+	int ret;
+
+	sz = sysconf(_SC_PAGESIZE);
+	if (sz < 0)
+		return sz;
+
+	buf = memalign(sz, 2 * sz);
+	if (buf == NULL)
+		return -ENOMEM;
+
+	ret = mprotect(buf, sz, PROT_READ | PROT_WRITE | PROT_EXEC);
+	free(buf);
+	return ret;
+}
+
+int exec_cmd(int *monitored_pid)
+{
+	int child_pid, child_status;
+
+	child_pid = fork();
+	if (child_pid == 0) {
+		*monitored_pid = getpid();
+		execvp(CMD_ARGS[0], CMD_ARGS);
+		return -EINVAL;
+	} else if (child_pid > 0) {
+		waitpid(child_pid, &child_status, 0);
+		return child_status;
+	}
+
+	return -EINVAL;
+}
+
+void test_test_lsm(void)
+{
+	struct lsm *skel = NULL;
+	int err, duration = 0;
+
+	skel = lsm__open_and_load();
+	if (CHECK(!skel, "skel_load", "lsm skeleton failed\n"))
+		goto close_prog;
+
+	err = lsm__attach(skel);
+	if (CHECK(err, "attach", "lsm attach failed: %d\n", err))
+		goto close_prog;
+
+	err = exec_cmd(&skel->bss->monitored_pid);
+	if (CHECK(err < 0, "exec_cmd", "err %d errno %d\n", err, errno))
+		goto close_prog;
+
+	CHECK(skel->bss->bprm_count != 1, "bprm_count", "bprm_count = %d\n",
+	      skel->bss->bprm_count);
+
+	skel->bss->monitored_pid = getpid();
+
+	err = heap_mprotect();
+	if (CHECK(errno != EPERM, "heap_mprotect", "want errno=EPERM, got %d\n",
+		  errno))
+		goto close_prog;
+
+	CHECK(skel->bss->mprotect_count != 1, "mprotect_count",
+	      "mprotect_count = %d\n", skel->bss->mprotect_count);
+
+close_prog:
+	lsm__destroy(skel);
+}

tools/testing/selftests/bpf/progs/lsm.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2020 Google LLC.
+ */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include  <errno.h>
+
+char _license[] SEC("license") = "GPL";
+
+int monitored_pid = 0;
+int mprotect_count = 0;
+int bprm_count = 0;
+
+SEC("lsm/file_mprotect")
+int BPF_PROG(test_int_hook, struct vm_area_struct *vma,
+	     unsigned long reqprot, unsigned long prot, int ret)
+{
+	if (ret != 0)
+		return ret;
+
+	__u32 pid = bpf_get_current_pid_tgid() >> 32;
+	int is_heap = 0;
+
+	is_heap = (vma->vm_start >= vma->vm_mm->start_brk &&
+		   vma->vm_end <= vma->vm_mm->brk);
+
+	if (is_heap && monitored_pid == pid) {
+		mprotect_count++;
+		ret = -EPERM;
+	}
+
+	return ret;
+}
+
+SEC("lsm/bprm_committed_creds")
+int BPF_PROG(test_void_hook, struct linux_binprm *bprm)
+{
+	__u32 pid = bpf_get_current_pid_tgid() >> 32;
+
+	if (monitored_pid == pid)
+		bprm_count++;
+
+	return 0;
+}