selftests/bpf: Check that bpf_kernel_read_file() denies reading IMA policy

Check that bpf_kernel_read_file() denies the reading of an IMA policy, by
ensuring that ima_setup.sh exits with an error.
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220302111404.193900-10-roberto.sassu@huawei.com

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

@@ -59,6 +59,7 @@ static void test_init(struct ima__bss *bss)
 	bss->use_ima_file_hash = false;
 	bss->enable_bprm_creds_for_exec = false;
 	bss->enable_kernel_read_file = false;
+	bss->test_deny = false;
 }
 
 void test_test_ima(void)
@@ -200,6 +201,22 @@ void test_test_ima(void)
 	ASSERT_NEQ(ima_hash_from_bpf[0], 0, "ima_hash");
 	ASSERT_NEQ(ima_hash_from_bpf[1], 0, "ima_hash");
 
+	/*
+	 * Test #6
+	 * - Goal: ensure that the kernel_read_file hook denies an operation
+	 * - Expected result: 0 samples
+	 */
+	test_init(skel->bss);
+	skel->bss->enable_kernel_read_file = true;
+	skel->bss->test_deny = true;
+	err = _run_measured_process(measured_dir, &skel->bss->monitored_pid,
+				    "load-policy");
+	if (CHECK(!err, "run_measured_process #6", "err = %d\n", err))
+		goto close_clean;
+
+	err = ring_buffer__consume(ringbuf);
+	ASSERT_EQ(err, 0, "num_samples_or_err");
+
 close_clean:
 	snprintf(cmd, sizeof(cmd), "./ima_setup.sh cleanup %s", measured_dir);
 	err = system(cmd);

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

@@ -21,6 +21,7 @@ char _license[] SEC("license") = "GPL";
 bool use_ima_file_hash;
 bool enable_bprm_creds_for_exec;
 bool enable_kernel_read_file;
+bool test_deny;
 
 static void ima_test_common(struct file *file)
 {
@@ -51,6 +52,17 @@ static void ima_test_common(struct file *file)
 	return;
 }
 
+static int ima_test_deny(void)
+{
+	u32 pid;
+
+	pid = bpf_get_current_pid_tgid() >> 32;
+	if (pid == monitored_pid && test_deny)
+		return -EPERM;
+
+	return 0;
+}
+
 SEC("lsm.s/bprm_committed_creds")
 void BPF_PROG(bprm_committed_creds, struct linux_binprm *bprm)
 {
@@ -71,6 +83,8 @@ SEC("lsm.s/kernel_read_file")
 int BPF_PROG(kernel_read_file, struct file *file, enum kernel_read_file_id id,
 	     bool contents)
 {
+	int ret;
+
 	if (!enable_kernel_read_file)
 		return 0;
 
@@ -80,6 +94,10 @@ int BPF_PROG(kernel_read_file, struct file *file, enum kernel_read_file_id id,
 	if (id != READING_POLICY)
 		return 0;
 
+	ret = ima_test_deny();
+	if (ret < 0)
+		return ret;
+
 	ima_test_common(file);
 	return 0;
 }