selftests/bpf: Add test for bpf_lsm_kernel_read_file()

Test the ability of bpf_lsm_kernel_read_file() to call the sleepable
functions bpf_ima_inode_hash() or bpf_ima_file_hash() to obtain a
measurement of a loaded IMA policy.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220302111404.193900-9-roberto.sassu@huawei.com

tools/testing/selftests/bpf/ima_setup.sh

@@ -12,7 +12,7 @@ LOG_FILE="$(mktemp /tmp/ima_setup.XXXX.log)"
 
 usage()
 {
-	echo "Usage: $0 <setup|cleanup|run|modify-bin|restore-bin> <existing_tmp_dir>"
+	echo "Usage: $0 <setup|cleanup|run|modify-bin|restore-bin|load-policy> <existing_tmp_dir>"
 	exit 1
 }
 
@@ -51,6 +51,7 @@ setup()
 
 	ensure_mount_securityfs
 	echo "measure func=BPRM_CHECK fsuuid=${mount_uuid}" > ${IMA_POLICY_FILE}
+	echo "measure func=BPRM_CHECK fsuuid=${mount_uuid}" > ${mount_dir}/policy_test
 }
 
 cleanup() {
@@ -95,6 +96,14 @@ restore_bin()
 	truncate -s -4 "${copied_bin_path}"
 }
 
+load_policy()
+{
+	local tmp_dir="$1"
+	local mount_dir="${tmp_dir}/mnt"
+
+	echo ${mount_dir}/policy_test > ${IMA_POLICY_FILE} 2> /dev/null
+}
+
 catch()
 {
 	local exit_code="$1"
@@ -127,6 +136,8 @@ main()
 		modify_bin "${tmp_dir}"
 	elif [[ "${action}" == "restore-bin" ]]; then
 		restore_bin "${tmp_dir}"
+	elif [[ "${action}" == "load-policy" ]]; then
+		load_policy "${tmp_dir}"
 	else
 		echo "Unknown action: ${action}"
 		exit 1

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

@@ -58,6 +58,7 @@ 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;
 }
 
 void test_test_ima(void)
@@ -181,6 +182,24 @@ void test_test_ima(void)
 	if (CHECK(err, "restore-bin #3", "err = %d\n", err))
 		goto close_clean;
 
+	/*
+	 * Test #5
+	 * - Goal: obtain a sample from the kernel_read_file hook
+	 * - Expected result: 2 samples (./ima_setup.sh, policy_test)
+	 */
+	test_init(skel->bss);
+	skel->bss->use_ima_file_hash = true;
+	skel->bss->enable_kernel_read_file = true;
+	err = _run_measured_process(measured_dir, &skel->bss->monitored_pid,
+				    "load-policy");
+	if (CHECK(err, "run_measured_process #5", "err = %d\n", err))
+		goto close_clean;
+
+	err = ring_buffer__consume(ringbuf);
+	ASSERT_EQ(err, 2, "num_samples_or_err");
+	ASSERT_NEQ(ima_hash_from_bpf[0], 0, "ima_hash");
+	ASSERT_NEQ(ima_hash_from_bpf[1], 0, "ima_hash");
+
 close_clean:
 	snprintf(cmd, sizeof(cmd), "./ima_setup.sh cleanup %s", measured_dir);
 	err = system(cmd);

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

@@ -20,6 +20,7 @@ char _license[] SEC("license") = "GPL";
 
 bool use_ima_file_hash;
 bool enable_bprm_creds_for_exec;
+bool enable_kernel_read_file;
 
 static void ima_test_common(struct file *file)
 {
@@ -65,3 +66,20 @@ int BPF_PROG(bprm_creds_for_exec, struct linux_binprm *bprm)
 	ima_test_common(bprm->file);
 	return 0;
 }
+
+SEC("lsm.s/kernel_read_file")
+int BPF_PROG(kernel_read_file, struct file *file, enum kernel_read_file_id id,
+	     bool contents)
+{
+	if (!enable_kernel_read_file)
+		return 0;
+
+	if (!contents)
+		return 0;
+
+	if (id != READING_POLICY)
+		return 0;
+
+	ima_test_common(file);
+	return 0;
+}