LSM: SafeSetID: add selftest

This patch adds a selftest for the SafeSetID LSM. The test requires
mounting securityfs if it isn't mounted, creating test users in
/etc/passwd, and configuring policies for the SafeSetID LSM through
writes to securityfs.
Signed-off-by: Micah Morton <mortonm@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>

tools/testing/selftests/safesetid/.gitignore

+safesetid-test

tools/testing/selftests/safesetid/Makefile

+# SPDX-License-Identifier: GPL-2.0
+# Makefile for mount selftests.
+CFLAGS = -Wall -lcap -O2
+
+TEST_PROGS := run_tests.sh
+TEST_GEN_FILES := safesetid-test
+
+include ../lib.mk

tools/testing/selftests/safesetid/config

+CONFIG_SECURITY=y
+CONFIG_SECURITYFS=y

tools/testing/selftests/safesetid/safesetid-test.c

+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <errno.h>
+#include <pwd.h>
+#include <string.h>
+#include <syscall.h>
+#include <sys/capability.h>
+#include <sys/types.h>
+#include <sys/mount.h>
+#include <sys/prctl.h>
+#include <sys/wait.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdarg.h>
+
+#ifndef CLONE_NEWUSER
+# define CLONE_NEWUSER 0x10000000
+#endif
+
+#define ROOT_USER 0
+#define RESTRICTED_PARENT 1
+#define ALLOWED_CHILD1 2
+#define ALLOWED_CHILD2 3
+#define NO_POLICY_USER 4
+
+char* add_whitelist_policy_file = "/sys/kernel/security/safesetid/add_whitelist_policy";
+
+static void die(char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+	exit(EXIT_FAILURE);
+}
+
+static bool vmaybe_write_file(bool enoent_ok, char *filename, char *fmt, va_list ap)
+{
+	char buf[4096];
+	int fd;
+	ssize_t written;
+	int buf_len;
+
+	buf_len = vsnprintf(buf, sizeof(buf), fmt, ap);
+	if (buf_len < 0) {
+		printf("vsnprintf failed: %s\n",
+		    strerror(errno));
+		return false;
+	}
+	if (buf_len >= sizeof(buf)) {
+		printf("vsnprintf output truncated\n");
+		return false;
+	}
+
+	fd = open(filename, O_WRONLY);
+	if (fd < 0) {
+		if ((errno == ENOENT) && enoent_ok)
+			return true;
+		return false;
+	}
+	written = write(fd, buf, buf_len);
+	if (written != buf_len) {
+		if (written >= 0) {
+			printf("short write to %s\n", filename);
+			return false;
+		} else {
+			printf("write to %s failed: %s\n",
+				filename, strerror(errno));
+			return false;
+		}
+	}
+	if (close(fd) != 0) {
+		printf("close of %s failed: %s\n",
+			filename, strerror(errno));
+		return false;
+	}
+	return true;
+}
+
+static bool write_file(char *filename, char *fmt, ...)
+{
+	va_list ap;
+	bool ret;
+
+	va_start(ap, fmt);
+	ret = vmaybe_write_file(false, filename, fmt, ap);
+	va_end(ap);
+
+	return ret;
+}
+
+static void ensure_user_exists(uid_t uid)
+{
+	struct passwd p;
+
+	FILE *fd;
+	char name_str[10];
+
+	if (getpwuid(uid) == NULL) {
+		memset(&p,0x00,sizeof(p));
+		fd=fopen("/etc/passwd","a");
+		if (fd == NULL)
+			die("couldn't open file\n");
+		if (fseek(fd, 0, SEEK_END))
+			die("couldn't fseek\n");
+		snprintf(name_str, 10, "%d", uid);
+		p.pw_name=name_str;
+		p.pw_uid=uid;
+		p.pw_gecos="Test account";
+		p.pw_dir="/dev/null";
+		p.pw_shell="/bin/false";
+		int value = putpwent(&p,fd);
+		if (value != 0)
+			die("putpwent failed\n");
+		if (fclose(fd))
+			die("fclose failed\n");
+	}
+}
+
+static void ensure_securityfs_mounted(void)
+{
+	int fd = open(add_whitelist_policy_file, O_WRONLY);
+	if (fd < 0) {
+		if (errno == ENOENT) {
+			// Need to mount securityfs
+			if (mount("securityfs", "/sys/kernel/security",
+						"securityfs", 0, NULL) < 0)
+				die("mounting securityfs failed\n");
+		} else {
+			die("couldn't find securityfs for unknown reason\n");
+		}
+	} else {
+		if (close(fd) != 0) {
+			die("close of %s failed: %s\n",
+				add_whitelist_policy_file, strerror(errno));
+		}
+	}
+}
+
+static void write_policies(void)
+{
+	ssize_t written;
+	int fd;
+
+	fd = open(add_whitelist_policy_file, O_WRONLY);
+	if (fd < 0)
+		die("cant open add_whitelist_policy file\n");
+	written = write(fd, "1:2", strlen("1:2"));
+	if (written != strlen("1:2")) {
+		if (written >= 0) {
+			die("short write to %s\n", add_whitelist_policy_file);
+		} else {
+			die("write to %s failed: %s\n",
+				add_whitelist_policy_file, strerror(errno));
+		}
+	}
+	written = write(fd, "1:3", strlen("1:3"));
+	if (written != strlen("1:3")) {
+		if (written >= 0) {
+			die("short write to %s\n", add_whitelist_policy_file);
+		} else {
+			die("write to %s failed: %s\n",
+				add_whitelist_policy_file, strerror(errno));
+		}
+	}
+	if (close(fd) != 0) {
+		die("close of %s failed: %s\n",
+			add_whitelist_policy_file, strerror(errno));
+	}
+}
+
+static bool test_userns(bool expect_success)
+{
+	uid_t uid;
+	char map_file_name[32];
+	size_t sz = sizeof(map_file_name);
+	pid_t cpid;
+	bool success;
+
+	uid = getuid();
+
+	int clone_flags = CLONE_NEWUSER;
+	cpid = syscall(SYS_clone, clone_flags, NULL);
+	if (cpid == -1) {
+	    printf("clone failed");
+	    return false;
+	}
+
+	if (cpid == 0) {	/* Code executed by child */
+		// Give parent 1 second to write map file
+		sleep(1);
+		exit(EXIT_SUCCESS);
+	} else {		/* Code executed by parent */
+		if(snprintf(map_file_name, sz, "/proc/%d/uid_map", cpid) < 0) {
+			printf("preparing file name string failed");
+			return false;
+		}
+		success = write_file(map_file_name, "0 0 1", uid);
+		return success == expect_success;
+	}
+
+	printf("should not reach here");
+	return false;
+}
+
+static void test_setuid(uid_t child_uid, bool expect_success)
+{
+	pid_t cpid, w;
+	int wstatus;
+
+	cpid = fork();
+	if (cpid == -1) {
+		die("fork\n");
+	}
+
+	if (cpid == 0) {	    /* Code executed by child */
+		setuid(child_uid);
+		if (getuid() == child_uid)
+			exit(EXIT_SUCCESS);
+		else
+			exit(EXIT_FAILURE);
+	} else {		 /* Code executed by parent */
+		do {
+			w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED);
+			if (w == -1) {
+				die("waitpid\n");
+			}
+
+			if (WIFEXITED(wstatus)) {
+				if (WEXITSTATUS(wstatus) == EXIT_SUCCESS) {
+					if (expect_success) {
+						return;
+					} else {
+						die("unexpected success\n");
+					}
+				} else {
+					if (expect_success) {
+						die("unexpected failure\n");
+					} else {
+						return;
+					}
+				}
+			} else if (WIFSIGNALED(wstatus)) {
+				if (WTERMSIG(wstatus) == 9) {
+					if (expect_success)
+						die("killed unexpectedly\n");
+					else
+						return;
+				} else {
+					die("unexpected signal: %d\n", wstatus);
+				}
+			} else {
+				die("unexpected status: %d\n", wstatus);
+			}
+		} while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
+	}
+
+	die("should not reach here\n");
+}
+
+static void ensure_users_exist(void)
+{
+	ensure_user_exists(ROOT_USER);
+	ensure_user_exists(RESTRICTED_PARENT);
+	ensure_user_exists(ALLOWED_CHILD1);
+	ensure_user_exists(ALLOWED_CHILD2);
+	ensure_user_exists(NO_POLICY_USER);
+}
+
+static void drop_caps(bool setid_retained)
+{
+	cap_value_t cap_values[] = {CAP_SETUID, CAP_SETGID};
+	cap_t caps;
+
+	caps = cap_get_proc();
+	if (setid_retained)
+		cap_set_flag(caps, CAP_EFFECTIVE, 2, cap_values, CAP_SET);
+	else
+		cap_clear(caps);
+	cap_set_proc(caps);
+	cap_free(caps);
+}
+
+int main(int argc, char **argv)
+{
+	ensure_users_exist();
+	ensure_securityfs_mounted();
+	write_policies();
+
+	if (prctl(PR_SET_KEEPCAPS, 1L))
+		die("Error with set keepcaps\n");
+
+	// First test to make sure we can write userns mappings from a user
+	// that doesn't have any restrictions (as long as it has CAP_SETUID);
+	setuid(NO_POLICY_USER);
+	setgid(NO_POLICY_USER);
+
+	// Take away all but setid caps
+	drop_caps(true);
+
+	// Need PR_SET_DUMPABLE flag set so we can write /proc/[pid]/uid_map
+	// from non-root parent process.
+	if (prctl(PR_SET_DUMPABLE, 1, 0, 0, 0))
+		die("Error with set dumpable\n");
+
+	if (!test_userns(true)) {
+		die("test_userns failed when it should work\n");
+	}
+
+	setuid(RESTRICTED_PARENT);
+	setgid(RESTRICTED_PARENT);
+
+	test_setuid(ROOT_USER, false);
+	test_setuid(ALLOWED_CHILD1, true);
+	test_setuid(ALLOWED_CHILD2, true);
+	test_setuid(NO_POLICY_USER, false);
+
+	if (!test_userns(false)) {
+		die("test_userns worked when it should fail\n");
+	}
+
+	// Now take away all caps
+	drop_caps(false);
+	test_setuid(2, false);
+	test_setuid(3, false);
+	test_setuid(4, false);
+
+	// NOTE: this test doesn't clean up users that were created in
+	// /etc/passwd or flush policies that were added to the LSM.
+	return EXIT_SUCCESS;
+}

tools/testing/selftests/safesetid/safesetid-test.sh

+#!/bin/bash
+
+TCID="safesetid-test.sh"
+errcode=0
+
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
+check_root()
+{
+	uid=$(id -u)
+	if [ $uid -ne 0 ]; then
+		echo $TCID: must be run as root >&2
+		exit $ksft_skip
+	fi
+}
+
+main_function()
+{
+  check_root
+  ./safesetid-test
+}
+
+main_function
+echo "$TCID: done"
+exit $errcode