selftest/interpreter: Add tests for trusted_for(2) policies

Test that checks performed by trusted_for(2) on file descriptors are
consistent with noexec mount points and file execute permissions,
according to the policy configured with the fs.trust_policy sysctl.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220104155024.48023-5-mic@digikod.net

tools/testing/selftests/Makefile

@@ -21,6 +21,7 @@ TARGETS += ftrace
 TARGETS += futex
 TARGETS += gpio
 TARGETS += intel_pstate
+TARGETS += interpreter
 TARGETS += ipc
 TARGETS += ir
 TARGETS += kcmp

tools/testing/selftests/interpreter/.gitignore

@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/*_test

tools/testing/selftests/interpreter/Makefile

@@ -0,0 +1,21 @@
+# SPDX-License-Identifier: GPL-2.0
+
+CFLAGS += -Wall -O2 -I$(khdr_dir)
+LDLIBS += -lcap
+
+src_test := $(wildcard *_test.c)
+TEST_GEN_PROGS := $(src_test:.c=)
+
+KSFT_KHDR_INSTALL := 1
+include ../lib.mk
+
+khdr_dir = $(top_srcdir)/usr/include
+
+$(khdr_dir)/asm-generic/unistd.h: khdr
+	@:
+
+$(khdr_dir)/linux/trusted-for.h: khdr
+	@:
+
+$(OUTPUT)/%_test: %_test.c $(khdr_dir)/asm-generic/unistd.h $(khdr_dir)/linux/trusted-for.h ../kselftest_harness.h
+	$(LINK.c) $< $(LDLIBS) -o $@

tools/testing/selftests/interpreter/config

@@ -0,0 +1 @@
+CONFIG_SYSCTL=y

tools/testing/selftests/interpreter/trust_policy_test.c

@@ -0,0 +1,362 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test trusted_for(2) with fs.trusted_for_policy sysctl
+ *
+ * Copyright © 2018-2020 ANSSI
+ *
+ * Author: Mickaël Salaün <mic@digikod.net>
+ */
+
+#define _GNU_SOURCE
+#include <asm-generic/unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/trusted-for.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/capability.h>
+#include <sys/mman.h>
+#include <sys/mount.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/sysmacros.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "../kselftest_harness.h"
+
+#ifndef trusted_for
+static int trusted_for(const int fd, const enum trusted_for_usage usage,
+		const __u32 flags)
+{
+	errno = 0;
+	return syscall(__NR_trusted_for, fd, usage, flags);
+}
+#endif
+
+static const char sysctl_path[] = "/proc/sys/fs/trusted_for_policy";
+
+static const char workdir_path[] = "./test-mount";
+static const char reg_file_path[] = "./test-mount/regular_file";
+static const char dir_path[] = "./test-mount/directory";
+static const char block_dev_path[] = "./test-mount/block_device";
+static const char char_dev_path[] = "./test-mount/character_device";
+static const char fifo_path[] = "./test-mount/fifo";
+
+static void ignore_dac(struct __test_metadata *_metadata, int override)
+{
+	cap_t caps;
+	const cap_value_t cap_val[2] = {
+		CAP_DAC_OVERRIDE,
+		CAP_DAC_READ_SEARCH,
+	};
+
+	caps = cap_get_proc();
+	ASSERT_NE(NULL, caps);
+	ASSERT_EQ(0, cap_set_flag(caps, CAP_EFFECTIVE, 2, cap_val,
+				override ? CAP_SET : CAP_CLEAR));
+	ASSERT_EQ(0, cap_set_proc(caps));
+	EXPECT_EQ(0, cap_free(caps));
+}
+
+static void ignore_sys_admin(struct __test_metadata *_metadata, int override)
+{
+	cap_t caps;
+	const cap_value_t cap_val[1] = {
+		CAP_SYS_ADMIN,
+	};
+
+	caps = cap_get_proc();
+	ASSERT_NE(NULL, caps);
+	ASSERT_EQ(0, cap_set_flag(caps, CAP_EFFECTIVE, 1, cap_val,
+				override ? CAP_SET : CAP_CLEAR));
+	ASSERT_EQ(0, cap_set_proc(caps));
+	EXPECT_EQ(0, cap_free(caps));
+}
+
+static void test_omx(struct __test_metadata *_metadata,
+		const char *const path, const int err_access)
+{
+	int flags = O_RDONLY | O_CLOEXEC;
+	int fd, access_ret, access_errno;
+
+	/* Do not block on pipes. */
+	if (path == fifo_path)
+		flags |= O_NONBLOCK;
+
+	fd = open(path, flags);
+	ASSERT_LE(0, fd) {
+		TH_LOG("Failed to open %s: %s", path, strerror(errno));
+	}
+	access_ret = trusted_for(fd, TRUSTED_FOR_EXECUTION, 0);
+	access_errno = errno;
+	if (err_access) {
+		ASSERT_EQ(err_access, access_errno) {
+			TH_LOG("Wrong error for trusted_for(2) with %s: %s",
+					path, strerror(access_errno));
+		}
+		ASSERT_EQ(-1, access_ret);
+	} else {
+		ASSERT_EQ(0, access_ret) {
+			TH_LOG("Access denied for %s: %s", path, strerror(access_errno));
+		}
+	}
+
+	/* Tests unsupported trusted usage. */
+	access_ret = trusted_for(fd, 0, 0);
+	ASSERT_EQ(-1, access_ret);
+	ASSERT_EQ(EINVAL, errno);
+
+	access_ret = trusted_for(fd, 2, 0);
+	ASSERT_EQ(-1, access_ret);
+	ASSERT_EQ(EINVAL, errno);
+
+	EXPECT_EQ(0, close(fd));
+}
+
+static void test_policy_fd(struct __test_metadata *_metadata, const int fd,
+		const bool has_policy)
+{
+	const int ret = trusted_for(fd, TRUSTED_FOR_EXECUTION, 0);
+
+	if (has_policy) {
+		ASSERT_EQ(-1, ret);
+		ASSERT_EQ(EACCES, errno) {
+			TH_LOG("Wrong error for trusted_for(2) with FD: %s", strerror(errno));
+		}
+	} else {
+		ASSERT_EQ(0, ret) {
+			TH_LOG("Access denied for FD: %s", strerror(errno));
+		}
+	}
+}
+
+FIXTURE(access) {
+	char initial_sysctl_value;
+	int memfd, pipefd;
+	int pipe_fds[2], socket_fds[2];
+};
+
+static void test_file_types(struct __test_metadata *_metadata, FIXTURE_DATA(access) *self,
+		const int err_code, const bool has_policy)
+{
+	/* Tests are performed on a tmpfs mount point. */
+	test_omx(_metadata, reg_file_path, err_code);
+	test_omx(_metadata, dir_path, has_policy ? EACCES : 0);
+	test_omx(_metadata, block_dev_path, has_policy ? EACCES : 0);
+	test_omx(_metadata, char_dev_path, has_policy ? EACCES : 0);
+	test_omx(_metadata, fifo_path, has_policy ? EACCES : 0);
+
+	/* Checks that exec is denied for any socket FD. */
+	test_policy_fd(_metadata, self->socket_fds[0], has_policy);
+
+	/* Checks that exec is denied for any memfd. */
+	test_policy_fd(_metadata, self->memfd, has_policy);
+
+	/* Checks that exec is denied for any pipefs FD. */
+	test_policy_fd(_metadata, self->pipefd, has_policy);
+}
+
+static void test_files(struct __test_metadata *_metadata, FIXTURE_DATA(access) *self,
+		const int err_code, const bool has_policy)
+{
+	/* Tests as root. */
+	ignore_dac(_metadata, 1);
+	test_file_types(_metadata, self, err_code, has_policy);
+
+	/* Tests without bypass. */
+	ignore_dac(_metadata, 0);
+	test_file_types(_metadata, self, err_code, has_policy);
+}
+
+static void sysctl_write_char(struct __test_metadata *_metadata, const char value)
+{
+	int fd;
+
+	fd = open(sysctl_path, O_WRONLY | O_CLOEXEC);
+	ASSERT_LE(0, fd);
+	ASSERT_EQ(1, write(fd, &value, 1));
+	EXPECT_EQ(0, close(fd));
+}
+
+static char sysctl_read_char(struct __test_metadata *_metadata)
+{
+	int fd;
+	char sysctl_value;
+
+	fd = open(sysctl_path, O_RDONLY | O_CLOEXEC);
+	ASSERT_LE(0, fd);
+	ASSERT_EQ(1, read(fd, &sysctl_value, 1));
+	EXPECT_EQ(0, close(fd));
+	return sysctl_value;
+}
+
+FIXTURE_VARIANT(access) {
+	const bool mount_exec;
+	const bool file_exec;
+	const int sysctl_err_code[3];
+};
+
+FIXTURE_VARIANT_ADD(access, mount_exec_file_exec) {
+	.mount_exec = true,
+	.file_exec = true,
+	.sysctl_err_code = {0, 0, 0},
+};
+
+FIXTURE_VARIANT_ADD(access, mount_exec_file_noexec)
+{
+	.mount_exec = true,
+	.file_exec = false,
+	.sysctl_err_code = {0, EACCES, EACCES},
+};
+
+FIXTURE_VARIANT_ADD(access, mount_noexec_file_exec)
+{
+	.mount_exec = false,
+	.file_exec = true,
+	.sysctl_err_code = {EACCES, 0, EACCES},
+};
+
+FIXTURE_VARIANT_ADD(access, mount_noexec_file_noexec)
+{
+	.mount_exec = false,
+	.file_exec = false,
+	.sysctl_err_code = {EACCES, EACCES, EACCES},
+};
+
+FIXTURE_SETUP(access)
+{
+	int procfd_path_size;
+	static const char path_template[] = "/proc/self/fd/%d";
+	char procfd_path[sizeof(path_template) + 10];
+
+	/*
+	 * Cleans previous workspace if any error previously happened (don't
+	 * check errors).
+	 */
+	umount(workdir_path);
+	rmdir(workdir_path);
+
+	/* Creates a clean mount point. */
+	ASSERT_EQ(0, mkdir(workdir_path, 00700));
+	ASSERT_EQ(0, mount("test", workdir_path, "tmpfs", MS_MGC_VAL |
+				(variant->mount_exec ? 0 : MS_NOEXEC),
+				"mode=0700,size=4k"));
+
+	/* Creates a regular file. */
+	ASSERT_EQ(0, mknod(reg_file_path, S_IFREG | (variant->file_exec ? 0500 : 0400), 0));
+	/* Creates a directory. */
+	ASSERT_EQ(0, mkdir(dir_path, variant->file_exec ? 0500 : 0400));
+	/* Creates a character device: /dev/null. */
+	ASSERT_EQ(0, mknod(char_dev_path, S_IFCHR | 0400, makedev(1, 3)));
+	/* Creates a block device: /dev/loop0 */
+	ASSERT_EQ(0, mknod(block_dev_path, S_IFBLK | 0400, makedev(7, 0)));
+	/* Creates a fifo. */
+	ASSERT_EQ(0, mknod(fifo_path, S_IFIFO | 0400, 0));
+
+	/* Creates a regular file without user mount point. */
+	self->memfd = memfd_create("test-interpreted", MFD_CLOEXEC);
+	ASSERT_LE(0, self->memfd);
+	/* Sets mode, which must be ignored by the exec check. */
+	ASSERT_EQ(0, fchmod(self->memfd, variant->file_exec ? 0500 : 0400));
+
+	/* Creates a pipefs file descriptor. */
+	ASSERT_EQ(0, pipe(self->pipe_fds));
+	procfd_path_size = snprintf(procfd_path, sizeof(procfd_path),
+			path_template, self->pipe_fds[0]);
+	ASSERT_LT(procfd_path_size, sizeof(procfd_path));
+	self->pipefd = open(procfd_path, O_RDONLY | O_CLOEXEC);
+	ASSERT_LE(0, self->pipefd);
+	ASSERT_EQ(0, fchmod(self->pipefd, variant->file_exec ? 0500 : 0400));
+
+	/* Creates a socket file descriptor. */
+	ASSERT_EQ(0, socketpair(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0, self->socket_fds));
+
+	/* Saves initial sysctl value. */
+	self->initial_sysctl_value = sysctl_read_char(_metadata);
+
+	/* Prepares for sysctl writes. */
+	ignore_sys_admin(_metadata, 1);
+}
+
+FIXTURE_TEARDOWN(access)
+{
+	EXPECT_EQ(0, close(self->memfd));
+	EXPECT_EQ(0, close(self->pipefd));
+	EXPECT_EQ(0, close(self->pipe_fds[0]));
+	EXPECT_EQ(0, close(self->pipe_fds[1]));
+	EXPECT_EQ(0, close(self->socket_fds[0]));
+	EXPECT_EQ(0, close(self->socket_fds[1]));
+
+	/* Restores initial sysctl value. */
+	sysctl_write_char(_metadata, self->initial_sysctl_value);
+
+	/* There is no need to unlink the test files. */
+	ASSERT_EQ(0, umount(workdir_path));
+	ASSERT_EQ(0, rmdir(workdir_path));
+}
+
+TEST_F(access, sysctl_0)
+{
+	/* Do not enforce anything. */
+	sysctl_write_char(_metadata, '0');
+	test_files(_metadata, self, 0, false);
+}
+
+TEST_F(access, sysctl_1)
+{
+	/* Enforces mount exec check. */
+	sysctl_write_char(_metadata, '1');
+	test_files(_metadata, self, variant->sysctl_err_code[0], true);
+}
+
+TEST_F(access, sysctl_2)
+{
+	/* Enforces file exec check. */
+	sysctl_write_char(_metadata, '2');
+	test_files(_metadata, self, variant->sysctl_err_code[1], true);
+}
+
+TEST_F(access, sysctl_3)
+{
+	/* Enforces mount and file exec check. */
+	sysctl_write_char(_metadata, '3');
+	test_files(_metadata, self, variant->sysctl_err_code[2], true);
+}
+
+FIXTURE(cleanup) {
+	char initial_sysctl_value;
+};
+
+FIXTURE_SETUP(cleanup)
+{
+	/* Saves initial sysctl value. */
+	self->initial_sysctl_value = sysctl_read_char(_metadata);
+}
+
+FIXTURE_TEARDOWN(cleanup)
+{
+	/* Restores initial sysctl value. */
+	ignore_sys_admin(_metadata, 1);
+	sysctl_write_char(_metadata, self->initial_sysctl_value);
+}
+
+TEST_F(cleanup, sysctl_access_write)
+{
+	int fd;
+	ssize_t ret;
+
+	ignore_sys_admin(_metadata, 1);
+	sysctl_write_char(_metadata, '0');
+
+	ignore_sys_admin(_metadata, 0);
+	fd = open(sysctl_path, O_WRONLY | O_CLOEXEC);
+	ASSERT_LE(0, fd);
+	ret = write(fd, "0", 1);
+	ASSERT_EQ(-1, ret);
+	ASSERT_EQ(EPERM, errno);
+	EXPECT_EQ(0, close(fd));
+}
+
+TEST_HARNESS_MAIN