selftests/exec: Add 32 tests for AT_EXECVE_CHECK and exec securebits

Test that checks performed by execveat(..., AT_EXECVE_CHECK) are
consistent with noexec mount points and file execute permissions.

Test that SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE are
inherited by child processes and that they can be pinned with the
appropriate SECBIT_EXEC_RESTRICT_FILE_LOCKED and
SECBIT_EXEC_DENY_INTERACTIVE_LOCKED bits.

Cc: Al Viro <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Paul Moore <[e-mail]>
Cc: Serge Hallyn <[e-mail]>
Signed-off-by: Mickaël Salaün <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

tools/testing/selftests/exec/.gitignore

···
 execveat.denatured
 non-regular
 null-argv
+/check-exec
+/false
 /load_address.*
 !load_address.c
 /recursion-depth

tools/testing/selftests/exec/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 CFLAGS = -Wall
 CFLAGS += -Wno-nonnull
+CFLAGS += $(KHDR_INCLUDES)
+
+LDLIBS += -lcap
 
 ALIGNS := 0x1000 0x200000 0x1000000
 ALIGN_PIES        := $(patsubst %,load_address.%,$(ALIGNS))
···
 
 TEST_PROGS := binfmt_script.py
 TEST_GEN_PROGS := execveat non-regular $(ALIGNMENT_TESTS)
+TEST_GEN_PROGS_EXTENDED := false
 TEST_GEN_FILES := execveat.symlink execveat.denatured script subdir
 # Makefile is a run-time dependency, since it's accessed by the execveat test
 TEST_FILES := Makefile
 
 TEST_GEN_PROGS += recursion-depth
 TEST_GEN_PROGS += null-argv
+TEST_GEN_PROGS += check-exec
 
 EXTRA_CLEAN := $(OUTPUT)/subdir.moved $(OUTPUT)/execveat.moved $(OUTPUT)/xxxxx*	\
 	       $(OUTPUT)/S_I*.test
···
 $(OUTPUT)/load_address.static.0x%: load_address.c
 	$(CC) $(CFLAGS) $(LDFLAGS) -Wl,-z,max-page-size=$(lastword $(subst ., ,$@)) \
 		-fPIE -static-pie $< -o $@
+$(OUTPUT)/false: false.c
+	$(CC) $(CFLAGS) $(LDFLAGS) -static $< -o $@

tools/testing/selftests/exec/check-exec.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test execveat(2) with AT_EXECVE_CHECK, and prctl(2) with
+ * SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE, and their locked
+ * counterparts.
+ *
+ * Copyright © 2018-2020 ANSSI
+ * Copyright © 2024 Microsoft Corporation
+ *
+ * Author: Mickaël Salaün <[e-mail]>
+ */
+
+#include <asm-generic/unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/prctl.h>
+#include <linux/securebits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/capability.h>
+#include <sys/mount.h>
+#include <sys/prctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/sysmacros.h>
+#include <unistd.h>
+
+/* Defines AT_EXECVE_CHECK without type conflicts. */
+#define _ASM_GENERIC_FCNTL_H
+#include <linux/fcntl.h>
+
+#include "../kselftest_harness.h"
+
+static void drop_privileges(struct __test_metadata *const _metadata)
+{
+	const unsigned int noroot = SECBIT_NOROOT | SECBIT_NOROOT_LOCKED;
+	cap_t cap_p;
+
+	if ((cap_get_secbits() & noroot) != noroot)
+		EXPECT_EQ(0, cap_set_secbits(noroot));
+
+	cap_p = cap_get_proc();
+	EXPECT_NE(NULL, cap_p);
+	EXPECT_NE(-1, cap_clear(cap_p));
+
+	/*
+	 * Drops everything, especially CAP_SETPCAP, CAP_DAC_OVERRIDE, and
+	 * CAP_DAC_READ_SEARCH.
+	 */
+	EXPECT_NE(-1, cap_set_proc(cap_p));
+	EXPECT_NE(-1, cap_free(cap_p));
+}
+
+static int test_secbits_set(const unsigned int secbits)
+{
+	int err;
+
+	err = prctl(PR_SET_SECUREBITS, secbits);
+	if (err)
+		return errno;
+	return 0;
+}
+
+FIXTURE(access)
+{
+	int memfd, pipefd;
+	int pipe_fds[2], socket_fds[2];
+};
+
+FIXTURE_VARIANT(access)
+{
+	const bool mount_exec;
+	const bool file_exec;
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(access, mount_exec_file_exec) {
+	/* clang-format on */
+	.mount_exec = true,
+	.file_exec = true,
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(access, mount_exec_file_noexec) {
+	/* clang-format on */
+	.mount_exec = true,
+	.file_exec = false,
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(access, mount_noexec_file_exec) {
+	/* clang-format on */
+	.mount_exec = false,
+	.file_exec = true,
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(access, mount_noexec_file_noexec) {
+	/* clang-format on */
+	.mount_exec = false,
+	.file_exec = false,
+};
+
+static const char binary_path[] = "./false";
+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";
+
+FIXTURE_SETUP(access)
+{
+	int procfd_path_size;
+	static const char path_template[] = "/proc/self/fd/%d";
+	char procfd_path[sizeof(path_template) + 10];
+
+	/* Makes sure we are not already restricted nor locked. */
+	EXPECT_EQ(0, test_secbits_set(0));
+
+	/*
+	 * 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=9m"));
+
+	/* Creates a regular file. */
+	ASSERT_EQ(0, mknod(reg_file_path,
+			   S_IFREG | (variant->file_exec ? 0700 : 0600), 0));
+	/* Creates a directory. */
+	ASSERT_EQ(0, mkdir(dir_path, variant->file_exec ? 0700 : 0600));
+	/* 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 | 0600, 0));
+
+	/* Creates a regular file without user mount point. */
+	self->memfd = memfd_create("test-exec-probe", 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 ? 0700 : 0600));
+
+	/* 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_RDWR | O_CLOEXEC);
+	ASSERT_LE(0, self->pipefd);
+	ASSERT_EQ(0, fchmod(self->pipefd, variant->file_exec ? 0700 : 0600));
+
+	/* Creates a socket file descriptor. */
+	ASSERT_EQ(0, socketpair(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0,
+				self->socket_fds));
+}
+
+FIXTURE_TEARDOWN_PARENT(access)
+{
+	/* There is no need to unlink the test files. */
+	EXPECT_EQ(0, umount(workdir_path));
+	EXPECT_EQ(0, rmdir(workdir_path));
+}
+
+static void fill_exec_fd(struct __test_metadata *_metadata, const int fd_out)
+{
+	char buf[1024];
+	size_t len;
+	int fd_in;
+
+	fd_in = open(binary_path, O_CLOEXEC | O_RDONLY);
+	ASSERT_LE(0, fd_in);
+	/* Cannot use copy_file_range(2) because of EXDEV. */
+	len = read(fd_in, buf, sizeof(buf));
+	EXPECT_LE(0, len);
+	while (len > 0) {
+		EXPECT_EQ(len, write(fd_out, buf, len))
+		{
+			TH_LOG("Failed to write: %s (%d)", strerror(errno),
+			       errno);
+		}
+		len = read(fd_in, buf, sizeof(buf));
+		EXPECT_LE(0, len);
+	}
+	EXPECT_EQ(0, close(fd_in));
+}
+
+static void fill_exec_path(struct __test_metadata *_metadata,
+			   const char *const path)
+{
+	int fd_out;
+
+	fd_out = open(path, O_CLOEXEC | O_WRONLY);
+	ASSERT_LE(0, fd_out)
+	{
+		TH_LOG("Failed to open %s: %s", path, strerror(errno));
+	}
+	fill_exec_fd(_metadata, fd_out);
+	EXPECT_EQ(0, close(fd_out));
+}
+
+static void test_exec_fd(struct __test_metadata *_metadata, const int fd,
+			 const int err_code)
+{
+	char *const argv[] = { "", NULL };
+	int access_ret, access_errno;
+
+	/*
+	 * If we really execute fd, filled with the "false" binary, the current
+	 * thread will exits with an error, which will be interpreted by the
+	 * test framework as an error.  With AT_EXECVE_CHECK, we only check a
+	 * potential successful execution.
+	 */
+	access_ret =
+		execveat(fd, "", argv, NULL, AT_EMPTY_PATH | AT_EXECVE_CHECK);
+	access_errno = errno;
+	if (err_code) {
+		EXPECT_EQ(-1, access_ret);
+		EXPECT_EQ(err_code, access_errno)
+		{
+			TH_LOG("Wrong error for execveat(2): %s (%d)",
+			       strerror(access_errno), errno);
+		}
+	} else {
+		EXPECT_EQ(0, access_ret)
+		{
+			TH_LOG("Access denied: %s", strerror(access_errno));
+		}
+	}
+}
+
+static void test_exec_path(struct __test_metadata *_metadata,
+			   const char *const path, const int err_code)
+{
+	int flags = O_CLOEXEC;
+	int fd;
+
+	/* Do not block on pipes. */
+	if (path == fifo_path)
+		flags |= O_NONBLOCK;
+
+	fd = open(path, flags | O_RDONLY);
+	ASSERT_LE(0, fd)
+	{
+		TH_LOG("Failed to open %s: %s", path, strerror(errno));
+	}
+	test_exec_fd(_metadata, fd, err_code);
+	EXPECT_EQ(0, close(fd));
+}
+
+/* Tests that we don't get ENOEXEC. */
+TEST_F(access, regular_file_empty)
+{
+	const int exec = variant->mount_exec && variant->file_exec;
+
+	test_exec_path(_metadata, reg_file_path, exec ? 0 : EACCES);
+
+	drop_privileges(_metadata);
+	test_exec_path(_metadata, reg_file_path, exec ? 0 : EACCES);
+}
+
+TEST_F(access, regular_file_elf)
+{
+	const int exec = variant->mount_exec && variant->file_exec;
+
+	fill_exec_path(_metadata, reg_file_path);
+
+	test_exec_path(_metadata, reg_file_path, exec ? 0 : EACCES);
+
+	drop_privileges(_metadata);
+	test_exec_path(_metadata, reg_file_path, exec ? 0 : EACCES);
+}
+
+/* Tests that we don't get ENOEXEC. */
+TEST_F(access, memfd_empty)
+{
+	const int exec = variant->file_exec;
+
+	test_exec_fd(_metadata, self->memfd, exec ? 0 : EACCES);
+
+	drop_privileges(_metadata);
+	test_exec_fd(_metadata, self->memfd, exec ? 0 : EACCES);
+}
+
+TEST_F(access, memfd_elf)
+{
+	const int exec = variant->file_exec;
+
+	fill_exec_fd(_metadata, self->memfd);
+
+	test_exec_fd(_metadata, self->memfd, exec ? 0 : EACCES);
+
+	drop_privileges(_metadata);
+	test_exec_fd(_metadata, self->memfd, exec ? 0 : EACCES);
+}
+
+TEST_F(access, non_regular_files)
+{
+	test_exec_path(_metadata, dir_path, EACCES);
+	test_exec_path(_metadata, block_dev_path, EACCES);
+	test_exec_path(_metadata, char_dev_path, EACCES);
+	test_exec_path(_metadata, fifo_path, EACCES);
+	test_exec_fd(_metadata, self->socket_fds[0], EACCES);
+	test_exec_fd(_metadata, self->pipefd, EACCES);
+}
+
+/* clang-format off */
+FIXTURE(secbits) {};
+/* clang-format on */
+
+FIXTURE_VARIANT(secbits)
+{
+	const bool is_privileged;
+	const int error;
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(secbits, priv) {
+	/* clang-format on */
+	.is_privileged = true,
+	.error = 0,
+};
+
+/* clang-format off */
+FIXTURE_VARIANT_ADD(secbits, unpriv) {
+	/* clang-format on */
+	.is_privileged = false,
+	.error = EPERM,
+};
+
+FIXTURE_SETUP(secbits)
+{
+	/* Makes sure no exec bits are set. */
+	EXPECT_EQ(0, test_secbits_set(0));
+	EXPECT_EQ(0, prctl(PR_GET_SECUREBITS));
+
+	if (!variant->is_privileged)
+		drop_privileges(_metadata);
+}
+
+FIXTURE_TEARDOWN(secbits)
+{
+}
+
+TEST_F(secbits, legacy)
+{
+	EXPECT_EQ(variant->error, test_secbits_set(0));
+}
+
+#define CHILD(...)                     \
+	do {                           \
+		pid_t child = vfork(); \
+		EXPECT_LE(0, child);   \
+		if (child == 0) {      \
+			__VA_ARGS__;   \
+			_exit(0);      \
+		}                      \
+	} while (0)
+
+TEST_F(secbits, exec)
+{
+	unsigned int secbits = prctl(PR_GET_SECUREBITS);
+
+	secbits |= SECBIT_EXEC_RESTRICT_FILE;
+	EXPECT_EQ(0, test_secbits_set(secbits));
+	EXPECT_EQ(secbits, prctl(PR_GET_SECUREBITS));
+	CHILD(EXPECT_EQ(secbits, prctl(PR_GET_SECUREBITS)));
+
+	secbits |= SECBIT_EXEC_DENY_INTERACTIVE;
+	EXPECT_EQ(0, test_secbits_set(secbits));
+	EXPECT_EQ(secbits, prctl(PR_GET_SECUREBITS));
+	CHILD(EXPECT_EQ(secbits, prctl(PR_GET_SECUREBITS)));
+
+	secbits &= ~(SECBIT_EXEC_RESTRICT_FILE | SECBIT_EXEC_DENY_INTERACTIVE);
+	EXPECT_EQ(0, test_secbits_set(secbits));
+	EXPECT_EQ(secbits, prctl(PR_GET_SECUREBITS));
+	CHILD(EXPECT_EQ(secbits, prctl(PR_GET_SECUREBITS)));
+}
+
+TEST_F(secbits, check_locked_set)
+{
+	unsigned int secbits = prctl(PR_GET_SECUREBITS);
+
+	secbits |= SECBIT_EXEC_RESTRICT_FILE;
+	EXPECT_EQ(0, test_secbits_set(secbits));
+	secbits |= SECBIT_EXEC_RESTRICT_FILE_LOCKED;
+	EXPECT_EQ(0, test_secbits_set(secbits));
+
+	/* Checks lock set but unchanged. */
+	EXPECT_EQ(variant->error, test_secbits_set(secbits));
+	CHILD(EXPECT_EQ(variant->error, test_secbits_set(secbits)));
+
+	secbits &= ~SECBIT_EXEC_RESTRICT_FILE;
+	EXPECT_EQ(EPERM, test_secbits_set(0));
+	CHILD(EXPECT_EQ(EPERM, test_secbits_set(0)));
+}
+
+TEST_F(secbits, check_locked_unset)
+{
+	unsigned int secbits = prctl(PR_GET_SECUREBITS);
+
+	secbits |= SECBIT_EXEC_RESTRICT_FILE_LOCKED;
+	EXPECT_EQ(0, test_secbits_set(secbits));
+
+	/* Checks lock unset but unchanged. */
+	EXPECT_EQ(variant->error, test_secbits_set(secbits));
+	CHILD(EXPECT_EQ(variant->error, test_secbits_set(secbits)));
+
+	secbits &= ~SECBIT_EXEC_RESTRICT_FILE;
+	EXPECT_EQ(EPERM, test_secbits_set(0));
+	CHILD(EXPECT_EQ(EPERM, test_secbits_set(0)));
+}
+
+TEST_F(secbits, restrict_locked_set)
+{
+	unsigned int secbits = prctl(PR_GET_SECUREBITS);
+
+	secbits |= SECBIT_EXEC_DENY_INTERACTIVE;
+	EXPECT_EQ(0, test_secbits_set(secbits));
+	secbits |= SECBIT_EXEC_DENY_INTERACTIVE_LOCKED;
+	EXPECT_EQ(0, test_secbits_set(secbits));
+
+	/* Checks lock set but unchanged. */
+	EXPECT_EQ(variant->error, test_secbits_set(secbits));
+	CHILD(EXPECT_EQ(variant->error, test_secbits_set(secbits)));
+
+	secbits &= ~SECBIT_EXEC_DENY_INTERACTIVE;
+	EXPECT_EQ(EPERM, test_secbits_set(0));
+	CHILD(EXPECT_EQ(EPERM, test_secbits_set(0)));
+}
+
+TEST_F(secbits, restrict_locked_unset)
+{
+	unsigned int secbits = prctl(PR_GET_SECUREBITS);
+
+	secbits |= SECBIT_EXEC_DENY_INTERACTIVE_LOCKED;
+	EXPECT_EQ(0, test_secbits_set(secbits));
+
+	/* Checks lock unset but unchanged. */
+	EXPECT_EQ(variant->error, test_secbits_set(secbits));
+	CHILD(EXPECT_EQ(variant->error, test_secbits_set(secbits)));
+
+	secbits &= ~SECBIT_EXEC_DENY_INTERACTIVE;
+	EXPECT_EQ(EPERM, test_secbits_set(0));
+	CHILD(EXPECT_EQ(EPERM, test_secbits_set(0)));
+}
+
+TEST_HARNESS_MAIN

tools/testing/selftests/exec/config

···
+CONFIG_BLK_DEV=y
+CONFIG_BLK_DEV_LOOP=y

tools/testing/selftests/exec/false.c

···
+// SPDX-License-Identifier: GPL-2.0
+int main(void)
+{
+	return 1;
+}