selftests: add openat2(2) selftests

Test all of the various openat2(2) flags. A small stress-test of a
symlink-rename attack is included to show that the protections against
".."-based attacks are sufficient.

The main things these self-tests are enforcing are:

  * The struct+usize ABI for openat2(2) and copy_struct_from_user() to
    ensure that upgrades will be handled gracefully (in addition,
    ensuring that misaligned structures are also handled correctly).

  * The -EINVAL checks for openat2(2) are all correctly handled to avoid
    userspace passing unknown or conflicting flag sets (most
    importantly, ensuring that invalid flag combinations are checked).

  * All of the RESOLVE_* semantics (including errno values) are
    correctly handled with various combinations of paths and flags.

  * RESOLVE_IN_ROOT correctly protects against the symlink rename(2)
    attack that has been responsible for several CVEs (and likely will
    be responsible for several more).

Cc: Shuah Khan <[email protected]>
Signed-off-by: Aleksa Sarai <[email protected]>
Signed-off-by: Al Viro <[email protected]>

Author: cyphar

tools/testing/selftests/Makefile

@@ -40,6 +40,7 @@ TARGETS += powerpc
 TARGETS += proc
 TARGETS += pstore
 TARGETS += ptrace
+TARGETS += openat2
 TARGETS += rseq
 TARGETS += rtc
 TARGETS += seccomp

tools/testing/selftests/openat2/.gitignore

@@ -0,0 +1 @@
+/*_test

tools/testing/selftests/openat2/Makefile

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+CFLAGS += -Wall -O2 -g -fsanitize=address -fsanitize=undefined
+TEST_GEN_PROGS := openat2_test resolve_test rename_attack_test
+
+include ../lib.mk
+
+$(TEST_GEN_PROGS): helpers.c

tools/testing/selftests/openat2/helpers.c

@@ -0,0 +1,109 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Author: Aleksa Sarai <[email protected]>
+ * Copyright (C) 2018-2019 SUSE LLC.
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <string.h>
+#include <syscall.h>
+#include <limits.h>
+
+#include "helpers.h"
+
+bool needs_openat2(const struct open_how *how)
+{
+	return how->resolve != 0;
+}
+
+int raw_openat2(int dfd, const char *path, void *how, size_t size)
+{
+	int ret = syscall(__NR_openat2, dfd, path, how, size);
+	return ret >= 0 ? ret : -errno;
+}
+
+int sys_openat2(int dfd, const char *path, struct open_how *how)
+{
+	return raw_openat2(dfd, path, how, sizeof(*how));
+}
+
+int sys_openat(int dfd, const char *path, struct open_how *how)
+{
+	int ret = openat(dfd, path, how->flags, how->mode);
+	return ret >= 0 ? ret : -errno;
+}
+
+int sys_renameat2(int olddirfd, const char *oldpath,
+		  int newdirfd, const char *newpath, unsigned int flags)
+{
+	int ret = syscall(__NR_renameat2, olddirfd, oldpath,
+					  newdirfd, newpath, flags);
+	return ret >= 0 ? ret : -errno;
+}
+
+int touchat(int dfd, const char *path)
+{
+	int fd = openat(dfd, path, O_CREAT);
+	if (fd >= 0)
+		close(fd);
+	return fd;
+}
+
+char *fdreadlink(int fd)
+{
+	char *target, *tmp;
+
+	E_asprintf(&tmp, "/proc/self/fd/%d", fd);
+
+	target = malloc(PATH_MAX);
+	if (!target)
+		ksft_exit_fail_msg("fdreadlink: malloc failed\n");
+	memset(target, 0, PATH_MAX);
+
+	E_readlink(tmp, target, PATH_MAX);
+	free(tmp);
+	return target;
+}
+
+bool fdequal(int fd, int dfd, const char *path)
+{
+	char *fdpath, *dfdpath, *other;
+	bool cmp;
+
+	fdpath = fdreadlink(fd);
+	dfdpath = fdreadlink(dfd);
+
+	if (!path)
+		E_asprintf(&other, "%s", dfdpath);
+	else if (*path == '/')
+		E_asprintf(&other, "%s", path);
+	else
+		E_asprintf(&other, "%s/%s", dfdpath, path);
+
+	cmp = !strcmp(fdpath, other);
+
+	free(fdpath);
+	free(dfdpath);
+	free(other);
+	return cmp;
+}
+
+bool openat2_supported = false;
+
+void __attribute__((constructor)) init(void)
+{
+	struct open_how how = {};
+	int fd;
+
+	BUILD_BUG_ON(sizeof(struct open_how) != OPEN_HOW_SIZE_VER0);
+
+	/* Check openat2(2) support. */
+	fd = sys_openat2(AT_FDCWD, ".", &how);
+	openat2_supported = (fd >= 0);
+
+	if (fd >= 0)
+		close(fd);
+}

tools/testing/selftests/openat2/helpers.h

@@ -0,0 +1,106 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Author: Aleksa Sarai <[email protected]>
+ * Copyright (C) 2018-2019 SUSE LLC.
+ */
+
+#ifndef __RESOLVEAT_H__
+#define __RESOLVEAT_H__
+
+#define _GNU_SOURCE
+#include <stdint.h>
+#include <errno.h>
+#include <linux/types.h>
+#include "../kselftest.h"
+
+#define ARRAY_LEN(X) (sizeof (X) / sizeof (*(X)))
+#define BUILD_BUG_ON(e) ((void)(sizeof(struct { int:(-!!(e)); })))
+
+#ifndef SYS_openat2
+#ifndef __NR_openat2
+#define __NR_openat2 437
+#endif /* __NR_openat2 */
+#define SYS_openat2 __NR_openat2
+#endif /* SYS_openat2 */
+
+/*
+ * Arguments for how openat2(2) should open the target path. If @resolve is
+ * zero, then openat2(2) operates very similarly to openat(2).
+ *
+ * However, unlike openat(2), unknown bits in @flags result in -EINVAL rather
+ * than being silently ignored. @mode must be zero unless one of {O_CREAT,
+ * O_TMPFILE} are set.
+ *
+ * @flags: O_* flags.
+ * @mode: O_CREAT/O_TMPFILE file mode.
+ * @resolve: RESOLVE_* flags.
+ */
+struct open_how {
+	__u64 flags;
+	__u64 mode;
+	__u64 resolve;
+};
+
+#define OPEN_HOW_SIZE_VER0	24 /* sizeof first published struct */
+#define OPEN_HOW_SIZE_LATEST	OPEN_HOW_SIZE_VER0
+
+bool needs_openat2(const struct open_how *how);
+
+#ifndef RESOLVE_IN_ROOT
+/* how->resolve flags for openat2(2). */
+#define RESOLVE_NO_XDEV		0x01 /* Block mount-point crossings
+					(includes bind-mounts). */
+#define RESOLVE_NO_MAGICLINKS	0x02 /* Block traversal through procfs-style
+					"magic-links". */
+#define RESOLVE_NO_SYMLINKS	0x04 /* Block traversal through all symlinks
+					(implies OEXT_NO_MAGICLINKS) */
+#define RESOLVE_BENEATH		0x08 /* Block "lexical" trickery like
+					"..", symlinks, and absolute
+					paths which escape the dirfd. */
+#define RESOLVE_IN_ROOT		0x10 /* Make all jumps to "/" and ".."
+					be scoped inside the dirfd
+					(similar to chroot(2)). */
+#endif /* RESOLVE_IN_ROOT */
+
+#define E_func(func, ...)						\
+	do {								\
+		if (func(__VA_ARGS__) < 0)				\
+			ksft_exit_fail_msg("%s:%d %s failed\n", \
+					   __FILE__, __LINE__, #func);\
+	} while (0)
+
+#define E_asprintf(...)		E_func(asprintf,	__VA_ARGS__)
+#define E_chmod(...)		E_func(chmod,		__VA_ARGS__)
+#define E_dup2(...)		E_func(dup2,		__VA_ARGS__)
+#define E_fchdir(...)		E_func(fchdir,		__VA_ARGS__)
+#define E_fstatat(...)		E_func(fstatat,		__VA_ARGS__)
+#define E_kill(...)		E_func(kill,		__VA_ARGS__)
+#define E_mkdirat(...)		E_func(mkdirat,		__VA_ARGS__)
+#define E_mount(...)		E_func(mount,		__VA_ARGS__)
+#define E_prctl(...)		E_func(prctl,		__VA_ARGS__)
+#define E_readlink(...)		E_func(readlink,	__VA_ARGS__)
+#define E_setresuid(...)	E_func(setresuid,	__VA_ARGS__)
+#define E_symlinkat(...)	E_func(symlinkat,	__VA_ARGS__)
+#define E_touchat(...)		E_func(touchat,		__VA_ARGS__)
+#define E_unshare(...)		E_func(unshare,		__VA_ARGS__)
+
+#define E_assert(expr, msg, ...)					\
+	do {								\
+		if (!(expr))						\
+			ksft_exit_fail_msg("ASSERT(%s:%d) failed (%s): " msg "\n", \
+					   __FILE__, __LINE__, #expr, ##__VA_ARGS__); \
+	} while (0)
+
+int raw_openat2(int dfd, const char *path, void *how, size_t size);
+int sys_openat2(int dfd, const char *path, struct open_how *how);
+int sys_openat(int dfd, const char *path, struct open_how *how);
+int sys_renameat2(int olddirfd, const char *oldpath,
+		  int newdirfd, const char *newpath, unsigned int flags);
+
+int touchat(int dfd, const char *path);
+char *fdreadlink(int fd);
+bool fdequal(int fd, int dfd, const char *path);
+
+extern bool openat2_supported;
+
+#endif /* __RESOLVEAT_H__ */