xref: /linux/tools/testing/selftests/filesystems/failfs/failfs_test.c (revision df4b2889ea9a1ef809bfb7e1401e2cdc475956c8)

// SPDX-License-Identifier: GPL-2.0
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <link.h>
#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/vfs.h>
#include <sys/wait.h>
#include <unistd.h>

#include "../../kselftest_harness.h"

#ifndef __NR_fchroot
#define __NR_fchroot 472
#endif

#ifndef FD_PIDFS_ROOT
#define FD_PIDFS_ROOT -10002
#endif

#ifndef FD_NSFS_ROOT
#define FD_NSFS_ROOT -10003
#endif

#ifndef FD_FAILFS_ROOT
#define FD_FAILFS_ROOT -10004
#endif

#define NOBODY_UID 65534

/* Child sentinel exit code: the exec was blocked as expected. */
#define FAILFS_EXEC_BLOCKED 99

/* Stack for the CLONE_FS helper in fchroot_sentinel_shared_fs_struct. */
#define FAILFS_CLONE_STACK (64 * 1024)

static int sys_fchroot(int fd, unsigned int flags)
{
	return syscall(__NR_fchroot, fd, flags);
}

/*
 * Raw syscall: glibc's getcwd() rejects the kernel's "(unreachable)"
 * result and falls back to a generic implementation.
 */
static long sys_getcwd(char *buf, size_t size)
{
	return syscall(__NR_getcwd, buf, size);
}

static int drop_to_nobody(void)
{
	return setresuid(NOBODY_UID, NOBODY_UID, NOBODY_UID);
}

/* Parked CLONE_FS child; dies with its parent so it never leaks. */
static int failfs_park(void *arg)
{
	pid_t parent = (pid_t)(long)arg;

	prctl(PR_SET_PDEATHSIG, SIGKILL);
	/* The parent may have died before the death signal was armed. */
	if (getppid() != parent)
		_exit(0);
	pause();
	return 0;
}

/* Is fd a dynamically linked ELF with an absolute PT_INTERP interpreter? */
static int elf_has_absolute_interp(int fd)
{
	ElfW(Ehdr) ehdr;
	ElfW(Phdr) phdr;
	char interp;
	int i;

	if (pread(fd, &ehdr, sizeof(ehdr), 0) != sizeof(ehdr))
		return 0;
	if (memcmp(ehdr.e_ident, ELFMAG, SELFMAG) != 0)
		return 0;

	for (i = 0; i < ehdr.e_phnum; i++) {
		if (pread(fd, &phdr, sizeof(phdr),
			  ehdr.e_phoff + i * sizeof(phdr)) != sizeof(phdr))
			return 0;
		if (phdr.p_type != PT_INTERP)
			continue;
		if (pread(fd, &interp, 1, phdr.p_offset) != 1)
			return 0;
		return interp == '/';
	}

	return 0;
}

TEST(fchdir_sentinel)
{
	char buf[PATH_MAX];
	int fd;

	ASSERT_EQ(fchdir(FD_FAILFS_ROOT), 0);

	/* The working directory is unreachable from the process root. */
	ASSERT_GT(sys_getcwd(buf, sizeof(buf)), 0);
	ASSERT_EQ(strncmp(buf, "(unreachable)", 13), 0);

	/* Every AT_FDCWD-relative lookup fails. */
	ASSERT_EQ(openat(AT_FDCWD, "foo", O_RDONLY), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);
	ASSERT_EQ(openat(AT_FDCWD, ".", O_RDONLY), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);
	ASSERT_EQ(openat(AT_FDCWD, "..", O_RDONLY), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);
	ASSERT_EQ(openat(AT_FDCWD, "foo", O_WRONLY | O_CREAT, 0600), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/* The cwd cannot be pinned by following /proc/self/cwd into it. */
	ASSERT_EQ(open("/proc/self/cwd", O_PATH), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/* The root is untouched so absolute lookups keep working... */
	fd = open("/", O_RDONLY | O_DIRECTORY);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(close(fd), 0);

	/* ... and the working directory can be recovered. */
	ASSERT_EQ(chdir("/"), 0);
	ASSERT_GT(sys_getcwd(buf, sizeof(buf)), 0);
	ASSERT_EQ(strcmp(buf, "/"), 0);
}

TEST(fchdir_rejects_other_sentinels)
{
	ASSERT_EQ(fchdir(FD_PIDFS_ROOT), -1);
	ASSERT_EQ(errno, EBADF);
	ASSERT_EQ(fchdir(FD_NSFS_ROOT), -1);
	ASSERT_EQ(errno, EBADF);
	ASSERT_EQ(fchdir(-10009), -1);
	ASSERT_EQ(errno, EBADF);
}

TEST(fchroot_flags)
{
	int fd;

	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 1), -1);
	ASSERT_EQ(errno, EINVAL);

	fd = open("/", O_PATH | O_DIRECTORY);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(sys_fchroot(fd, 1), -1);
	ASSERT_EQ(errno, EINVAL);
	ASSERT_EQ(close(fd), 0);
}

TEST(fchroot_bad_fd)
{
	ASSERT_EQ(sys_fchroot(-1, 0), -1);
	ASSERT_EQ(errno, EBADF);

	/* Only FD_FAILFS_ROOT is a valid sentinel. */
	ASSERT_EQ(sys_fchroot(FD_PIDFS_ROOT, 0), -1);
	ASSERT_EQ(errno, EBADF);
	ASSERT_EQ(sys_fchroot(FD_NSFS_ROOT, 0), -1);
	ASSERT_EQ(errno, EBADF);
}

TEST(fchroot_notdir)
{
	int fd;

	fd = open("/proc/self/status", O_RDONLY);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(sys_fchroot(fd, 0), -1);
	ASSERT_EQ(errno, ENOTDIR);
	ASSERT_EQ(close(fd), 0);
}

TEST(fchroot_realfd_requires_cap)
{
	int fd;

	if (geteuid() == 0)
		ASSERT_EQ(drop_to_nobody(), 0);

	fd = open("/", O_PATH | O_DIRECTORY);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(sys_fchroot(fd, 0), -1);
	ASSERT_EQ(errno, EPERM);
	ASSERT_EQ(close(fd), 0);
}

TEST(fchroot_realfd)
{
	char template[] = "/tmp/failfs_test.XXXXXX";
	char path[PATH_MAX];
	struct stat st;
	int tmpfd, dfd, fd;

	if (geteuid() != 0)
		SKIP(return, "fchroot() with a regular fd requires CAP_SYS_CHROOT");

	tmpfd = open("/tmp", O_PATH | O_DIRECTORY);
	ASSERT_GE(tmpfd, 0);

	ASSERT_NE(mkdtemp(template), NULL);
	snprintf(path, sizeof(path), "%s/canary", template);
	fd = open(path, O_WRONLY | O_CREAT, 0600);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(close(fd), 0);

	dfd = open(template, O_PATH | O_DIRECTORY);
	ASSERT_GE(dfd, 0);
	ASSERT_EQ(sys_fchroot(dfd, 0), 0);
	ASSERT_EQ(close(dfd), 0);

	ASSERT_EQ(stat("/canary", &st), 0);

	/* Best-effort cleanup: dirfd-anchored I/O works with the new root. */
	snprintf(path, sizeof(path), "%s/canary", template + strlen("/tmp/"));
	unlinkat(tmpfd, path, 0);
	unlinkat(tmpfd, template + strlen("/tmp/"), AT_REMOVEDIR);
}

TEST(fchroot_sentinel)
{
	char template[] = "/tmp/failfs_test.XXXXXX";
	struct stat realroot, st;
	struct statfs sfs;
	char buf[PATH_MAX];
	int procfd, tmpfd, dfd, fd;
	struct {
		struct file_handle handle;
		unsigned char f_handle[MAX_HANDLE_SZ];
	} fh;
	int mntid;
	ssize_t ret;

	if (geteuid() != 0)
		SKIP(return, "privileged fchroot(FD_FAILFS_ROOT) requires CAP_SYS_CHROOT");

	ASSERT_EQ(stat("/", &realroot), 0);
	procfd = open("/proc", O_PATH | O_DIRECTORY);
	ASSERT_GE(procfd, 0);
	tmpfd = open("/tmp", O_PATH | O_DIRECTORY);
	ASSERT_GE(tmpfd, 0);
	ASSERT_NE(mkdtemp(template), NULL);
	dfd = open(template, O_RDONLY | O_DIRECTORY);
	ASSERT_GE(dfd, 0);

	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), 0);

	/* Absolute lookups fail. */
	ASSERT_EQ(open("/etc/passwd", O_RDONLY), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);
	ASSERT_EQ(mkdir("/foo", 0700), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/*
	 * The root cannot be referenced at all - not even an O_PATH open,
	 * which skips ->permission(), because it lands on the root as a
	 * jumped walk terminal that ->d_weak_revalidate() refuses.
	 */
	ASSERT_EQ(open("/", O_RDONLY | O_DIRECTORY), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);
	ASSERT_EQ(open("/", O_PATH), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);
	ASSERT_EQ(statfs("/", &sfs), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/*
	 * It cannot be pinned by following /proc/self/root into it either
	 * (only the root is in failfs here, so self/cwd is still real).
	 */
	ASSERT_EQ(openat(procfd, "self/root", O_PATH), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/* Nor encoded into a file handle. */
	fh.handle.handle_bytes = MAX_HANDLE_SZ;
	ASSERT_EQ(name_to_handle_at(AT_FDCWD, "/", &fh.handle, &mntid, 0), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/* The working directory is now unreachable from the root. */
	ASSERT_GT(sys_getcwd(buf, sizeof(buf)), 0);
	ASSERT_EQ(strncmp(buf, "(unreachable)", 13), 0);

	/* Lookups anchored at real directories keep working. */
	fd = openat(AT_FDCWD, ".", O_RDONLY | O_DIRECTORY);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(close(fd), 0);
	fd = openat(dfd, "canary", O_WRONLY | O_CREAT, 0600);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(write(fd, "x", 1), 1);
	ASSERT_EQ(close(fd), 0);
	fd = openat(dfd, "canary", O_RDONLY);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(close(fd), 0);

	/* ".." walks clamp at the top of the mount tree, not at failfs. */
	fd = openat(AT_FDCWD, "../../../../../../../../../..", O_PATH);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(fstat(fd, &st), 0);
	ASSERT_EQ(st.st_dev, realroot.st_dev);
	ASSERT_EQ(st.st_ino, realroot.st_ino);
	ASSERT_EQ(close(fd), 0);

	/* readlink of the magic link still works: it does not follow. */
	ret = readlinkat(procfd, "self/root", buf, sizeof(buf) - 1);
	ASSERT_GT(ret, 0);
	buf[ret] = '\0';
	TH_LOG("/proc/self/root points to '%s'", buf);
	/* d_path() names the failfs root synthetically, never as a real path. */
	ASSERT_EQ(strcmp(buf, "failfs:/"), 0);

	/* But following it into failfs is refused. */
	ASSERT_EQ(fstatat(procfd, "self/root", &st, 0), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/* Best-effort cleanup via the pre-opened dirfds. */
	unlinkat(dfd, "canary", 0);
	unlinkat(tmpfd, template + strlen("/tmp/"), AT_REMOVEDIR);
}

TEST(fchroot_sentinel_absolute_symlink)
{
	char template[] = "/tmp/failfs_test.XXXXXX";
	int tmpfd, dfd, fd;

	if (geteuid() != 0)
		SKIP(return, "privileged fchroot(FD_FAILFS_ROOT) requires CAP_SYS_CHROOT");

	tmpfd = open("/tmp", O_PATH | O_DIRECTORY);
	ASSERT_GE(tmpfd, 0);
	ASSERT_NE(mkdtemp(template), NULL);
	dfd = open(template, O_RDONLY | O_DIRECTORY);
	ASSERT_GE(dfd, 0);

	fd = openat(dfd, "target", O_WRONLY | O_CREAT, 0600);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(close(fd), 0);
	ASSERT_EQ(symlinkat("target", dfd, "rel"), 0);
	ASSERT_EQ(symlinkat("/etc", dfd, "abs"), 0);

	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), 0);

	/* Relative symlinks keep resolving within the dirfd-anchored walk... */
	fd = openat(dfd, "rel", O_RDONLY);
	ASSERT_GE(fd, 0);
	ASSERT_EQ(close(fd), 0);

	/* ... absolute symlinks restart the walk at the failfs root. */
	ASSERT_EQ(openat(dfd, "abs", O_RDONLY), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/* Best-effort cleanup via the pre-opened dirfds. */
	unlinkat(dfd, "abs", 0);
	unlinkat(dfd, "rel", 0);
	unlinkat(dfd, "target", 0);
	unlinkat(tmpfd, template + strlen("/tmp/"), AT_REMOVEDIR);
}

TEST(fchroot_sentinel_unprivileged)
{
	char buf[PATH_MAX];

	if (geteuid() == 0)
		ASSERT_EQ(drop_to_nobody(), 0);

	/* Without no_new_privs entering failfs is not allowed... */
	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), -1);
	ASSERT_EQ(errno, EPERM);

	/* ... with no_new_privs set it is allowed. */
	ASSERT_EQ(prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0), 0);
	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), 0);

	ASSERT_EQ(open("/etc/passwd", O_RDONLY), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/* The task counts as chrooted: no user namespaces anymore. */
	ASSERT_EQ(unshare(CLONE_NEWUSER), -1);
	ASSERT_EQ(errno, EPERM);

	/* With both root and cwd in failfs getcwd() reports "/". */
	ASSERT_EQ(fchdir(FD_FAILFS_ROOT), 0);
	ASSERT_GT(sys_getcwd(buf, sizeof(buf)), 0);
	ASSERT_EQ(strcmp(buf, "/"), 0);
}

TEST(fchroot_sentinel_rejected_when_chrooted)
{
	char template[] = "/tmp/failfs_test.XXXXXX";
	int tmpfd;

	if (geteuid() != 0)
		SKIP(return, "chroot() requires CAP_SYS_CHROOT");

	tmpfd = open("/tmp", O_PATH | O_DIRECTORY);
	ASSERT_GE(tmpfd, 0);
	ASSERT_NE(mkdtemp(template), NULL);
	ASSERT_EQ(chroot(template), 0);
	ASSERT_EQ(chdir("/"), 0);

	/* Remove the jail while still privileged; sticky /tmp blocks nobody. */
	unlinkat(tmpfd, template + strlen("/tmp/"), AT_REMOVEDIR);

	ASSERT_EQ(drop_to_nobody(), 0);
	ASSERT_EQ(prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0), 0);

	/* An unprivileged chrooted task must not lift its ".." barrier. */
	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), -1);
	ASSERT_EQ(errno, EPERM);
}

TEST(fchroot_sentinel_shared_fs_struct)
{
	char stack[FAILFS_CLONE_STACK];
	pid_t pid;

	if (geteuid() == 0)
		ASSERT_EQ(drop_to_nobody(), 0);

	/* A CLONE_FS sibling shares the fs_struct: bump fs->users to 2. */
	pid = clone(failfs_park, stack + sizeof(stack), CLONE_FS | SIGCHLD,
		    (void *)(long)getpid());
	ASSERT_GE(pid, 0);

	ASSERT_EQ(prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0), 0);

	/*
	 * A sibling without no_new_privs could exec a setuid binary with
	 * the failfs root, so a shared fs_struct is refused even with
	 * no_new_privs set.
	 */
	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), -1);
	ASSERT_EQ(errno, EINVAL);

	ASSERT_EQ(kill(pid, SIGKILL), 0);
	ASSERT_EQ(waitpid(pid, NULL, 0), pid);
}

TEST(fchroot_sentinel_no_overmount)
{
	if (geteuid() != 0)
		SKIP(return, "mounting requires privileges");

	/*
	 * Contain the blast radius: if failfs ever regressed and "/"
	 * resolved to the real root, the tmpfs mount below must not touch
	 * the host. A private mount namespace keeps it local to this child.
	 */
	ASSERT_EQ(unshare(CLONE_NEWNS), 0);
	ASSERT_EQ(mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL), 0);

	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), 0);

	/*
	 * Nothing can be mounted on top of the failfs root. It cannot even
	 * be named as a mount target: resolving "/" is refused before the
	 * mount machinery (which, failfs being in no mount namespace, would
	 * reject it anyway) is ever reached. open_tree(OPEN_TREE_CLONE) is
	 * likewise moot since no fd to the root can be obtained.
	 */
	ASSERT_EQ(mount("none", "/", "tmpfs", 0, NULL), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);
}

TEST(fchroot_sentinel_setns_escape)
{
	struct stat realroot, st;
	int nsfd;

	if (geteuid() != 0)
		SKIP(return, "setns() to a mount namespace requires privileges");

	ASSERT_EQ(stat("/", &realroot), 0);
	nsfd = open("/proc/self/ns/mnt", O_RDONLY);
	ASSERT_GE(nsfd, 0);

	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), 0);
	ASSERT_EQ(open("/etc", O_PATH), -1);
	ASSERT_EQ(errno, EOPNOTSUPP);

	/* A mount namespace fd is the key out: it resets root and cwd. */
	ASSERT_EQ(setns(nsfd, CLONE_NEWNS), 0);
	ASSERT_EQ(close(nsfd), 0);

	ASSERT_EQ(stat("/", &st), 0);
	ASSERT_EQ(st.st_dev, realroot.st_dev);
	ASSERT_EQ(st.st_ino, realroot.st_ino);
}

TEST(fchroot_sentinel_exec)
{
	pid_t pid;
	int status;

	if (geteuid() != 0)
		SKIP(return, "privileged fchroot(FD_FAILFS_ROOT) requires CAP_SYS_CHROOT");

	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), 0);

	/*
	 * Exec in a child: a wrongly successful exec would replace the test
	 * image and its exit code would not match the sentinel below.
	 */
	pid = fork();
	ASSERT_GE(pid, 0);
	if (pid == 0) {
		execl("/bin/true", "true", NULL);
		_exit(errno == EOPNOTSUPP ? FAILFS_EXEC_BLOCKED : 1);
	}
	ASSERT_EQ(waitpid(pid, &status, 0), pid);
	ASSERT_TRUE(WIFEXITED(status));
	ASSERT_EQ(WEXITSTATUS(status), FAILFS_EXEC_BLOCKED);
}

TEST(fchroot_sentinel_exec_interpreter)
{
	static const char * const argv[] = { "failfs_test", NULL };
	static const char * const envp[] = { NULL };
	pid_t pid;
	int status, exefd;

	if (geteuid() != 0)
		SKIP(return, "privileged fchroot(FD_FAILFS_ROOT) requires CAP_SYS_CHROOT");

	/* Exec ourselves: the one binary guaranteed to be around. */
	exefd = open("/proc/self/exe", O_RDONLY);
	ASSERT_GE(exefd, 0);
	if (!elf_has_absolute_interp(exefd))
		SKIP(return, "test binary has no absolute PT_INTERP interpreter");

	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), 0);

	/*
	 * The binary itself needs no path lookup - it is executed by fd -
	 * but loading it fails on opening the absolute PT_INTERP
	 * interpreter. Run it in a child so a wrongly successful exec does
	 * not replace the test image and masquerade as a pass.
	 */
	pid = fork();
	ASSERT_GE(pid, 0);
	if (pid == 0) {
		syscall(__NR_execveat, exefd, "", argv, envp, AT_EMPTY_PATH);
		_exit(errno == EOPNOTSUPP ? FAILFS_EXEC_BLOCKED : 1);
	}
	ASSERT_EQ(waitpid(pid, &status, 0), pid);
	ASSERT_TRUE(WIFEXITED(status));
	ASSERT_EQ(WEXITSTATUS(status), FAILFS_EXEC_BLOCKED);
}

TEST(fchroot_sentinel_inherited)
{
	pid_t pid;
	int status;

	if (geteuid() != 0)
		SKIP(return, "privileged fchroot(FD_FAILFS_ROOT) requires CAP_SYS_CHROOT");

	ASSERT_EQ(sys_fchroot(FD_FAILFS_ROOT, 0), 0);

	pid = fork();
	ASSERT_GE(pid, 0);
	if (pid == 0) {
		if (open("/etc", O_PATH) != -1 || errno != EOPNOTSUPP)
			_exit(1);
		_exit(0);
	}
	ASSERT_EQ(waitpid(pid, &status, 0), pid);
	ASSERT_TRUE(WIFEXITED(status));
	ASSERT_EQ(WEXITSTATUS(status), 0);
}

TEST_HARNESS_MAIN