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// SPDX-License-Identifier: GPL-2.0
/*
* A recursive bind mount of a namespace file that lives in another mount
* namespace copies whatever is stacked on top of it there. If that includes
* the file of the caller's own mount namespace, or of an older one, the copy
* would pin the namespace it is put in forever. The bind mount has to be
* refused, a plain bind mount of the file itself still works.
*/
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include "../../kselftest_harness.h"
#define DIR_LEN 64
#define PATH_LEN 128
/* Child exit codes. */
enum {
CHILD_OK,
CHILD_UNSHARE, /* could not create the newer mount namespace */
CHILD_PIPE, /* the parent went away */
CHILD_TMPFS, /* could not mount the tmpfs in the new namespace */
CHILD_REC_ALLOWED, /* the recursive bind mount was not refused */
CHILD_REC_ERRNO, /* it was refused with the wrong error */
CHILD_PLAIN_REFUSED, /* the plain bind mount of the file was refused */
};
static int write_file(const char *path, const char *s)
{
ssize_t n = -1;
int fd;
fd = open(path, O_WRONLY | O_CLOEXEC);
if (fd >= 0) {
n = write(fd, s, strlen(s));
close(fd);
}
return n == (ssize_t)strlen(s) ? 0 : -1;
}
static int create_file(const char *path)
{
int fd = open(path, O_WRONLY | O_CREAT | O_CLOEXEC, 0644);
if (fd < 0)
return -1;
close(fd);
return 0;
}
/* Become root in a new user namespace with a private mount namespace. */
static int enter_userns(void)
{
uid_t uid = getuid();
gid_t gid = getgid();
char map[32];
if (unshare(CLONE_NEWUSER | CLONE_NEWNS))
return -1;
if (write_file("/proc/self/setgroups", "deny") && errno != ENOENT)
return -1;
snprintf(map, sizeof(map), "0 %d 1", uid);
if (write_file("/proc/self/uid_map", map))
return -1;
snprintf(map, sizeof(map), "0 %d 1", gid);
if (write_file("/proc/self/gid_map", map))
return -1;
if (setgid(0) || setuid(0))
return -1;
return mount("", "/", NULL, MS_REC | MS_PRIVATE, NULL);
}
static int send_msg(int fd, char msg)
{
return write(fd, &msg, 1) == 1 ? 0 : -1;
}
static char recv_msg(int fd)
{
char msg;
if (read(fd, &msg, 1) != 1)
return 0;
return msg;
}
FIXTURE(nsfs_rbind_loop) {
char dir[DIR_LEN];
char x[PATH_LEN];
};
FIXTURE_SETUP(nsfs_rbind_loop)
{
snprintf(self->dir, sizeof(self->dir), "/tmp/nsfs_rbind_loop.XXXXXX");
ASSERT_NE(mkdtemp(self->dir), NULL);
if (enter_userns()) {
rmdir(self->dir);
SKIP(return, "test requires user namespaces");
}
ASSERT_EQ(mount("tmpfs", self->dir, "tmpfs", 0, NULL), 0);
snprintf(self->x, sizeof(self->x), "%s/x", self->dir);
ASSERT_EQ(create_file(self->x), 0);
}
FIXTURE_TEARDOWN(nsfs_rbind_loop)
{
umount2(self->dir, MNT_DETACH);
rmdir(self->dir);
}
/*
* The child in the newer mount namespace binds the network namespace file
* mount of the parent through @fd. Recursively that would copy the mount of
* its own mount namespace file that the parent stacked on top.
*/
static int newer_ns_child(const char *dir, int fd, int to_parent, int from_parent)
{
char src[32], y[PATH_LEN];
if (unshare(CLONE_NEWNS))
return CHILD_UNSHARE;
if (send_msg(to_parent, 'r') || recv_msg(from_parent) != 'g')
return CHILD_PIPE;
snprintf(y, sizeof(y), "%s/y", dir);
if (mount("tmpfs", y, "tmpfs", 0, NULL))
return CHILD_TMPFS;
snprintf(src, sizeof(src), "/proc/self/fd/%d", fd);
snprintf(y, sizeof(y), "%s/y/f", dir);
if (create_file(y))
return CHILD_TMPFS;
if (!mount(src, y, NULL, MS_BIND | MS_REC, NULL))
return CHILD_REC_ALLOWED;
if (errno != EINVAL)
return CHILD_REC_ERRNO;
if (mount(src, y, NULL, MS_BIND, NULL))
return CHILD_PLAIN_REFUSED;
umount2(y, MNT_DETACH);
return CHILD_OK;
}
TEST_F(nsfs_rbind_loop, own_ns_file_below_foreign_source)
{
int to_child[2], to_parent[2], fd, status;
char p[PATH_LEN];
pid_t pid;
snprintf(p, sizeof(p), "%s/y", self->dir);
ASSERT_EQ(mkdir(p, 0755), 0);
/* M, a mount of our network namespace file, held by a descriptor */
ASSERT_EQ(mount("/proc/self/ns/net", self->x, NULL, MS_BIND, NULL), 0);
fd = open(self->x, O_PATH | O_CLOEXEC);
ASSERT_GE(fd, 0);
ASSERT_EQ(pipe(to_child), 0);
ASSERT_EQ(pipe(to_parent), 0);
pid = fork();
ASSERT_GE(pid, 0);
if (pid == 0) {
close(to_child[1]);
close(to_parent[0]);
_exit(newer_ns_child(self->dir, fd, to_parent[1], to_child[0]));
}
close(to_child[0]);
close(to_parent[1]);
ASSERT_EQ(recv_msg(to_parent[0]), 'r');
/* the child's mount namespace file on top of M */
snprintf(p, sizeof(p), "/proc/%d/ns/mnt", pid);
ASSERT_EQ(mount(p, self->x, NULL, MS_BIND, NULL), 0);
ASSERT_EQ(send_msg(to_child[1], 'g'), 0);
ASSERT_EQ(waitpid(pid, &status, 0), pid);
ASSERT_TRUE(WIFEXITED(status));
ASSERT_EQ(WEXITSTATUS(status), CHILD_OK);
close(fd);
ASSERT_EQ(umount2(self->x, MNT_DETACH), 0);
ASSERT_EQ(umount2(self->x, MNT_DETACH), 0);
}
TEST_HARNESS_MAIN
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