userns_child_exec.c

/* userns_child_exec.c

Licensed under GNU General Public License v2 or later

Create a child process that executes a shell command in new
namespace(s); allow UID and GID mappings to be specified when
creating a user namespace.
*/

#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <signal.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>
#include <errno.h>

/* A simple error-handling function: print an error message based
   on the value in 'errno' and terminate the calling process */

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE);   \
  } while (0)

struct child_args {
  char **argv;        /* Command to be executed by child, with args */
  int    pipe_fd[2];  /* Pipe used to synchronize parent and child */
};

static int verbose;

static void
usage(char *pname)
{
  fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
  fprintf(stderr, "Create a child process that executes a shell "
          "command in a new user namespace,\n"
          "and possibly also other new namespace(s).\n\n");
  fprintf(stderr, "Options can be:\n\n");
#define fpe(str) fprintf(stderr, "    %s", str);
  fpe("-i          New IPC namespace\n");
  fpe("-m          New mount namespace\n");
  fpe("-n          New network namespace\n");
  fpe("-p          New PID namespace\n");
  fpe("-u          New UTS namespace\n");
  fpe("-U          New user namespace\n");
  fpe("-M uid_map  Specify UID map for user namespace\n");
  fpe("-G gid_map  Specify GID map for user namespace\n");
  fpe("-z          Map user's UID and GID to 0 in user namespace\n");
  fpe("            (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
  fpe("-v          Display verbose messages\n");
  fpe("\n");
  fpe("If -z, -M, or -G is specified, -U is required.\n");
  fpe("It is not permitted to specify both -z and either -M or -G.\n");
  fpe("\n");
  fpe("Map strings for -M and -G consist of records of the form:\n");
  fpe("\n");
  fpe("    ID-inside-ns   ID-outside-ns   len\n");
  fpe("\n");
  fpe("A map string can contain multiple records, separated"
      " by commas;\n");
  fpe("the commas are replaced by newlines before writing"
      " to map files.\n");

  exit(EXIT_FAILURE);
}

/* Update the mapping file 'map_file', with the value provided in
   'mapping', a string that defines a UID or GID mapping. A UID or
   GID mapping consists of one or more newline-delimited records
   of the form:

   ID_inside-ns    ID-outside-ns   length

   Requiring the user to supply a string that contains newlines is
   of course inconvenient for command-line use. Thus, we permit the
   use of commas to delimit records in this string, and replace them
   with newlines before writing the string to the file. */

static void
update_map(char *mapping, char *map_file)
{
  int fd, j;
  size_t map_len;     /* Length of 'mapping' */

  /* Replace commas in mapping string with newlines */

  map_len = strlen(mapping);
  for (j = 0; j < (int)map_len; j++)
    if (mapping[j] == ',')
      mapping[j] = '\n';

  fd = open(map_file, O_RDWR);
  if (fd == -1) {
    fprintf(stderr, "ERROR: open %s: %s\n", map_file,
            strerror(errno));
    exit(EXIT_FAILURE);
  }

  if (write(fd, mapping, map_len) != (int)map_len) {
    fprintf(stderr, "ERROR: write %s: %s\n", map_file,
            strerror(errno));
    exit(EXIT_FAILURE);
  }

  close(fd);
}

/* Linux 3.19 made a change in the handling of setgroups(2) and the
   'gid_map' file to address a security issue. The issue allowed
   *unprivileged* users to employ user namespaces in order to drop
   The upshot of the 3.19 changes is that in order to update the
   'gid_maps' file, use of the setgroups() system call in this
   user namespace must first be disabled by writing "deny" to one of
   the /proc/PID/setgroups files for this namespace.  That is the
   purpose of the following function. */

static void
proc_setgroups_write(pid_t child_pid, char *str)
{
  char setgroups_path[PATH_MAX];
  int fd;

  snprintf(setgroups_path, PATH_MAX, "/proc/%ld/setgroups",
           (long) child_pid);

  fd = open(setgroups_path, O_RDWR);
  if (fd == -1) {

    /* We may be on a system that doesn't support
       /proc/PID/setgroups. In that case, the file won't exist,
       and the system won't impose the restrictions that Linux 3.19
       added. That's fine: we don't need to do anything in order
       to permit 'gid_map' to be updated.

       However, if the error from open() was something other than
       the ENOENT error that is expected for that case,  let the
       user know. */

    if (errno != ENOENT)
      fprintf(stderr, "ERROR: open %s: %s\n", setgroups_path,
              strerror(errno));
    return;
  }

  if (write(fd, str, strlen(str)) == -1)
    fprintf(stderr, "ERROR: write %s: %s\n", setgroups_path,
            strerror(errno));

  close(fd);
}

static int              /* Start function for cloned child */
childFunc(void *arg)
{
  struct child_args *args = (struct child_args *) arg;
  char ch;

  /* Wait until the parent has updated the UID and GID mappings.
     See the comment in main(). We wait for end of file on a
     pipe that will be closed by the parent process once it has
     updated the mappings. */

  close(args->pipe_fd[1]);    /* Close our descriptor for the write
                                 end of the pipe so that we see EOF
                                 when parent closes its descriptor */
  if (read(args->pipe_fd[0], &ch, 1) != 0) {
    fprintf(stderr,
            "Failure in child: read from pipe returned != 0\n");
    exit(EXIT_FAILURE);
  }

  close(args->pipe_fd[0]);

  /* Execute a shell command */

  printf("About to exec %s\n", args->argv[0]);
  execvp(args->argv[0], args->argv);
  errExit("execvp");
}

#define STACK_SIZE (1024 * 1024)

static char child_stack[STACK_SIZE];    /* Space for child's stack */

int
main(int argc, char *argv[])
{
  int flags, opt, map_zero;
  pid_t child_pid;
  struct child_args args;
  char *uid_map, *gid_map;
  const int MAP_BUF_SIZE = 100;
  char map_buf[MAP_BUF_SIZE];
  char map_path[PATH_MAX];

  /* Parse command-line options. The initial '+' character in
     the final getopt() argument prevents GNU-style permutation
     of command-line options. That's useful, since sometimes
     the 'command' to be executed by this program itself
     has command-line options. We don't want getopt() to treat
     those as options to this program. */

  flags = 0;
  verbose = 0;
  gid_map = NULL;
  uid_map = NULL;
  map_zero = 0;
  while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
    switch (opt) {
    case 'i': flags |= CLONE_NEWIPC;        break;
    case 'm': flags |= CLONE_NEWNS;         break;
    case 'n': flags |= CLONE_NEWNET;        break;
    case 'p': flags |= CLONE_NEWPID;        break;
    case 'u': flags |= CLONE_NEWUTS;        break;
    case 'v': verbose = 1;                  break;
    case 'z': map_zero = 1;                 break;
    case 'M': uid_map = optarg;             break;
    case 'G': gid_map = optarg;             break;
    case 'U': flags |= CLONE_NEWUSER;       break;
    default:  usage(argv[0]);
    }
  }

  /* -M or -G without -U is nonsensical */

  if (((uid_map != NULL || gid_map != NULL || map_zero) &&
       !(flags & CLONE_NEWUSER)) ||
      (map_zero && (uid_map != NULL || gid_map != NULL)))
    usage(argv[0]);

  args.argv = &argv[optind];

  /* We use a pipe to synchronize the parent and child, in order to
     ensure that the parent sets the UID and GID maps before the child
     calls execve(). This ensures that the child maintains its
     capabilities during the execve() in the common case where we
     want to map the child's effective user ID to 0 in the new user
     namespace. Without this synchronization, the child would lose
     its capabilities if it performed an execve() with nonzero
     user IDs (see the capabilities(7) man page for details of the
     transformation of a process's capabilities during execve()). */

  if (pipe(args.pipe_fd) == -1)
    errExit("pipe");

  /* Create the child in new namespace(s) */

  child_pid = clone(childFunc, child_stack + STACK_SIZE,
                    flags | SIGCHLD, &args);
  if (child_pid == -1)
    errExit("clone");

  /* Parent falls through to here */

  if (verbose)
    printf("%s: PID of child created by clone() is %ld\n",
           argv[0], (long) child_pid);

  /* Update the UID and GID maps in the child */

  if (uid_map != NULL || map_zero) {
    snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
             (long) child_pid);
    if (map_zero) {
      snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getuid());
      uid_map = map_buf;
    }
    update_map(uid_map, map_path);
  }

  if (gid_map != NULL || map_zero) {
    proc_setgroups_write(child_pid, "deny");

    snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
             (long) child_pid);
    if (map_zero) {
      snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getgid());
      gid_map = map_buf;
    }
    update_map(gid_map, map_path);
  }

  /* Close the write end of the pipe, to signal to the child that we
     have updated the UID and GID maps */
  close(args.pipe_fd[1]);

  if (waitpid(child_pid, NULL, 0) == -1)      /* Wait for child */
    errExit("waitpid");

  if (verbose)
    printf("%s: terminating\n", argv[0]);

  exit(EXIT_SUCCESS);
}