glibc/sysdeps/unix/sysv/linux/tst-getcwd-smallbuff.c

/* Verify that getcwd returns ERANGE for size 1 byte and does not underflow
   buffer when the CWD is too long and is also a mount target of /.  See bug
   #28769 or CVE-2021-3999 for more context.
   Copyright The GNU Toolchain Authors.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#include <errno.h>
#include <fcntl.h>
#include <intprops.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>

#include <sys/socket.h>
#include <sys/un.h>
#include <support/check.h>
#include <support/temp_file.h>
#include <support/test-driver.h>
#include <support/xsched.h>
#include <support/xunistd.h>

static char *base;
#define BASENAME "tst-getcwd-smallbuff"
#define MOUNT_NAME "mpoint"
static int sockfd[2];

static void
do_cleanup (void)
{
  support_chdir_toolong_temp_directory (base);
  TEST_VERIFY_EXIT (rmdir (MOUNT_NAME) == 0);
  free (base);
}

static void
send_fd (const int sock, const int fd)
{
  struct msghdr msg = {0};
  union
    {
      struct cmsghdr hdr;
      char buf[CMSG_SPACE (sizeof (int))];
    } cmsgbuf = {0};
  struct cmsghdr *cmsg;
  struct iovec vec;
  char ch = 'A';
  ssize_t n;

  msg.msg_control = &cmsgbuf.buf;
  msg.msg_controllen = sizeof (cmsgbuf.buf);

  cmsg = CMSG_FIRSTHDR (&msg);
  cmsg->cmsg_len = CMSG_LEN (sizeof (int));
  cmsg->cmsg_level = SOL_SOCKET;
  cmsg->cmsg_type = SCM_RIGHTS;
  memcpy (CMSG_DATA (cmsg), &fd, sizeof (fd));

  vec.iov_base = &ch;
  vec.iov_len = 1;
  msg.msg_iov = &vec;
  msg.msg_iovlen = 1;

  while ((n = sendmsg (sock, &msg, 0)) == -1 && errno == EINTR);

  TEST_VERIFY_EXIT (n == 1);
}

static int
recv_fd (const int sock)
{
  struct msghdr msg = {0};
  union
    {
      struct cmsghdr hdr;
      char buf[CMSG_SPACE(sizeof(int))];
    } cmsgbuf = {0};
  struct cmsghdr *cmsg;
  struct iovec vec;
  ssize_t n;
  char ch = '\0';
  int fd = -1;

  vec.iov_base = &ch;
  vec.iov_len = 1;
  msg.msg_iov = &vec;
  msg.msg_iovlen = 1;

  msg.msg_control = &cmsgbuf.buf;
  msg.msg_controllen = sizeof (cmsgbuf.buf);

  while ((n = recvmsg (sock, &msg, 0)) == -1 && errno == EINTR);
  if (n != 1 || ch != 'A')
    return -1;

  cmsg = CMSG_FIRSTHDR (&msg);
  if (cmsg == NULL)
    return -1;
  if (cmsg->cmsg_type != SCM_RIGHTS)
    return -1;
  memcpy (&fd, CMSG_DATA (cmsg), sizeof (fd));
  if (fd < 0)
    return -1;
  return fd;
}

static int
child_func (void * const arg)
{
  xclose (sockfd[0]);
  const int sock = sockfd[1];
  char ch;

  TEST_VERIFY_EXIT (read (sock, &ch, 1) == 1);
  TEST_VERIFY_EXIT (ch == '1');

  if (mount ("/", MOUNT_NAME, NULL, MS_BIND | MS_REC, NULL))
    FAIL_EXIT1 ("mount failed: %m\n");
  const int fd = xopen ("mpoint",
			O_RDONLY | O_PATH | O_DIRECTORY | O_NOFOLLOW, 0);

  send_fd (sock, fd);
  xclose (fd);

  TEST_VERIFY_EXIT (read (sock, &ch, 1) == 1);
  TEST_VERIFY_EXIT (ch == 'a');

  xclose (sock);
  return 0;
}

static void
update_map (char * const mapping, const char * const map_file)
{
  const size_t map_len = strlen (mapping);

  const int fd = xopen (map_file, O_WRONLY, 0);
  xwrite (fd, mapping, map_len);
  xclose (fd);
}

static void
proc_setgroups_write (const long child_pid, const char * const str)
{
  const size_t str_len = strlen(str);

  char setgroups_path[sizeof ("/proc//setgroups") + INT_STRLEN_BOUND (long)];

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

  const int fd = open (setgroups_path, O_WRONLY);

  if (fd < 0)
    {
      TEST_VERIFY_EXIT (errno == ENOENT);
      FAIL_UNSUPPORTED ("/proc/%ld/setgroups not found\n", child_pid);
    }

  xwrite (fd, str, str_len);
  xclose(fd);
}

static char child_stack[1024 * 1024];

int
do_test (void)
{
  base = support_create_and_chdir_toolong_temp_directory (BASENAME);

  xmkdir (MOUNT_NAME, S_IRWXU);
  atexit (do_cleanup);

  /* Check whether user namespaces are supported.  */
  {
    pid_t pid = xfork ();
    if (pid == 0)
      {
	if (unshare (CLONE_NEWUSER | CLONE_NEWNS) != 0)
	  _exit (EXIT_UNSUPPORTED);
	else
	  _exit (0);
      }
    int status;
    xwaitpid (pid, &status, 0);
    TEST_VERIFY_EXIT (WIFEXITED (status));
    if (WEXITSTATUS (status) != 0)
      return WEXITSTATUS (status);
  }

  TEST_VERIFY_EXIT (socketpair (AF_UNIX, SOCK_STREAM, 0, sockfd) == 0);
  pid_t child_pid = xclone (child_func, NULL, child_stack,
			    sizeof (child_stack),
			    CLONE_NEWUSER | CLONE_NEWNS | SIGCHLD);

  xclose (sockfd[1]);
  const int sock = sockfd[0];

  char map_path[sizeof ("/proc//uid_map") + INT_STRLEN_BOUND (long)];
  char map_buf[sizeof ("0  1") + INT_STRLEN_BOUND (long)];

  snprintf (map_path, sizeof (map_path), "/proc/%ld/uid_map",
	    (long) child_pid);
  snprintf (map_buf, sizeof (map_buf), "0 %ld 1", (long) getuid());
  update_map (map_buf, map_path);

  proc_setgroups_write ((long) child_pid, "deny");
  snprintf (map_path, sizeof (map_path), "/proc/%ld/gid_map",
	    (long) child_pid);
  snprintf (map_buf, sizeof (map_buf), "0 %ld 1", (long) getgid());
  update_map (map_buf, map_path);

  TEST_VERIFY_EXIT (send (sock, "1", 1, MSG_NOSIGNAL) == 1);
  const int fd = recv_fd (sock);
  TEST_VERIFY_EXIT (fd >= 0);
  TEST_VERIFY_EXIT (fchdir (fd) == 0);

  static char buf[2 * 10 + 1];
  memset (buf, 'A', sizeof (buf));

  /* Finally, call getcwd and check if it resulted in a buffer underflow.  */
  char * cwd = getcwd (buf + sizeof (buf) / 2, 1);
  TEST_VERIFY (cwd == NULL);
  TEST_VERIFY (errno == ERANGE);

  for (int i = 0; i < sizeof (buf); i++)
    if (buf[i] != 'A')
      {
	printf ("buf[%d] = %02x\n", i, (unsigned int) buf[i]);
	support_record_failure ();
      }

  TEST_VERIFY_EXIT (send (sock, "a", 1, MSG_NOSIGNAL) == 1);
  xclose (sock);
  TEST_VERIFY_EXIT (xwaitpid (child_pid, NULL, 0) == child_pid);

  return 0;
}

#define CLEANUP_HANDLER do_cleanup
#include <support/test-driver.c>