Re: [PATCH v3 3/3] getcwd: Set errno to ERANGE for size == 1 (CVE-2021-3999)

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 20/01/2022 06:32, Siddhesh Poyarekar wrote:
> No valid path returned by getcwd would fit into 1 byte, so reject the
> size early and return NULL with errno set to ERANGE.  This change is
> prompted by CVE-2021-3999, which describes a single byte buffer
> underflow and overflow when all of the following conditions are met:
> 
> - The buffer size (i.e. the second argument of getcwd) is 1 byte
> - The current working directory is too long
> - '/' is also mounted on the current working directory
> 
> Sequence of events:
> 
> - In sysdeps/unix/sysv/linux/getcwd.c, the syscall returns ENAMETOOLONG
>   because the linux kernel checks for name length before it checks
>   buffer size
> 
> - The code falls back to the generic getcwd in sysdeps/posix
> 
> - In the generic func, the buf[0] is set to '\0' on line 250
> 
> - this while loop on line 262 is bypassed:
> 
>     while (!(thisdev == rootdev && thisino == rootino))
> 
>   since the rootfs (/) is bind mounted onto the directory and the flow
>   goes on to line 449, where it puts a '/' in the byte before the
>   buffer.
> 
> - Finally on line 458, it moves 2 bytes (the underflowed byte and the
>   '\0') to the buf[0] and buf[1], resulting in a 1 byte buffer overflow.
> 
> - buf is returned on line 469 and errno is not set.
> 
> This resolves BZ #28769.
> 
> Signed-off-by: Qualys Security Advisory <qsa@qualys.com>
> Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

Look good with just two fixed below for CMSG_DATA and a couple of comments.
Ok with the fixes, the comments would be good but it is a blocker.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  NEWS                                          |   6 +
>  sysdeps/posix/getcwd.c                        |   7 +
>  sysdeps/unix/sysv/linux/Makefile              |   7 +-
>  .../unix/sysv/linux/tst-getcwd-smallbuff.c    | 245 ++++++++++++++++++
>  4 files changed, 264 insertions(+), 1 deletion(-)
>  create mode 100644 sysdeps/unix/sysv/linux/tst-getcwd-smallbuff.c
> 
> diff --git a/NEWS b/NEWS
> index 4c392a445e..07e9eac52d 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -170,6 +170,12 @@ Security related changes:
>    function could result in a memory leak and potential access of
>    uninitialized memory.  Reported by Qualys.
>  
> +  CVE-2021-3999: Passing a buffer of size exactly 1 byte to the getcwd
> +  function may result in an off-by-one buffer underflow and overflow
> +  when the current working directory is longer than PATH_MAX and also
> +  corresponds to the / directory through an unprivileged mount
> +  namespace.  Reported by Qualys.
> +
>  The following bugs are resolved with this release:
>  
>    [The release manager will add the list generated by
> diff --git a/sysdeps/posix/getcwd.c b/sysdeps/posix/getcwd.c
> index e147a31a81..9d5787b6f4 100644
> --- a/sysdeps/posix/getcwd.c
> +++ b/sysdeps/posix/getcwd.c
> @@ -187,6 +187,13 @@ __getcwd_generic (char *buf, size_t size)
>    size_t allocated = size;
>    size_t used;
>  
> +  /* A size of 1 byte is never useful.  */
> +  if (allocated == 1)
> +    {
> +      __set_errno (ERANGE);
> +      return NULL;
> +    }
> +
>  #if HAVE_MINIMALLY_WORKING_GETCWD
>    /* If AT_FDCWD is not defined, the algorithm below is O(N**2) and
>       this is much slower than the system getcwd (at least on
> diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
> index 85fc8cbf75..7ca9350c99 100644
> --- a/sysdeps/unix/sysv/linux/Makefile
> +++ b/sysdeps/unix/sysv/linux/Makefile
> @@ -346,7 +346,12 @@ sysdep_routines += xstatconv internal_statvfs \
>  
>  sysdep_headers += bits/fcntl-linux.h
>  
> -tests += tst-fallocate tst-fallocate64 tst-o_path-locks
> +tests += \
> +  tst-fallocate \
> +  tst-fallocate64 \
> +  tst-getcwd-smallbuff \
> +  tst-o_path-locks \
> +# tests
>  endif
>  
>  ifeq ($(subdir),elf)
> diff --git a/sysdeps/unix/sysv/linux/tst-getcwd-smallbuff.c b/sysdeps/unix/sysv/linux/tst-getcwd-smallbuff.c
> new file mode 100644
> index 0000000000..791dfe4d02
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/tst-getcwd-smallbuff.c
> @@ -0,0 +1,245 @@
> +/* 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 <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/xsched.h>
> +#include <support/xunistd.h>
> +
> +#ifndef PATH_MAX
> +# define PATH_MAX 1024
> +#endif

No need since it is a Linux only test and PATH_MAX is always defined.

> +
> +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;
> +  union
> +    {
> +      struct cmsghdr hdr;
> +      char buf[CMSG_SPACE (sizeof (int))];
> +    } cmsgbuf;

Maybe zero-initialize both first to avoid the memset below?

> +  struct cmsghdr *cmsg;
> +  struct iovec vec;
> +  char ch = 'A';
> +  ssize_t n;
> +
> +  memset (&msg, 0, sizeof (msg));
> +  memset (&cmsgbuf, 0, sizeof (cmsgbuf));
> +  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;
> +  *(int *) CMSG_DATA (cmsg) = fd;

I think CMSG_DATA does not guarantee the alignment, so I think it would be 
safe to use memcpy here:

  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);
> +}
> +

Ok.

> +static int
> +recv_fd (const int sock)
> +{
> +  struct msghdr msg;

Maybe also zero-initialize here.

> +  union
> +    {
> +      struct cmsghdr hdr;
> +      char buf[CMSG_SPACE(sizeof(int))];
> +    } cmsgbuf;
> +  struct cmsghdr *cmsg;
> +  struct iovec vec;
> +  ssize_t n;
> +  char ch = '\0';
> +  int fd = -1;
> +
> +  memset (&msg, 0, sizeof (msg));
> +  vec.iov_base = &ch;
> +  vec.iov_len = 1;
> +  msg.msg_iov = &vec;
> +  msg.msg_iovlen = 1;
> +
> +  memset (&cmsgbuf, 0, sizeof (cmsgbuf));
> +  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;
> +  fd = *(const int *) CMSG_DATA (cmsg);

Same as before, I think you will need to copy to a temporary using memcpy.

> +  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[64];


Maybe define the size as:

  /* The path is the form /proc/%ld/setgroups.  */
  char map_path[sizeof("/proc/setgroups") + INT_STRLEN_BOUND (long int)];


> +  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);
> +
> +  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[64], map_buf[64];

Same comment as for setgroups_path.

> +  snprintf (map_path, sizeof (map_path),  ,
> +	    (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));


Space before (.

> +
> +  /* 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>