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

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

On 21/01/2022 22:11, Adhemerval Zanella wrote:
> 
> 
> 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.

Ahh, I didn't actually need it anyway, leftover from a previous 
iteration.  I'll remove it.

> 
>> +
>> +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?

OK.

> 
>> +  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));

OK.

> 
>> +
>> +  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.

Why not just:

     memcpy (&fd, CMSG_DATA (cmsg), sizeof (fd));

i.e., is a temporary necessary?

> 
>> +  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)];
> 

OK.

>> +  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>
>