commit 23e0e8f5f1fb5ed150253d986ecccdc90c2dcd5e

Author: Siddhesh Poyarekar <siddhesh@sourceware.org>

Date:   Fri Jan 21 23:32:56 2022 +0530

    getcwd: Set errno to ERANGE for size == 1 (CVE-2021-3999)

    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.

    Reviewed-by: Andreas Schwab <schwab@linux-m68k.org>

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

    Signed-off-by: Qualys Security Advisory <qsa@qualys.com>

    Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

# Conflicts:

#	sysdeps/posix/getcwd.c

#	sysdeps/unix/sysv/linux/Makefile

diff --git a/sysdeps/posix/getcwd.c b/sysdeps/posix/getcwd.c

index b53433a2dc77fafa..fcd7aaea79c6477b 100644

--- a/sysdeps/posix/getcwd.c

+++ b/sysdeps/posix/getcwd.c

@@ -238,6 +238,13 @@ __getcwd (char *buf, size_t size)

   bool fd_needs_closing = false;

   int fd = AT_FDCWD;

+  /* A size of 1 byte is never useful.  */

+  if (size == 1)

+    {

+      __set_errno (ERANGE);

+      return NULL;

+    }

+

   char *path;

 #ifndef NO_ALLOCATION

   size_t allocated = size;

diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile

index 688cf9fa9dea23a6..bb055f9d6b841ff5 100644

--- a/sysdeps/unix/sysv/linux/Makefile

+++ b/sysdeps/unix/sysv/linux/Makefile

@@ -180,7 +180,11 @@ sysdep_routines += xstatconv internal_statvfs internal_statvfs64 \

 sysdep_headers += bits/fcntl-linux.h

-tests += tst-fallocate tst-fallocate64

+tests += \

+  tst-fallocate \

+  tst-fallocate64 \

+  tst-getcwd-smallbuff \

+# 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 0000000000000000..d460d6e7662dc5e4

--- /dev/null

+++ b/sysdeps/unix/sysv/linux/tst-getcwd-smallbuff.c

@@ -0,0 +1,241 @@

+/* 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/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);

+

+  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>