commit 472e799a5f2102bc0c3206dbd5a801765fceb39c
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>
(cherry picked from commit 23e0e8f5f1fb5ed150253d986ecccdc90c2dcd5e)
diff --git a/sysdeps/posix/getcwd.c b/sysdeps/posix/getcwd.c
index 13680026ffecbd51..b6984a382c3e1711 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 d30d21898b402d1e..cdc01a3f023ec09a 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -342,7 +342,12 @@ 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 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>