From 52a713fdd0a30e1bd79818e2e3c4ab44ddca1a94 Mon Sep 17 00:00:00 2001
From: "Dmitry V. Levin" <ldv@altlinux.org>
Date: Sun, 7 Jan 2018 02:03:41 +0000
Subject: [PATCH] linux: make getcwd(3) fail if it cannot obtain an absolute
path [BZ #22679]
Currently getcwd(3) can succeed without returning an absolute path
because the underlying getcwd syscall, starting with linux commit
v2.6.36-rc1~96^2~2, may succeed without returning an absolute path.
This is a conformance issue because "The getcwd() function shall
place an absolute pathname of the current working directory
in the array pointed to by buf, and return buf".
This is also a security issue because a non-absolute path returned
by getcwd(3) causes a buffer underflow in realpath(3).
Fix this by checking the path returned by getcwd syscall and falling
back to generic_getcwd if the path is not absolute, effectively making
getcwd(3) fail with ENOENT. The error code is chosen for consistency
with the case when the current directory is unlinked.
[BZ #22679]
CVE-2018-1000001
* sysdeps/unix/sysv/linux/getcwd.c (__getcwd): Fall back to
generic_getcwd if the path returned by getcwd syscall is not absolute.
* io/tst-getcwd-abspath.c: New test.
* io/Makefile (tests): Add tst-getcwd-abspath.
---
ChangeLog | 9 ++++++
NEWS | 4 +++
io/Makefile | 2 +-
io/tst-getcwd-abspath.c | 66 ++++++++++++++++++++++++++++++++++++++++
sysdeps/unix/sysv/linux/getcwd.c | 8 ++---
5 files changed, 84 insertions(+), 5 deletions(-)
create mode 100644 io/tst-getcwd-abspath.c
diff --git a/io/Makefile b/io/Makefile
index 2f26bf5..566f03c 100644
--- a/io/Makefile
+++ b/io/Makefile
@@ -70,7 +70,8 @@
tst-symlinkat tst-linkat tst-readlinkat tst-mkdirat \
tst-mknodat tst-mkfifoat tst-ttyname_r bug-ftw5 \
tst-posix_fallocate tst-posix_fallocate64 \
- tst-fts tst-fts-lfs tst-open-tmpfile
+ tst-fts tst-fts-lfs tst-open-tmpfile \
+ tst-getcwd-abspath \
ifeq ($(run-built-tests),yes)
tests-special += $(objpfx)ftwtest.out
diff --git a/io/tst-getcwd-abspath.c b/io/tst-getcwd-abspath.c
new file mode 100644
index 0000000..3a3636f
--- /dev/null
+++ b/io/tst-getcwd-abspath.c
@@ -0,0 +1,66 @@
+/* BZ #22679 getcwd(3) should not succeed without returning an absolute path.
+
+ Copyright (C) 2018 Free Software Foundation, Inc.
+ 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 <stdio.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/test-driver.h>
+#include <support/xunistd.h>
+#include <unistd.h>
+
+static char *chroot_dir;
+
+/* The actual test. Run it in a subprocess, so that the test harness
+ can remove the temporary directory in --direct mode. */
+static void
+getcwd_callback (void *closure)
+{
+ xchroot (chroot_dir);
+
+ errno = 0;
+ char *cwd = getcwd (NULL, 0);
+ TEST_COMPARE (errno, ENOENT);
+ TEST_VERIFY (cwd == NULL);
+
+ errno = 0;
+ cwd = realpath (".", NULL);
+ TEST_COMPARE (errno, ENOENT);
+ TEST_VERIFY (cwd == NULL);
+
+ _exit (0);
+}
+
+static int
+do_test (void)
+{
+ support_become_root ();
+ if (!support_can_chroot ())
+ return EXIT_UNSUPPORTED;
+
+ chroot_dir = support_create_temp_directory ("tst-getcwd-abspath-");
+ support_isolate_in_subprocess (getcwd_callback, NULL);
+
+ return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/unix/sysv/linux/getcwd.c b/sysdeps/unix/sysv/linux/getcwd.c
index f545106..866b9d2 100644
--- a/sysdeps/unix/sysv/linux/getcwd.c
+++ b/sysdeps/unix/sysv/linux/getcwd.c
@@ -76,7 +76,7 @@ __getcwd (char *buf, size_t size)
int retval;
retval = INLINE_SYSCALL (getcwd, 2, path, alloc_size);
- if (retval >= 0)
+ if (retval > 0 && path[0] == '/')
{
#ifndef NO_ALLOCATION
if (buf == NULL && size == 0)
@@ -92,10 +92,10 @@ __getcwd (char *buf, size_t size)
return buf;
}
- /* The system call cannot handle paths longer than a page.
- Neither can the magic symlink in /proc/self. Just use the
+ /* The system call either cannot handle paths longer than a page
+ or can succeed without returning an absolute path. Just use the
generic implementation right away. */
- if (errno == ENAMETOOLONG)
+ if (retval >= 0 || errno == ENAMETOOLONG)
{
#ifndef NO_ALLOCATION
if (buf == NULL && size == 0)
--
2.9.3