commit 52a713fdd0a30e1bd79818e2e3c4ab44ddca1a94
Author: Dmitry V. Levin <ldv@altlinux.org>
Date:   Sun Jan 7 02:03:41 2018 +0000

    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.

Index: glibc-2.17-c758a686/io/Makefile
===================================================================
--- glibc-2.17-c758a686.orig/io/Makefile
+++ glibc-2.17-c758a686/io/Makefile
@@ -70,7 +70,8 @@ tests		:= test-utime test-stat test-stat
 		   tst-symlinkat tst-linkat tst-readlinkat tst-mkdirat \
 		   tst-mknodat tst-mkfifoat tst-ttyname_r bug-ftw5 \
 		   tst-posix_fallocate \
-		   tst-open-tmpfile
+		   tst-open-tmpfile \
+		   tst-getcwd-abspath
 
 include ../Rules
 
Index: glibc-2.17-c758a686/io/tst-getcwd-abspath.c
===================================================================
--- /dev/null
+++ glibc-2.17-c758a686/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>
Index: glibc-2.17-c758a686/sysdeps/unix/sysv/linux/getcwd.c
===================================================================
--- glibc-2.17-c758a686.orig/sysdeps/unix/sysv/linux/getcwd.c
+++ glibc-2.17-c758a686/sysdeps/unix/sysv/linux/getcwd.c
@@ -79,7 +79,7 @@ __getcwd (char *buf, size_t size)
   int retval;
 
   retval = INLINE_SYSCALL (getcwd, 2, CHECK_STRING (path), alloc_size);
-  if (retval >= 0)
+  if (retval >= 0 && path[0] == '/')
     {
 #ifndef NO_ALLOCATION
       if (buf == NULL && size == 0)
@@ -95,10 +95,10 @@ __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)