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.

1 files changed, 9 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index fb280c1de5..971257a4bd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2018-01-12  Dmitry V. Levin  <ldv@altlinux.org>
+
+	[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.
+
 2018-01-12  Istvan Kurucsai  <pistukem@gmail.com>
 
 	* malloc/malloc.c (malloc_consolidate): Add size check.