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, 1 insertions, 1 deletions

diff --git a/io/Makefile b/io/Makefile
index 09b4f468b0..2117cb6b62 100644
--- a/io/Makefile
+++ b/io/Makefile
@@ -71,7 +71,7 @@ tests		:= test-utime test-stat test-stat2 test-lfs tst-getcwd \
 		   tst-mknodat tst-mkfifoat tst-ttyname_r bug-ftw5 \
 		   tst-posix_fallocate tst-posix_fallocate64 \
 		   tst-fts tst-fts-lfs tst-open-tmpfile \
-		   tst-copy_file_range \
+		   tst-copy_file_range tst-getcwd-abspath \
 
 # This test includes the compat implementation of copy_file_range,
 # which uses internal, unexported libc functions.