chroot: perform chdir("/") again unless new --skip-chdir is specified

Since commit v8.22-94-g99960ee, chroot(1) skips the chroot(2) syscall
for "/" arguments (and synonyms).  The problem is that it also skips
the following chdir("/") call in that case.  The latter breaks existing
scripts which expect "/" to be the working directory inside the chroot.
While the first part of the change - i.e., skipping chroot("/") - is
okay for consistency with systems where it might succeed for a non-root
user, the second part might be malicious, e.g.

  cd /home/user && chroot '/' bin/foo

In the "best" case, chroot(1) could not execute 'bin/foo' with ENOENT,
but in the worst case, chroot(1) would execute '/home/user/bin/foo' in
the case that exists - instead of '/bin/foo'.

Revert that second part of the patch, i.e., perform the chdir("/)
in the common case again - unless the new --skip-chdir option is
specified.  Restrict this new option to the case of "/" arguments.

* src/chroot.c (SKIP_CHDIR): Add enum.
(long_opts): Add entry for the new --skip-chdir option.
(usage): Add --skip-chdir option, and while at it, move the other
to options into alphabetical order.
(main): Accept the above new option, allowing it only in the case
when NEWROOT is the old "/".
Move down the chdir() call after the if-clause to ensure it is
run in any case - unless --skip-chdir is specified.
Add a 'newroot' variable for the new root directory as it is used
in a couple of places now.
* tests/misc/chroot-fail.sh: Invert the last tests which check the
working directory of the execvp()ed program when a "/"-like
argument was passed: now expect it to be "/" - unless --skip-chdir
is given.
* doc/coreutils.texi (chroot invocation): Document the new option.
Document that chroot(1) usually calls chdir("/") unless the new
--skip-chdir option is specified.  Sort options.
* NEWS (Changes in behavior): Mention the fix.
(New features): Mention the new option.
* init.cfg (nonroot_has_perm_): Add chroot's new --skip-chdir option.
* tests/cp/preserve-gid.sh (t1): Likewise.
* tests/cp/special-bits.sh: Likewise.
* tests/id/setgid.sh: Likewise.
* tests/misc/truncate-owned-by-other.sh: Likewise.
* tests/mv/sticky-to-xpart.sh: Likewise.
* tests/rm/fail-2eperm.sh: Likewise.
* tests/rm/no-give-up.sh: Likewise.
* tests/touch/now-owned-by-other.sh: Likewise.

Reported by Andreas Schwab in http://bugs.gnu.org/18062

1 files changed, 13 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 511e626bc..ba0d3d771 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,19 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** New features
+
+  chroot accepts the new --skip-chdir option to not change the working directory
+  to "/" after changing into the chroot(2) jail, thus retaining the current wor-
+  king directory.  The new option is only permitted if the new root directory is
+  the old "/", and therefore is useful with the --group and --userspec options.
+
+** Changes in behavior
+
+  chroot changes the current directory to "/" in again - unless the above new
+  --skip-chdir option is specified.
+  [bug introduced in coreutils-8.23]
+
 
 * Noteworthy changes in release 8.23 (2014-07-18) [stable]