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, 24 insertions, 12 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 96f07816f..7c8671971 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -16113,7 +16113,10 @@ On many systems, only the super-user can do this.@footnote{However,
 some systems (e.g., FreeBSD) can be configured to allow certain regular
 users to use the @code{chroot} system call, and hence to run this program.
 Also, on Cygwin, anyone can run the @command{chroot} command, because the
-underlying function is non-privileged due to lack of support in MS-Windows.}
+underlying function is non-privileged due to lack of support in MS-Windows.
+Furthermore, the @command{chroot} command avoids the @code{chroot} system call
+when @var{newroot} is identical to the old @file{/} directory for consistency
+with systems where this is allowed for non-privileged users.}.
 Synopses:
 
 @example
@@ -16123,10 +16126,11 @@ chroot @var{option}
 
 Ordinarily, file names are looked up starting at the root of the
 directory structure, i.e., @file{/}.  @command{chroot} changes the root to
-the directory @var{newroot} (which must exist) and then runs
-@var{command} with optional @var{args}.  If @var{command} is not
-specified, the default is the value of the @env{SHELL} environment
-variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
+the directory @var{newroot} (which must exist), then changes the working
+directory to @file{/}, and finally runs @var{command} with optional @var{args}.
+If @var{command} is not specified, the default is the value of the @env{SHELL}
+environment variable or @command{/bin/sh} if not set, invoked with the
+@option{-i} option.
 @var{command} must not be a special built-in utility
 (@pxref{Special built-in utilities}).
 
@@ -16135,6 +16139,14 @@ Options must precede operands.
 
 @table @samp
 
+@item --groups=@var{groups}
+@opindex --groups
+Use this option to override the supplementary @var{groups} to be
+used by the new process.
+The items in the list (names or numeric IDs) must be separated by commas.
+Use @samp{--groups=''} to disable the supplementary group look-up
+implicit in the @option{--userspec} option.
+
 @item --userspec=@var{user}[:@var{group}]
 @opindex --userspec
 By default, @var{command} is run with the same credentials
@@ -16145,13 +16157,13 @@ If a @var{user} is specified then the supplementary groups
 are set according to the system defined list for that user,
 unless overridden with the @option{--groups} option.
 
-@item --groups=@var{groups}
-@opindex --groups
-Use this option to override the supplementary @var{groups} to be
-used by the new process.
-The items in the list (names or numeric IDs) must be separated by commas.
-Use @samp{--groups=''} to disable the supplementary group look-up
-implicit in the @option{--userspec} option.
+@item --skip-chdir
+@opindex --skip-chdir
+Use this option to not change the working directory to @file{/} after changing
+the root directory to @var{newroot}, i.e., inside the chroot.
+This option is only permitted when @var{newroot} is the old @file{/} directory,
+and therefore is mostly useful together with the @option{--groups} and
+@option{--userspec} options to retain the previous working directory.
 
 @end table