It's now possible to run commands as other users without shell
interpolation by using "--exec":
Read /etc/shadow as root without specifying user:
```
su --exec /bin/cat -- /etc/shadow
```
Or specify user:
```
su --exec /bin/cat root -- /etc/shadow
```
Mechanical rename distinguishing this variable from intended changes
supporting executing commands without using an interpretive shell
(i.e. no '/bin/sh -c').
In preparation for supporting --exec I was testing the robustness
of "--" handling and it became apparent that things are currently
a bit broken in `su`.
Since "--" is currently of limited utility, as the subsequent
words are simply passed to the shell after "-c","command_string",
it seems to have gone unnoticed for ages.
However, with --exec, it's expected that "--" would be an almost
required separator with every such usage, considering the
following flags must be passed verbatim to execve() and will
likely begin with hyphens looking indistinguishable from any
other flags in lieu of shell interpolation to worry about.
For some practical context of the existing situation, this
invocation doesn't work today:
```
$ su --command ls -- flags for shell
No passwd entry for user 'flags'
$
```
This should just run ls as root with "flags","for","shell"
forwarded to the shell after "-c","ls".
The "--" should block "flags" from being treated as the user.
That particular issue isn't a getopt one per-se, it's arguably
just a bug in su.c's implementation.
It *seemed* like an easy fix for this would be to add a check if
argv[optind-1] were "--" before treating argv[optind] as USER.
But testing that fix revealed getopt was rearranging things when
encountering "--", the "--" would always separate the handled
opts from the unhandled ones. USER would become shifted to
*after* "--" even when it occurred before it!
If we change the command to specify the user, it works as-is:
```
$ su --command ls root -- flags for shell
Password:
testfile
$
```
But what's rather surprising is how that works; the argv winds up:
"su","--command","ls","--","root","flags","for","shell"
with optind pointing at "root".
That arrangement of argv is indistinguishable from omitting the
user and having "root","flags","for","shell" as the stuff after
"--".
This makes it non-trivial to fix the bug of omitting user
treating the first word after "--" as the user, which one could
argue is a potentially serious security bug if you omit the user,
expect the command to run as root, and the first word after "--"
is a valid user, and what follows that something valid and
potentially destructive not only running in unintended form but
as whatever user happened to be the first word after "--".
So, it seems like something important to fix, and getopt seems to
be getting in the way of fixing it properly without being more
trouble than replacing getopt.
In disbelief of what I was seeing getopt doing with argv here, I
took a glance at the getopt source and found the following:
```
/* The special ARGV-element '--' means premature end of options.
Skip it like a null option,
then exchange with previous non-options as if it were an option,
then skip everything else like a non-option. */
if (d->optind != argc && !strcmp (argv[d->optind], "--"))
```
I basically never use getopt personally because ages ago it
annoyed me with its terrible API for what little it brought to
the table, and this brings it to a whole new level of awful.
When "su -l" is used the behaviour is described as similar to
a direct login. However login.c is doing a setup_env(pw) and then a
pam_getenvlist() in this scenario. But su.c is doing it the other
way around. Which means that the value of PATH from /etc/environment
is overriden. I think this is a bug because:
The man-page claims that "-l": "provides an environment similar
to what the user would expect had the user logged in directly."
And login.c is using the PATH from /etc/environment.
This will fix:
https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/984390
Do not reset the pid_child to 0 if the child process is still
running. This else-condition can be reached with pid being -1,
therefore explicitly test this condition.
This is a regression fix for CVE-2017-2616. If su receives a
signal like SIGTERM, it is not propagated to the child.
Reported-by: Radu Duta <raduduta@gmail.com>
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
If su is compiled with PAM support, it is possible for any local user
to send SIGKILL to other processes with root privileges. There are
only two conditions. First, the user must be able to perform su with
a successful login. This does NOT have to be the root user, even using
su with the same id is enough, e.g. "su $(whoami)". Second, SIGKILL
can only be sent to processes which were executed after the su process.
It is not possible to send SIGKILL to processes which were already
running. I consider this as a security vulnerability, because I was
able to write a proof of concept which unlocked a screen saver of
another user this way.
Some of the supplied tools use functions which are not signal-safe.
Most of the times it's exit() vs. _exit().
In other times it's how the standard output or standard error is
handled. FILE-related functions shall be avoided, therefore I replaced
them with write().
Also there is no need to call closelog(). At worst, it allows to
trigger a deadlock by issuing different signal types at bad timings.
But as these fixes are about race conditions, expect bad timings in
general for these bugs to be triggered. :)
* src/su.c: Terminate the child (if needed) before closing the PAM
session. This is probably more correct, and avoid reporting
termination from signals possibly sent by PAM modules (e.g. former
versions of pam_systemd). Debian#670132
* src/su.c: When a SIGTSTP is caught, reset caught to 0. There is
no need to kill the child in such case after su is resumed. This
remove the "Session terminated, terminating shell...
...terminated." messages in such case.
save_caller_context() is allocated and freed.
* src/su.c: Added missing #endif indication
* src/su.c save_caller_context(): password only needed if
SU_ACCESS and !USE_PAM.
* src/su.c: Added function prototypes.
* src/su.c: Rename shellstr parameter to shellname to avoid
collision with static variable.
* NEWS, src/su.c: Added support for PAM modules which change
PAM_USER.
child and listening for signal in the parent from run_shell().
prepare_pam_close_session() is now executed before the creation of
the pam session and before the UID is changed. This allows to
close the session as root.
use the pwent global variable to communicate between APIs of
different files. Added boolean parameter su_to_root to
check_su_auth().
* src/su.c (check_perms): Return the passwd entry of the finally
authenticated user. Remove usage of the pwent variable.
* src/su.c: The password of the caller is the one from the
spwd structure only if the passwd's password is 'x'.
after the switch to the final subsystem. The previous architecture
forced to always change the environment (the shell starts with a
'*' and was thus restricted, and change_environment could not be
reset to false).
definition of shellstr, PATH and IFS is not influenced (getenv,
getdef, restricted_shell) by and does not influence (addenv does
not change environ) the authentication. And the authentication
does not overwrite those definitions. This will ease an extraction
from the big main() function.
