1) Introduction

The dnf5daemon-server component offers a collection of D-Bus interfaces to interact with the dnf5 package manager on the system. An openSUSE community packager wanted to add the additional D-Bus component to the openSUSE Tumbleweed distribution. New D-Bus system services require a review by the SUSE security team. In the course of this review I found the issues described in this report.

The version of dnf5 I reviewed for this is 5.1.9, and any source code references below are based on the corresponding version tag in the upstream Git repository.

2) D-Bus Interface Design

The dnf5daemon-server offers a main interface “org.rpm.dnf.v0.SessionManager” on which clients can create a new session, which results in a new dynamically allocated D-Bus object being registered on the bus. This session object provides a set of additional D-Bus interfaces for modifying package manager configuration, for installing or removing packages or for inspecting metadata about packages and repositories on the system.

The dnf5daemon-server is running as root and can be autostarted via the D-Bus system bus if it is not already running. Any other users with access to the D-Bus system bus may talk to it.

For certain privileged operations the D-Bus service implements Polkit authentication. Only three operations are protected by Polkit:

  • org.rpm.dnf.v0.rpm.RepoConf.write
  • org.rpm.dnf.v0.rpm.execute_transaction
  • org.rpm.dnf.v0.rpm.Repo.confirm_key

These relate to changing the repository configuration, executing transactions or importing new trusted signing keys. Transactions cover all kinds of changes introduced through the package manager.

All of these operations require auth_admin privileges on Polkit level. The integration of the Polkit authorization logic is correct as far as I can tell.

The per-session D-Bus interface provided by dnf5daemon-server is rather large and many calls take additional key/value maps to tune the behaviour of the package manager logic contained in libdnf5. In summary, the libdnf5 library is attached very closely to the D-Bus system bus via dnf5daemon-server.

3) Local Root Exploit via Configuration Dictionary (CVE-2024-1929)

While the privileged operations mentioned above are correctly protected by Polkit, there are issues with the D-Bus interface long before Polkit is even invoked.

The org.rpm.dnf.v0.SessionManager.open_session method takes a key/value map of configuration entries. A sub-entry in this map, placed under the “config” key, is another key/value map. The configuration values found in it will be forwarded as configuration overrides to the libdnf5::Base configuration. The spot where this happens is found in session.cpp:63.

Practically all libdnf5 configuration aspects can be influenced here, as can be seen in the ConfigMain class in config_main.hpp. Already when opening the session via D-Bus, the libdnf5 will be initialized using these override configuration values. There is no sanity checking of the content of this “config” map, which is untrusted data.

There are surely a lot of different ways to exploit this possibility to influence the libdnf5 configuration. The simplest approach to get full root privileges I found is to trick the library into loading a plug-in shared library under control of the unprivileged user.

To do this, the “pluginpath” and “pluginconfpath” configuration entries need to be supplied and need to point to a user-controlled path. There the unprivileged user can place a configuration file that in turn points towards a user controlled shared library. The library will then dlopen() this user controlled shared library, which will lead to full code execution in the context of the root user.

3.1) Proof of Concept

A proof of concept I wrote shows this vulnerability in action. I successfully tested this exploit also on Fedora 39 using dnf5daemon-server version 5.1.10. The only precondition for this exploit is that the dnf5daemon-server package is installed on the system. Any local user, even nobody, can obtain root privileges this way.

3.2) Bugfix

To fix this, I suggested to enforce a whitelist of configuration parameters that are allowed to be overridden. Only a very small subset of values should be allowed for unprivileged clients.

In upstream commit e51bf2f0d such a whitelist enforcement has been implemented for dnf5daemon-server.

3.3) CVE Assignment

Red Hat Product Security assigned CVE-2024-1929 for this issue.

4) No Limit on Number of Open Sessions / Bad Session Close Behaviour (CVE-2024-1930)

There is no limit on how many sessions D-Bus clients may create using the open_session() D-Bus method. In my tests I was able to quickly create about 4,500 sessions and keep them open. For each session a thread is created in dnf5daemon-server. This spends a couple of hundred megabytes of memory in the process. Further connections will become impossible, likely because no more threads can be spawned by the D-Bus service.

In some cases I even managed to cause the D-Bus service to abort() as a result of hitting resource limits. If the service continues running and the client disconnects, then the cleanup code found in SessionManager::on_name_owner_changed() runs for each session that has been created. Each Session holds a ThreadsManager, where the thread associated with each session originates from. It is a thread that is busy-waiting to join other threads, the code for this is found in threads_manager.cpp:33. Since there is a sleep of one second in this thread’s loop it takes about ~4,500 seconds for all the threads from the 4,500 sessions to be joined one by one. The service will be unreachable for more than an hour.

4.1) Bugfix

To fix this, I suggested to limit the number of sessions for each unprivileged user in the system to a sensible value. Also the busy-wait loop in ThreadsManager seems ill-devised. Maybe using a condition variable to inform the cleanup thread of new work would be more efficient. Having a dedicated ThreadsManager and join-thread for each session seems a bit overkill, too.

In upstream commit c090ffeb79 the maximum number of sessions is limited to three sessions. The problematic thread joining behaviour has not been addressed as of now.

4.2) CVE Assignment

Red Hat Product Security assigned CVE-2024-1930 for this issue.

5) Untrusted locale Setting for each Session

Another part of the per-session setup is the use of an untrusted locale setting in session.cpp:54. This string is passed to the C library’s newlocale() function in threads_manager.cpp:92. A danger here is that arbitrary user controlled files or symlinks could be processed by the C library, which could lead to various issues like information leaks, denial of service or even code execution. I looked into the GNU glibc implementation of newlocale(), and luckily it already implements a very careful locale lookup algorithm, that prevents that arbitrary paths are accessed if crafted locale strings are passed to it. This outcome might change if a different C library is used.

Whether anything bad could happen, apart from file system issues, when exotic locales are selected for a thread running in dnf5daemon-server is another question that I did not investigate more closely.

For hardening purposes I suggested that dnf5daemon-server performs a sanity check of the locale string to prevent a string that contains e.g. a slash character / from being used. I don’t know of any upstream commits that address this yet, but upstream stated that they intend to work on this in the future.

6) dnfdaemon-client Demands Full Root

Although the D-Bus service implements Polkit for privileged operations, the dnf5daemon-client refuses to perform privileged operations for non-root users. For example:

user$ dnf5daemon-client distro-sync
This command has to be run with superuser privileges (under the root user on most systems).

The code for this is found in various sub-command implementations:

$ grep -r 'throw UnprivilegedUserError' -C 1
dnf5daemon-client/commands/downgrade/downgrade.cpp-    if (!libdnf5::utils::am_i_root()) {
dnf5daemon-client/commands/downgrade/downgrade.cpp:        throw UnprivilegedUserError();
dnf5daemon-client/commands/downgrade/downgrade.cpp-    }
dnf5daemon-client/commands/distro-sync/distro-sync.cpp-    if (!libdnf5::utils::am_i_root()) {
dnf5daemon-client/commands/distro-sync/distro-sync.cpp:        throw UnprivilegedUserError();
dnf5daemon-client/commands/distro-sync/distro-sync.cpp-    }
dnf5daemon-client/commands/install/install.cpp-    if (!libdnf5::utils::am_i_root()) {
dnf5daemon-client/commands/install/install.cpp:        throw UnprivilegedUserError();
dnf5daemon-client/commands/install/install.cpp-    }
dnf5daemon-client/commands/remove/remove.cpp-    if (!libdnf5::utils::am_i_root()) {
dnf5daemon-client/commands/remove/remove.cpp:        throw UnprivilegedUserError();
dnf5daemon-client/commands/remove/remove.cpp-    }
dnf5daemon-client/commands/upgrade/upgrade.cpp-    if (!libdnf5::utils::am_i_root()) {
dnf5daemon-client/commands/upgrade/upgrade.cpp:        throw UnprivilegedUserError();
dnf5daemon-client/commands/upgrade/upgrade.cpp-    }
dnf5daemon-client/commands/reinstall/reinstall.cpp-    if (!libdnf5::utils::am_i_root()) {
dnf5daemon-client/commands/reinstall/reinstall.cpp:        throw UnprivilegedUserError();
dnf5daemon-client/commands/reinstall/reinstall.cpp-    }

It doesn’t make sense that the client forces users to run as root (and thereby increase the attack surface by running also the client code as root) when the service could authenticate the user via Polkit.

I suggested to drop this root-check code in the client, and rely on the authentication logic in the service side code. If authentication fails, then the client code can still hint at the possibility to run the program as root. I am not aware on any upstream commits that address this yet, but upstream stated that they intend to work on this in the future.

7) General Review Summary

In summary my impression is that the libdnf5 library is too closely connected to the D-Bus system bus. The library itself is unaware of the fact that it is running with partially untrusted input. The dnf5daemon-server code needs to carefully filter untrusted input before it is passed to the generic library code.

8) Timeline

2024-01-10 I reported the findings to secalert@redhat.com offering coordinated disclosure.
2024-01-16 We received a reply that the issue would affect Fedora only and it was suggested that I create a bug in their Bugzilla for direct communication with the dnf5 developers.
2024-01-18 I created a private Bugzilla bug as suggested.
2024-01-24 The bug did not receive any attention, so I asked Red Hat Product Security once more about the procedures going forward, as the offer for coordinated disclosure has not been accepted or denied yet.
2024-01-25 We received a reply that bugfixes are being worked on, but they would likely need the full 90 days maximum embargo period for publishing updates.
2024-02-26 From the openSUSE packager for dnf5 I learned that a bugfix for issue 3) was already public. So I contacted Red Hat Product Security once more to learn about the publication status of the issues.
2024-02-27 We received a reply with the two CVE assignments mentioned in this report and have been asked what our wishes are regarding a publication date.

9) References