Build User Takeover Vulnerability

Caleb Ristvedt — 21. Oktober 2024

A security issue has been identified in guix-daemon which allows for a local user to gain the privileges of any of the build users and subsequently use this to manipulate the output of any build. You are strongly advised to upgrade your daemon now (see instructions below), especially on multi-user systems.

This exploit requires the ability to start a derivation build and the ability to run arbitrary code with access to the store in the root PID namespace on the machine the build occurs on. As such, this represents an increased risk primarily to multi-user systems and systems using dedicated privilege-separation users for various daemons: without special sandboxing measures, any process of theirs can take advantage of this vulnerability.

Vulnerability

For a very long time, guix-daemon has helpfully made the outputs of failed derivation builds available at the same location they were at in the build container. This has aided greatly especially in situations where test suites require the package to already be installed in order to run, as it allows one to re-run the test suite interactively outside of the container when built with --keep-failed. This transferral of store items from inside the chroot to the real store was implemented with a simple rename, and no modification of the store item or any files it may contain.

If an attacker starts a build of a derivation that creates a binary with the setuid and/or setgid bit in an output directory, then, and the build fails, that binary will be accessible unaltered for anybody on the system. The attacker or a cooperating user can then execute the binary, gain the privileges, and from there use a combination of signals and procfs to freeze a builder, open any file it has open via /proc/$PID/fd, and overwrite it with whatever it wants. This manipulation of builds can happen regardless of which user started the build, so it can work not only for producing compromised outputs for commonly-used programs before anybody else uses them, but also for compromising any builds another user happens to start.

A related vulnerability was also discovered concerning the outputs of successful builds. These were moved - also via rename() - outside of the container prior to having their permissions, ownership, and timestamps canonicalized. This means that there also exists a window of time for a successful build's outputs during which a setuid/setgid binary can be executed.

In general, any time that a build user running a build for some submitter can get a setuid/setgid binary to a place the submitter can execute it, it is possible for the submitter to use it to take over the build user. This situation always occurs when --disable-chroot is passed to guix-daemon. This holds even in the case where there are no dedicated build users, and builds happen under the same user the daemon runs as, as happens during make check in the guix repository. Consequently, if a permissive umask that allows execute permission for untrusted users on directories all the way to a user's guix checkout is used, an attacker can use that user's test-environment daemon to gain control over their user while make check is running.

Mitigation

This security issue has been fixed by two commits. Users should make sure they have updated to the second commit to be protected from this vulnerability. Upgrade instructions are in the following section. If there is a possibility that a failed build has left a setuid/setgid binary lying around in the store by accident, run guix gc to remove all failed build outputs.

The fix was accomplished by sanitizing the permissions of all files in a failed build output prior to moving it to the store, and also by waiting to move successful build outputs to the store until after their permissions had been canonicalized. The sanitizing was done in such a way as to preserve as many non-security-critical properties of failed build outputs as possible to aid in debugging. After applying these two commits, the guix package in Guix was updated so that guix-daemon deployed using it would use the fixed version.

If you are using --disable-chroot, whether with dedicated build users or not, make sure that access to your daemon's socket is restricted to trusted users. This particularly affects anyone running make check and anyone running on GNU/Hurd. The former should either manually remove execute permission for untrusted users on their guix checkout or apply this patch, which restricts access to the test-environment daemon to the user running the tests. The latter should adjust the ownership and permissions of /var/guix/daemon-socket, which can be done for Guix System users using the new socket-directory-{perms,group,user} fields in this patch.

A proof of concept is available at the end of this post. One can run this code with:

guix repl -- setuid-exposure-vuln-check.scm

This will output whether the current guix-daemon being used is vulnerable or not. If it is not vulnerable, the last line will contain your system is not vulnerable, otherwise the last line will contain YOUR SYSTEM IS VULNERABLE.

Upgrading

Due to the severity of this security advisory, we strongly recommend all users to upgrade their guix-daemon immediately.

For Guix System, the procedure is to reconfigure the system after a guix pull, either restarting guix-daemon or rebooting. For example:

guix pull
sudo guix system reconfigure /run/current-system/configuration.scm
sudo herd restart guix-daemon

where /run/current-system/configuration.scm is the current system configuration but could, of course, be replaced by a system configuration file of a user's choice.

For Guix running as a package manager on other distributions, one needs to guix pull with sudo, as the guix-daemon runs as root, and restart the guix-daemon service, as documented. For example, on a system using systemd to manage services, run:

sudo --login guix pull
sudo systemctl restart guix-daemon.service

Note that for users with their distro's package of Guix (as opposed to having used the install script) you may need to take other steps or upgrade the Guix package as per other packages on your distro. Please consult the relevant documentation from your distro or contact the package maintainer for additional information or questions.

Conclusion

Even with the sandboxing features of modern kernels, it can be quite challenging to synthesize a situation in which two users on the same system who are determined to cooperate nevertheless cannot. Guix has an especially difficult job because it needs to not only realize such a situation, but also maintain the ability to interact with both users itself, while not allowing them to cooperate through itself in unintended ways. Keeping failed build outputs around for debugging introduced a vulnerability, but finding that vulnerability because of it enabled the discovery of an additional vulnerability that would have existed anyway, and prompted the use of mechanisms for securing access to the guix daemon.

I would like to thank Ludovic Courtès for giving feedback on these vulnerabilities and their fixes — discussion of which led to discovering the vulnerable time window with successful build outputs — and also for helping me to discover that my email server was broken.

Proof of Concept

Below is code to check if your guix-daemon is vulnerable to this exploit. Save this file as setuid-exposure-vuln-check.scm and run following the instructions above, in "Mitigation."

(use-modules (guix)
             (srfi srfi-34))

(define maybe-setuid-file
  ;; Attempt to create a setuid file in the store, with one of the build
  ;; users as its owner.
  (computed-file "maybe-setuid-file"
                 #~(begin
                     (call-with-output-file #$output (const #t))
                     (chmod #$output #o6000)

                     ;; Failing causes guix-daemon to copy the output from
                     ;; its temporary location back to the store.
                     (exit 1))))

(with-store store
  (let* ((drv (run-with-store store
                (lower-object maybe-setuid-file)))
         (out (derivation->output-path drv)))
    (guard (c (#t
               (if (zero? (logand #o6000 (stat:perms (stat out))))
                   (format #t "~a is not setuid: your system is not \
vulnerable.~%"
                           out)
                   (format #t "~a is setuid: YOUR SYSTEM IS VULNERABLE.

Run 'guix gc' to remove that file and upgrade.~%"
                           out))))
      (build-things store (list (derivation-file-name drv))))))