Risk of local privilege escalation via guix-daemon (CVE-2021-27851)

Ludovic Courtès and Leo Famulari — 18-a de Marto 2021

A security vulnerability that can lead to local privilege escalation has been found in guix-daemon. It affects multi-user setups in which guix-daemon runs locally.

It does not affect multi-user setups where guix-daemon runs on a separate machine and is accessed over the network via GUIX_DAEMON_SOCKET, as is customary on cluster setups. Exploitation is more difficult, but not impossible, on machines where the Linux protected hardlinks feature is enabled, which is common — this is the case when the contents of /proc/sys/fs/protected_hardlinks are 1.

Vulnerability

The attack consists in having an unprivileged user spawn a build process, for instance with guix build, that makes its build directory world-writable. The user then creates a hardlink to a root-owned file such as /etc/shadow in that build directory. If the user passed the --keep-failed option and the build eventually fails, the daemon changes ownership of the whole build tree, including the hardlink, to the user. At that point, the user has write access to the target file.

This is CVE-2021-27851.

Fix

This bug has been fixed. See below for upgrade instructions.

The fix consists in adding a root-owned “wrapper” directory in which the build directory itself is located. If the user passed the --keep-failed option and the build fails, the guix-daemon first changes ownership of the build directory, and then, in two stages, moves the build directory into the location where users expect to find failed builds, roughly like this:

1. chown -R USER /tmp/guix-build-foo.drv-0/top
2. mv /tmp/guix-build-foo.drv-0{,.pivot}
3. mv /tmp/guix-build-foo.drv-0.pivot/top /tmp/guix-build-foo.drv-0

In step #1, /tmp/guix-build-foo.drv-0 remains root-owned, with permissions of #o700. Thus, only root can change directory into it or into top. Likewise in step #2.

The build tree becomes accessible to the user once step #3 has succeeded, not before. These steps are performed after the package build scripts have stopped running.

Upgrading

On multi-user systems, we recommend upgrading the guix-daemon now.

To upgrade the daemon on Guix System, run something like:

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

On other distros, use something like this:

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

Conclusions

One of the flagship features of GNU Guix is to enable unprivileged package management, which includes building packages. Building occurs in an isolated build environment. This environment is isolated from the rest of the system not only to control the build process to implement the functional packaging model, but also to protect the system from package build scripts.

Despite our best efforts, there is always the possibility that we have overlooked something, as in this case.

This issue is tracked as bug #47229; you can read the thread for more information.

We are grateful to Nathan Nye of WhiteBeam Security for reporting this bug.

Please report any issues you may have to guix-devel@gnu.org. See the security web page for information on how to report security issues.

About GNU Guix

GNU Guix is a transactional package manager and an advanced distribution of the GNU system that respects user freedom. Guix can be used on top of any system running the Hurd or the Linux kernel, or it can be used as a standalone operating system distribution for i686, x86_64, ARMv7, and AArch64 machines.

In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection. When used as a standalone GNU/Linux distribution, Guix offers a declarative, stateless approach to operating system configuration management. Guix is highly customizable and hackable through Guile programming interfaces and extensions to the Scheme language.