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Date: Thu, 18 Mar 2021 17:27:09 -0400
From: Leo Famulari <>
Subject: Risk of local privilege escalation in GNU Guix

A security vulnerability that can lead to local privilege escalation has
been found in `guix-daemon` [0]. 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 [1].

Exploitation is more difficult, but not impossible, on machines where
the Linux protected hardlinks [2] 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.

# Fix

This bug [3] has been fixed [4].

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

More information may be available on the Guix blog:

We are grateful to Nathan Nye of WhiteBeam Security for reporting
this bug and discussing fixes with us!

Your feedback is welcome.

On behalf of the Guix team,
Leo Famulari


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