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January 24, 2017

Thoughts on the Systemd Root Exploit

Sebastian Krahmer of the SUSE Security Team has discovered a local root exploit in systemd v228. A local user on a system running systemd v228 can escalate to root privileges. That's bad.

At a high level, the exploit is trivial:

  1. Systemd uses -1 to represent an invalid mode_t (filesystem permissions) value.
  2. Systemd was accidentally passing this value to open when creating a new file, resulting in a file with all permission bits set: that is, world-writable, world-executable, and setuid-root.
  3. The attacker writes an arbitrary program to this file, which succeeds because it's world-writable.
  4. The attacker executes this file, which succeeds because it's world-executable.
  5. The attacker-supplied program runs as root, because the file is setuid-root.

In mitigation: The vulnerability was fixed a year ago and less than three months after it was introduced. It is present only in v228.

In aggravation: The vulnerability was mislabeled at the time as a local denial-of-service and the systemd team did not request a CVE-ID for it. Had they requested a CVE-ID, someone may have noticed that this was more than a DoS. (Krahmer accurately points out that the systemd commit log is "really huge," which makes it hard to spot security-relevant commits.)

In mitigation: The vulnerability depends on a yet-unfixed hole in how Linux clears a file's setuid and setgid bits when writing to it. Systemd merely creates an empty setuid-root file. Gaining root requires writing to this file, and when a non-root user writes to a setuid-root file, the setuid bit is supposed to be cleared. halfdog found a clever way to circumvent this by tricking a root process into writing to the file instead. This is an extremely interesting vulnerability in itself and I can't wait to dive deeper into it.

In aggravation: The vulnerability would have been prevented if systemd used a fail-safe umask rather than setting it to 0, something I called out last September as evidence of systemd's poor security hygiene. A more sensible umask, such as 022, would have caused open to create the setuid-root file without world-writable permissions, preventing exploitation. However, systemd maintainer David Strauss rejected a safe umask with a completely illogical argument that shows his cluelessness over how systemd uses umask.

Lastly, this is yet another example of "The Billion Dollar Mistake": systemd was using a magic value (-1) to represent an invalid mode_t value, and C's type system did not prevent passing it to the mode argument of open. A language with a better type system, such as Rust or C++ (which has std::optional) can help prevent this kind of error.

That said, this is not about programming languages. Dovecot (among a handful of others) has demonstrated that adherence to good coding practices can produce secure software written in C. Rewriting systemd in a safer language would not transform it into quality software, although certain classes of bugs would likely be reduced or eliminated.

Rather, this is about lock-in. Systemd is introducing unprecedented lock-in to the Linux userspace. They are replacing previously-independent userspace services with ones whose development is controlled by the systemd project and which only work if systemd is PID 1. They are defining their own non-standard protocols and encouraging applications to use them. They have even replaced DNS with a dbus-based protocol, which they "strongly recommend" applications use instead of DNS. Sadly, the most recent version of Ubuntu ships with this travesty.

Systemd's developers have repeatedly demonstrated their poor judgment and unfitness to hold such responsibility. Unfortunately, the lock-in they're creating will deprive people of the ability to vote with their feet and switch to better alternatives.

Posted on 2017-01-24 at 17:54:16 UTC | Comments