Comment by misja111
14 days ago
The root cause of some of the bugs seems to be the opaque nature of some of the Unix API. E.g.
> The trap is that get_user_by_name ends up loading shared libraries from the new root filesystem to resolve the username. An attacker who can plant a file in the chroot gets to run code as uid 0.
To me such a get_user_by_name function is like a booby trap, an accident that is waiting to happen. You need to have user data, you have this get_user_by_name function, and then it goes and starts loading shared libraries. This smells like mixing of concerns to me. I'd say, either split getting the user data and loading any shared libraries in two separate functions, or somehow make it clear in the function name what it is doing.
> The root cause of some of the bugs seems to be the opaque nature of some of the Unix API.
Some, maybe, but if you've decided to rewrite coreutils from scratch, understanding the POSIX APIs is literally your entire job.
And in any case, their test for whether a path was pointing to the fs root was `file == Path::new("/")`. That's not an API problem, the problem is that whoever wrote that is uniquely unqualified to be working on this project.
Interestingly, it looks like the `file == Path::new("/")` bit was basically unchanged from when it was introduced... 12 (!) years ago [0] (though back then it was `filename == "/"`). The change from comparing a filename to a path was part of a change made 8 months ago to handle non-UTF-8 filenames.
> That's not an API problem, the problem is that whoever wrote that is uniquely unqualified to be working on this project.
To be fair, uutils started out with far smaller ambitions. It was originally intended to be a way to learn Rust.
[0]: https://github.com/uutils/coreutils/commit/7abc6c007af75504f...
> Some, maybe, but if you've decided to rewrite coreutils from scratch, understanding the POSIX APIs is literally your entire job.
Yes, it is. But still such traps in API just unacceptable. If you design API that requires obscure knowledge to do it right, and if you do it wrong you'll get privilege escalation, it is just... just... I have no words for it. It is beyond stupidity. You are just making sure that your system will get these privilege escalations, and not just once, but multiple times.
No one is under any impression (or should be) that the POSIX API isn't old and legacy. That's not why we still use it.
> The root cause of some of the bugs seems to be the opaque nature of some of the Unix API.
Seems and smells is weasel words. The root cause is not thinking: Why is root chrooting into a directory they do not control?
Whatever you chroot into is under control of whoever made that chroot, and if you cannot understand this you have no business using chroot()
> To me such a get_user_by_name function is like a booby trap
> I'd say, either split getting the user data and loading any shared libraries in two separate functions, or somehow make it clear in the function name what it is doing.
You'd probably still be in the trap: there's usually very little difference between writing to newroot/etc/passwd and newroot/usr/lib/x86_64-linux-gnu/libnss_compat.so or newroot/bin/sh or anything else.
So I think there's no reason for /usr/sbin/chroot look up the user id in the first place (toybox chroot doesn't!), so I think the bug was doing anything at all.
> The root cause is not thinking: Why is root chrooting into a directory they do not control?
Because you can't call chroot(2) unless you're root. And "control a directory" is weasel words; root technically controls everything in one sense of the word. It can also gain full control (in a slightly different sense of the word) over a directory: kill every single process that's owned by the owner of that directory, then don't setuid into that user in this process and in any other process that the root currently executes, or will execute, until you're done with this directory. But that's just not useful for actual use, isn't it?
Secure things should be simple to do, and potentially unsafe things should be possible.
> And "control a directory" is weasel words;
I did not choose the term to confuse you, that's from the definition document linked to the CVE:
https://cwe.mitre.org/data/definitions/426.html
The CVE itself uses the language "If the NEWROOT is writable by an attacker" which could refer to a shared library (as indicated in the report), or even a passwd file as would have been true since the origin of chroot()
> root technically controls everything in one sense of the word.
But not the sense we're talking about.
> Because you can't call chroot(2) unless you're root
Well you can[1], but this is /usr/sbin/chroot aka chroot(8) when used with a non-numeric --userspec, and the point is to drop root to a user that root controls with setuid(2). Something needs to map user names to the numeric userids that setuid(2) uses, and that something is typically the NSS database.
Now: Which database should be used to map a username to a userid?
- The one from before the chroot(2)?
- Or the one that you're chroot(2)ing into
If you're the author of the code in-question, you chose the latter, and that is totally obvious to anyone who can read because that's the order the code appears in, but it's also obvious that only the first one* is under control of root, and so only the first one could be correct.
[1]: if you're curious: unshare(CLONE_USERNS|CLONE_FS) can be used. this is part of how rootless containers work.
2 replies →
Rather, I think that using a functional safe language tricks people into thinking that the data it deals with is stateless. Whereas many many things change in operating systems all the time.
Until we have a filesystem that can present a snapshot, everything has to checked all the time.
i.e. we need an API which gives input -> good result or failure. Not input -> good result or failure or error.
Yes thats one thing Musl libc removes.
If the attacker can control newroot/etc/passwd they _still_ get getpwnam to return whatever userid they want. The solution is to not lookup --userspec=username:group inside the chrooted-space, but from outside.
Also, hi how's things? :)
hi! good, how are you doing?
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Unix and POSIX are fractally a booby trap.