alt Hacker NewsThe 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.
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.