this post was submitted on 04 May 2025
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    174
    This week on "ancient unix hacks that are still somehow a core part of linux": Setuid (lemmy.world)
    submitted 19 hours ago by [email protected] to c/[email protected]
    13 comments fedilink hide all child comments
     

    Explanation for newbies: setuid is a special permission bit that makes an executable run with the permissions of its owner rather than the user executing it. This is often used to let a user run a specific program as root without having sudo access.

    If this sounds like a security nightmare, that's because it is.

    In linux, setuid is slowly being phased out by Capabilities. An example of this is the ping command which used to need setuid in order to create raw sockets, but now just needs the cap_net_raw capability. More info: https://unix.stackexchange.com/questions/382771/why-does-ping-need-setuid-permission. Nevertheless, many linux distros still ship with setuid executables, for example passwd from the shadow-utils package.

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    [–] [email protected] 15 points 13 hours ago (1 children)

    fork bomb still being possible out of the box in a couple of characters is funny to me

    [–] [email protected] 15 points 12 hours ago

    That's the thing about Linux. The developers generally assume you want to do the thing you're doing. So they don't stop you.

    [–] [email protected] 23 points 15 hours ago* (last edited 15 hours ago)

    Last time I was tempted to use suid, it was in order to allow an application I'd written to listen on 80 and 443. Fortunately I found the capabilities way of doing that (setcap 'cap_net_bind_service=+ep' executable) and that was the first I ever heard of capabilities. I consider myself pretty Linux-savvy, but it was pretty recently that I learned about capabilities.

    [–] [email protected] 29 points 18 hours ago (3 children)

    If this sounds like a security nightmare, that’s because it is.

    You can perfectly-reasonably implement suid binaries securely. They need to be simple and carefully constructed, and there shouldn't be many of them, but the assertion that suid is "a security nightmare" is ridiculous. sudo itself relies on the suid bit.

    [–] ricecake 22 points 16 hours ago (1 children)

    I would describe need to proactively go out of your way to ensure a program is simple, minimal, and carefully constructed to avoid interactions potentially outside of a restricted security scope as a "security nightmare".

    Being possible to do right or being necessary in some cases at the moment doesn't erase the downsides.

    It's the opposite of secure by default. It throws the door wide open and leaves it to the developer and distro maintainer to make sure there's nothing dangerous in the room and that only the right doors are opened. Since these are usually not coordinated, it's entirely possible for a change or oversight by the developer to open a hole in multiple distros.
    In a less nightmarish system a program starting to do something it wasn't before that should be restricted is for the user to get denied, not for it to fail open.

    https://www.cve.org/CVERecord/SearchResults?query=Setuid

    It may be possible, but it's got the hallmarks of a nightmare too.

    [–] [email protected] 5 points 15 hours ago* (last edited 15 hours ago)

    Hard agree. This is why rust is getting so much attention, and the c/c++ crowd are so mad. They're happy just blaming it on a "skill issue" while losing their shit over [the rust crowd] saying "how about we don't let you in the first place."

    [–] [email protected] 30 points 18 hours ago* (last edited 18 hours ago) (1 children)

    They need to be simple and carefully constructed

    Yeah, that's the difficult part. It's always better to go with the principle of least privilege (which is Capabilities is trying to do) than to just cross your fingers and hope that there are not bugs in your code. And who exactly is going to police people to make sure that their programs are "simple and carefully constructed"? The article I linked is about a setuid-related vuln in goddamn Xorg which is anything but.

    [–] Jajcus 4 points 17 hours ago

    Yes, Xorg being suid is stupid. That used to be needed due to several historical reasons, but is not any more.

    But for 'su' or 'sudo' suid is still the right mechanism to use. Capabilities won't help, when the tool is supposed to give one full privileges. Of course, in some use cases no such command is needed, then the system can run with no suid. Similar functionality could be implemented without suid too (e.g. ssh to localhost), but with its own security implications, usually bigger than those brought but a mechanism as simple as suid (the KISS rule).

    [–] [email protected] 2 points 14 hours ago (1 children)

    Does passwd rely on it as well? I'm curious to it's benefits, and what we're it's original use cases. Is it a necessary component of multi-user systems?

    [–] [email protected] 2 points 5 hours ago* (last edited 5 hours ago)

    passwd uses it to update your password in an root-only-writable file

    [–] unhrpetby 6 points 18 hours ago* (last edited 18 hours ago) (1 children)

    The nosuid mount option disables this behavior per mount. Just be sure you don't use suid binaries.

    Example: sudo or doas. I replaced those with switching to a tty with an already open root account on startup. Generally faster and more secure (you need physical access to get to the tty).

    [–] [email protected] 4 points 17 hours ago (1 children)

    Does run0 use suid? From my understanding, it shouldn't.

    [–] unhrpetby 3 points 13 hours ago

    From what I've read, no. Though it doesn't solve the fundamental problem of a root process handling untrusted input from a regular user.

    The TTY method is IMO better as it ties privileges to a piece of physical hardware, bypassing the complexities of userspace elevation of privileges.