linuxmemes @lemmy.world renzev @lemmy.world 2mo ago

This week on "ancient unix hacks that are still somehow a core part of linux": Setuid

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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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 (for me) more secure (you need physical access to get to the tty).
- Does run0 use suid? From my understanding, it shouldn't.
  
  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.
- How do you set up the tty method?
  
  All I do is have agetty --autologin root tty2 linux run as a service. It launches on startup, and I just hit CTRL + ALT + F2 if I ever need a root shell.
  
  All its doing is just auto logging-in as root on TTY2.
  
  Thanks! I hadn't heard about agetty before. Will save this for later.
- So you can't become root on your system unless you switch to that tty? That sounds like a gigantic pain in the ass.
  
  Your needs must be different than mine.
  
  I press one button combination and have root without ever entering a password. I press a similar combination and go back. Not sure how this is a pain in the ass.
  
  Yea, it sounds pretty nice actually. I'm considering doing that as well. Makes it obvious when you're running in a root shell too which is nice. I'd probably still keep sudo around though.
  
  With a programmable keyboard it can just be one button too!
  
  You can modify the keybinds in software too. You would need to change your console keymap (TTY) and your desktop environment keybindings. Programmable keyboard is most likely easier though.
  
  I played around with it and changed both to just use F1 = tty1 and so on, without requiring CTRL+ALT.
  
  You can also create a custom keyboard layout in Linux. From what I have written down, here is how to do it (can't double check because work computer).
  
  I suspect this could be X11 only because it is X keyboard extension, pop_OS! didn't adopt Wayland as early Ubuntu.
  
  Start by navigating to /usr/share/X11/xkb/symbols/.
  
  Open the file that corresponds to your keyboard layout (I think it is us for American).
  
  Add a new layout:
  
  partial alphanumeric_keys xkb_symbols "custom" { include "us(basic)" // includes another configuration to build on, see current file you are editing. name[Group1]="US (custom)"; // will be the name of your configration/layout. key <LSGT> { [ greater, less, bar, brokenbar ] }; // < key <SPCE> { [ space, space, nobreakspace, space ] }; // Space // Add more key maps as you see fit. };
  
  Go to /usr/share/X11/xkb/rules/
  
  Locate evdev.xml and base.xml
  
  Edit both and look for the following block
  
  <layout> <configItem> <name>us</name> 
  
  Add the following to both files on the row after the comment:
  
  <variant> <configItem> <name>custom</name> <description>US (custom)</description> </configItem> </variant>
  
  You might have to reboot afterwards.

36 comments