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I am getting the dreaded sudo: must be setuid root error even though the executable in /usr/bin/sudo has ownership root:root and mode 4755 (-rwsr-xr-x). /etc/sudoers is mode 440. My user is in sudoers with all appropriate settings. I've purged and re-installed the package, to no avail.

I didn't initially configure this machine; its upkeep has fallen to me by default. Is it possible that this behaviour is an interaction with PAM and, if so, how do I fix it? The machine is running Debian squeeze 2.6.26-2-686 SMP kernel. Nothing is written on failure to /var/log/auth.log so it's bailing out before that.

UPDATE: here's sudoers (with a lot of superfluous comment lines removed), but I'm pretty sure it's not the issue.

Defaults        env_reset
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:\

# Host alias specification

# User alias specification
User_Alias              SUDOERS=david
Defaults:SUDOERS        !lecture,!authenticate

# User privilege specification
root    ALL=(ALL:ALL) ALL

Yes, I know this is overly lax. I'll tighten it up when I get it working. As far as I can tell, it never even gets to the stage of reading this file, but dies when the sudo process itself tries to escalate its user ID. A near-identical setup works flawlessly on several (>10) other machines, so I think it must be an external configuration that is causing this to fail.

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What's the command you are trying to run? I get this with strace sudo as that won't take sudo's permissions into account. – Dennis Kaarsemaker Jan 18 '13 at 15:33
I'm trying to run sudo su – David G Jan 18 '13 at 15:36
Why don't you post your /etc/sudoers file so we can see what's in there? – val0x00ff Jan 18 '13 at 15:40
Have you tried to create another user, chmod/chown the sudo binary over again and try sudo su - or su - etc. I don't have an answer at this moment. – val0x00ff Jan 18 '13 at 16:28
Firstly, are you sure you're picking up the right sudo (ie, does the error persist when you do /usr/bin/sudo instead of letting PATH do the work)? If you are, could we see the output of mount, in case /usr has somehow got mounted nosuid? – MadHatter Jan 18 '13 at 16:37

Thanks to MadHatter, I found the solution. The filesystem had been created as one big root partition (not by me!) with /usr, /var, /home and so on simply as subdirectories. For some reason, it had also been mounted nosuid, presumably in a misguided attempt at security. I'm promoting this to an answer so it will show up more readily for anyone suffering the same issue. I should have checked mount options but didn't think to as it's such a silly situation to encounter.

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