Permission denied for 'sudo' after 'sudo chmod 666 /usr'

I have made a big mistake and now I cannot do anything in my work station. I tried to start VirtualBox and it complained that /usr had execution access. Then I executed, wrongly, sudo chmod 666 /usr and now I cannot execute 'sudo' anymore.

I have asked our technical department and they say that Ubuntu has no password for root so they cannot see any solution for my error but reinstall again Ubuntu which means I would lose all my work.

I hope any of you can think of a solution for this problem because. What do you think I should do to gain access again in the /usr or become root user?

2 Answers

If you just did sudo chmod 666 /usr then things are no too bad - since that will have changed the permissions only on the parent /usr directory, not any of the files of subdirectories within it.

If however you added the recursive flag -R then you are almost certainly better off re-installing the system. Even so, there should be no reason to lose data: you can boot from a live USB or DVD and backup your important files from there first.

If you only did a non-recursive sudo chmod 666 of the /usr directory, and you have physical access to the machine, then the simplest fix should be

1. Boot into recovery mode via the grub advanced menu and select 'Drop to root shell'

2. Remount the filesystem in read-write mode

   mount -o remount,rw /

3. Execute the command

   chmod 755 /usr

   Don't add any other command line options and don't use any shell wildcards.

4. Type exit to continue booting normally

Note, chmod lives in /bin rather than /usr/bin so shouldn't be affected by the permissions on /usr. If (for example) you'd change the permission bits on the /bin directory, or on the / directory itself, then this method would not be appropriate since you wouldn't be able to execute chomd from recovery mode - the best option in that case would probably be to boot a live CD/DVD/USB of any available Linux distribution (it doesn't need to be Ubuntu); identify and mount the broken system's root device (at /mnt in the live system, for example); and then run chmod 755 /mnt/usr from the live system.

Your command had possibly removed the SUID-Bit from /usr/bin/sudo The correct permissions should be:

-rwsr-xr-x 1 root root 136808 Aug 17 15:20 /usr/bin/sudo

You should get a recovery-cd, as mentioned in die comments above and follow the steps in section "finding your root partition". After you successfully mounted it set the SUID-Bit with : chmod 4755 /usr/bin/sudo