How to fix sudo after "chmod -R 777 /usr/bin"?

Question

I entered chmod -R 777 /usr/bin and now sudo is not working.

It says sudo must be setuid root.

Some advice online said to run chown root:root /usr/bin/sudo chmod 4755 /usr/bin/sudo.

On entering chown root:root /usr/bin/sudo it shows an opened in readonly mode error.

Answer 1

^{Accepted status notwithstanding, I'm now convinced this answer is wrong. (I hope to improve it soon, after consulting with the OP about the accept.) I think I recall others saying this worked, but I believe their problems were at least slightly different. The method described herein remains valuable for some situations where chmod -R 777 /usr/bin is interrupted with Ctrl+C or otherwise does not complete. But once it does, pkexec is un-setuid'd too and it won't work any better than sudo, as Damien Roche and Oli have rightly commented.}

On an Ubuntu desktop system, PolicyKit is installed, so pkexec can be used to repair a broken sudo executable or sudoers file. You do not need to boot into recovery mode and you do not need to boot from a live CD. You don't even need to reboot.

In this case, run the following commands:

pkexec chown root:root /usr/bin/sudo
pkexec chmod 4755 /usr/bin/sudo

See this question for more information.

Answer 2

Even when running from the live CD / Pendrive, you must prefix your chmod command with sudo. So your steps will be like the following:

1. boot from a live CD / Pendrive
2. check whether your disk was already automounted (and where to). If not, mount it (see below)
3. use sudo chmod 0755 <path> to adjust the permissions

How to figure out where your disk is mounted: from a terminal window, run mount (without arguments). This will list up all mounted devices. Check for the type listed -- you can skip everything not using a "real file system" (your disk probably uses either ext3 or ext4 -- you can for sure skip things like proc, sysfs and the like). If something sounds promising (looking like /dev/sda1 on /media/sda1 type ext3), check its contents using ls /media/sda1 to see if it's that.

If it is not mounted, you can check with the /dev entries where the disk could be (using ls /dev/ |grep '/dev/sd to check for available devices; your disk should look like /dev/sdaX, /dev/sdbX or the like -- with X being a number). Compare this with the list of mounted devices. If it's not there, try to mount it and check its contents (as shown above). To mount it, first create a mountpoint, e.g. sudo mkdir /mnt/mydisk, then try to mount the device using mount /dev/sda1 /mnt/mydisk and check its contents using ls /mnt/mydisk.

Once you get the right disk there, you can go to change the permissions back on your usr dir: sudo chmod 0755 /mnt/mydisk/usr.

Now you still might be in trouble if you originally ran the chmod command recursively, using the -R parameter. In that case you can either try to fix each entry manually -- or you can go straight for a fresh install...

Answer 3

I think Mat's correct, you must be root to add the bit to /usr/bin, but of course sudo is broken. If you have a root password you can use that to logon as root and then fix the permissions with the above command. If you don't, however, (and I don't either) it would probably be best to:

• boot from a Linux live CD
• become root there
• mount the partition with the above system
• then straighten out the permissions on that file system using a terminal.

Root is always user number 0 so root on any system can make changes allowed to root on other file systems.

Answer 4

I don't have much knowledge. But these steps solved my problem even without restarting my machine. Follow these steps:

su root
<enter root password>
cd /usr
chmod -R 755 *

Answer 5

In an attempt to set permissions for my local scripts, I broke the sudo permission and changed ownership, by mistake. I was able to change back the ownership of sudo to root by doing the following:

Step 1: Switch to ubuntu recovery mode. If you are unaware of the process, you can refer an answer here: https://askubuntu.com/a/172346/223901

Step 2: Once in recovery mode, select root - Drop to root shell prompt

Step 3: Do the following commands

mount -o remount,rw /
chown root:root /usr/bin/sudo
chmod 4755 /usr/bin/sudo
reboot

Wait for your system to boot normally and you will see the ownership of sudo back to root.

Answer 6

ramsudharsan is quite right ...
In my case Ubuntu 20.04

Step 1: Switch to ubuntu recovery mode. If you are unaware of the process, you can refer an answer here: https://askubuntu.com/a/172346/223901
My machine → reboot and [Esc] after splash screen. A little tricky, but after a few attemps GRUB show up ...

Step 2: Once in recovery mode, select root - Drop to root shell prompt

Step 3: Do the following commands

# mount -o remount,rw / → on my machine works without!
chown root:root /usr/bin/sudo
chmod 4755 /usr/bin/sudo
reboot

Step 4: After normal reboot/system start

sudo chmod 4755 /usr/bin/[files from the list below]

also the other files (no complet list):
/usr/bin:

• -rwsr-xr-x 1 root root 84K Jul 15 2021 chfn*
• -rwsr-xr-x 1 root root 52K Jul 15 2021 chsh*
• -rwsr-xr-x 1 root root 39K Mär 7 2020 fusermount*
• -rwsr-xr-x 1 root root 87K Jul 15 2021 gpasswd*
• -rwsr-xr-x 1 root root 310K Okt 11 2019 info*
• -rwsr-xr-x 1 root root 55K Jul 21 2020 mount*
• -rwsr-xr-x 1 root root 44K Jul 15 2021 newgrp*
• -rwsr-xr-x 1 root root 67K Jul 15 2021 passwd*
• -rwsr-xr-x 1 root root 31K Mai 26 2021 pkexec*
• -rwsr-xr-x 1 root root 67K Jul 21 2020 su*
• -rwsr-xr-x 1 root root 163K Jan 19 2021 sudo*
• -rwsr-xr-x 1 root root 39K Jul 21 2020 umount*
• -rwsr-xr-x 1 root root 26K Jan 11 2021 xdg-open*
• -rwsr-xr-x 1 root root 30 Nov 12 2017 xlinks2*

/usr/sbin/:
-rwsr-xr-x 1 root dip 386K Jul 23 2020 pppd

/usr/lib/policykit-1/:
-rwsr-xr-x 1 root root 23K Mai 26 2021 polkit-agent-helper-1

Thanks to the recovery mode, there is no need for a reinstallation! ...