Debug System Folder Permissions

You are helping the user debug systemwide folder permissions and ensure they are set appropriately.

Your tasks:

1. Gather information from user: Ask:

   • Are they experiencing specific permission errors?
   • Which directories or operations are affected?
   • What user/group should have access?

2. Check common system directories:

   Root filesystem:

   ls -ld /
   # Should be: drwxr-xr-x root root
   

   Essential system directories:

   ls -ld /bin /sbin /usr /usr/bin /usr/sbin /lib /lib64
   # Should be: drwxr-xr-x root root
   

   Variable data:

   ls -ld /var /var/log /var/tmp
   # /var: drwxr-xr-x root root
   # /var/log: drwxrwxr-x root syslog (or root root)
   # /var/tmp: drwxrwxrwt root root (sticky bit)
   

   Temporary directories:

   ls -ld /tmp
   # Should be: drwxrwxrwt root root (sticky bit important!)
   

   Home directories:

   ls -ld /home /home/$USER
   # /home: drwxr-xr-x root root
   # /home/$USER: drwxr-xr-x $USER $USER (or drwx------ for privacy)
   

3. Check for permission issues:

   World-writable directories without sticky bit (security risk):

   sudo find / -type d -perm -0002 ! -perm -1000 2>/dev/null
   

   Files with SUID bit (potential security issue if unexpected):

   sudo find / -type f -perm -4000 2>/dev/null
   

   Files with SGID bit:

   sudo find / -type f -perm -2000 2>/dev/null
   

4. Check /etc permissions:

   ls -la /etc | head -20
   # /etc itself: drwxr-xr-x root root
   # Most files should be 644 (rw-r--r--)
   # Some may be 640 or 600 for security
   

   Sensitive files:

   ls -l /etc/shadow /etc/gshadow /etc/ssh/sshd_config
   # /etc/shadow: -rw-r----- root shadow
   # /etc/ssh/sshd_config: -rw-r--r-- root root
   

5. Check user home directory structure:

   ls -la ~/ | grep "^d"
   

   Common directories and recommended permissions:

   • ~/.ssh: 700 (drwx------)
   • ~/.ssh/id_rsa: 600 (-rw-------)
   • ~/.ssh/id_rsa.pub: 644 (-rw-r--r--)
   • ~/.ssh/authorized_keys: 600 (-rw-------)
   • ~/.gnupg: 700 (drwx------)
   • ~/bin: 755 (drwxr-xr-x)
   • ~/.local: 755 (drwxr-xr-x)
   • ~/.config: 755 (drwxr-xr-x)

6. Check /opt and /usr/local:

   ls -ld /opt /usr/local /usr/local/bin
   # Typically: drwxr-xr-x root root
   # But may be group-writable for admin group
   

7. Check mount points:

   mount | grep "^/" | awk '{print $3}' | while read mp; do
     ls -ld "$mp"
   done
   

8. Check ownership of user files: Find files in home directory not owned by user:

   find ~/ -not -user $USER 2>/dev/null
   

9. Check group memberships:

   groups
   id
   

   Common groups users might need:

   • sudo - for administrative access
   • docker - for Docker access
   • video - for video devices
   • audio - for audio devices
   • plugdev - for removable devices
   • dialout - for serial ports

10. Fix common issues:

    Fix sticky bit on /tmp:

    sudo chmod 1777 /tmp
    

    Fix ~/.ssh permissions:

    chmod 700 ~/.ssh
    chmod 600 ~/.ssh/id_rsa
    chmod 644 ~/.ssh/id_rsa.pub
    chmod 600 ~/.ssh/authorized_keys
    chmod 600 ~/.ssh/config
    

    Fix ownership of home directory:

    sudo chown -R $USER:$USER ~/
    

    Fix common directories:

    chmod 755 ~/.local ~/.config ~/bin
    

11. Check for ACL (Access Control Lists):

    getfacl /path/to/directory
    

    If ACLs are in use (indicated by + in ls -l):

    ls -la | grep "+"
    

12. Check SELinux context (if enabled):

    getenforce
    ls -Z /path/to/directory
    

13. Check for immutable flags:

    lsattr /path/to/file
    

    If files have i flag, they can't be modified even by root:

    sudo chattr -i /path/to/file
    

14. Specific directory recommendations:

    /var/www (web server):

    sudo chown -R www-data:www-data /var/www
    sudo find /var/www -type d -exec chmod 755 {} \;
    sudo find /var/www -type f -exec chmod 644 {} \;
    

    /srv (service data):

    sudo chown -R root:root /srv
    sudo chmod 755 /srv
    

    Shared directories:

    sudo chown root:groupname /shared/directory
    sudo chmod 2775 /shared/directory  # SGID bit for group
    

15. Check logs for permission denials:

    sudo journalctl -p err | grep -i "permission denied"
    dmesg | grep -i "permission denied"
    sudo grep "permission denied" /var/log/syslog
    

16. Report findings: Summarize:

    • Incorrect permissions on system directories
    • Security issues (world-writable without sticky, unexpected SUID)
    • User home directory issues
    • Files/directories with wrong ownership
    • Missing group memberships
    • ACL or SELinux issues

17. Provide recommendations:

    • Fix commands for identified issues
    • Whether to add user to specific groups
    • Security improvements for sensitive directories
    • Standard permission schemes for common directories
    • Whether to use ACLs for complex permission needs

Important notes:

• Always backup or test in safe environment first
• Changing system permissions incorrectly can break the system
• Use sudo carefully when fixing permissions
• Don't recursively chmod/chown system directories without understanding
• Some non-standard permissions may be intentional
• Check application documentation for required permissions
• SELinux/AppArmor may also affect access beyond traditional permissions
• Sticky bit on /tmp is critical for security
• SUID/SGID bits on unexpected files are security risks