System Recovery – How to Recover from chmod -R 000 /bin

You can run the loader directly, and pass it the command you want to run:

/lib/ld-linux.so /bin/chmod +x /bin/chmod

Your path to the loader might vary. On a 64-bit system you need to choose the right one based on how chmod was compiled; the 64-bit version is named something like /lib64/ld-linux-x86-64.so.2

The good news is that all your data is still there. The mixed news is that your system installation may or may not be recoverable — it depends where chmod stopped.

You will need to boot into a rescue system to repair it. From the rescue system, mount your broken installation somewhere, say /mnt. Issue the following commands:

chmod 755 /mnt
find /mnt -type d -perm 644 >/mnt/bad-permissions
find /mnt -type d -exec chmod 755 {} +

The first find command saves a record of directories with bad permissions into a file. The purpose is to see where permissions have been modified. The second find command changes all directories to be publicly accessible.

You now have a system where all directories listed in /mnt/bad-permissions and all files in these directories are world-readable. Furthermore files in these directories are not executable. Depending on which files were affected, this may be easily repairable or not. See Wrongly set chmod / 777. Problems? for what you can try to get the system functional, to which you should add

chmod a+x /bin/* /sbin/* /usr/bin/* /usr/sbin/* /lib*/ld-*

But even if you manage to get something working, there's a high risk that some permissions are still wrong, so I recommend reinstalling a new system, then restoring your data. How do I replicate installed package selections from one Debian system to another? (Debian Wheezy) should help.