Unable to SU with root: `/bin/bash: permission denied`

Change the permission of these folders like this and now you can su to another user.

chmod 755 /
chmod 755 /bin
chmod 755 /lib

1. Check the permissions of /bin folder

   # ls -ld /bin
   drwxr-xr-x 2 root root 4096 May 27 21:39 /bin
   

2. Check the permissions of all shells available

   # ls -l /bin/*sh
   -rwxr-xr-x 1 root root 1037464 Sep  1  2015 /bin/bash
   -rwxr-xr-x 1 root root  154072 Feb 17 21:25 /bin/dash
   lrwxrwxrwx 1 root root       4 Sep  1  2015 /bin/rbash -> bash
   lrwxrwxrwx 1 root root       4 Feb 17 21:25 /bin/sh -> dash
   lrwxrwxrwx 1 root root       7 Aug 19  2015 /bin/static-sh -> busybox
   

   Some are links that we should check their targets

   # ls -lL /bin/*sh
   -rwxr-xr-x 1 root root 1037464 Sep  1  2015 /bin/bash
   -rwxr-xr-x 1 root root  154072 Feb 17 21:25 /bin/dash
   -rwxr-xr-x 1 root root 1037464 Sep  1  2015 /bin/rbash
   -rwxr-xr-x 1 root root  154072 Feb 17 21:25 /bin/sh
   -rwxr-xr-x 1 root root 1964536 Aug 19  2015 /bin/static-sh
   

3. Try another shell

   The best is busybox because it is a static build (No .so library needed)

   su newuser -s /bin/static-sh
   

   Next is dash, low dependencies and installed by default

   su newuser -s /bin/dash
   

4. Check permissions of libraries and their parent folders, you can get list using ldd

   # ldd /bin/bash
       linux-vdso.so.1 =>  (0x00007ffdefb5a000)
       libtinfo.so.5 => /lib/x86_64-linux-gnu/libtinfo.so.5 (0x00007f714bbbd000)
       libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f714b9b9000)
       libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f714b5ef000)
       /lib64/ld-linux-x86-64.so.2 (0x000055c6bc494000)
   
   # ls -ld /lib /lib/x86_64-linux-gnu /lib64
   drwxr-xr-x 26 root root  4096 May 15 07:41 /lib
   drwxr-xr-x  2 root root  4096 May 14 15:52 /lib64
   drwxr-xr-x  3 root root 16384 May 27 21:39 /lib/x86_64-linux-gnu
   
   # ls -l /lib/x86_64-linux-gnu/libtinfo.so.5 /lib/x86_64-linux-gnu/libdl.so.2 /lib/x86_64-linux-gnu/libc.so.6 /lib64/ld-linux-x86-64.so.2
   lrwxrwxrwx 1 root root 32 Apr 14 23:16 /lib64/ld-linux-x86-64.so.2 -> /lib/x86_64-linux-gnu/ld-2.23.so
   lrwxrwxrwx 1 root root 12 Apr 14 23:16 /lib/x86_64-linux-gnu/libc.so.6 -> libc-2.23.so
   lrwxrwxrwx 1 root root 13 Apr 14 23:16 /lib/x86_64-linux-gnu/libdl.so.2 -> libdl-2.23.so
   lrwxrwxrwx 1 root root 15 Feb 19 09:23 /lib/x86_64-linux-gnu/libtinfo.so.5 -> libtinfo.so.5.9
   

   They are just links we need to verify the target files

   # ls -lH /lib/x86_64-linux-gnu/libtinfo.so.5 /lib/x86_64-linux-gnu/libdl.so.2 /lib/x86_64-linux-gnu/libc.so.6 /lib64/ld-linux-x86-64.so.2
   ##or
   # ls -lL /lib/x86_64-linux-gnu/libtinfo.so.5 /lib/x86_64-linux-gnu/libdl.so.2 /lib/x86_64-linux-gnu/libc.so.6 /lib64/ld-linux-x86-64.so.2
   -rwxr-xr-x 1 root root  162632 Apr 14 23:16 /lib64/ld-linux-x86-64.so.2
   -rwxr-xr-x 1 root root 1864888 Apr 14 23:16 /lib/x86_64-linux-gnu/libc.so.6
   -rw-r--r-- 1 root root   14608 Apr 14 23:16 /lib/x86_64-linux-gnu/libdl.so.2
   -rw-r--r-- 1 root root  167240 Feb 19 09:23 /lib/x86_64-linux-gnu/libtinfo.so.5