I have just installed CentOS 7 and quickly mixed up my root passwd.
I did not enable my one non root user for sudo.

I get to single user mode by modifying ro to rw init=/sysroot/bin/sh

in the following line in the (grub2?) boot screen:

linuxefi /vmlinuz-3.10.0-229.e17.x86_64 root=/dev/mapper/centos-root ro rd.lvm.lv=centos/swap rd.lvm.lv=centos/usr rhgb quiet LANG=en.US-UTF-8

CTRL x to boot, then chroot /sysroot to be able to use passwd command.

I've changed root passwd, my one users passwd, created a second regular user tried not setting a passwd by replying with the enter key (this failed to change the passwd), stayed in single user mode and su - <normal_user>, then su - <second_normal_user>; always I get Authentication failure. The passwd command says it completed successfully but I can never use the new password. As of now all I can do is work in single user mode.

I also tried switching between not using /etc/shadow and using /etc/shadow with the pwconv and pwunconv commands. I get Authentication failure in both cases.

I am concerned about re-installing the Linux system because there is a Windows 10 also installed. On every machine but this one (three others) when I installed linux it wipped out windows. The difference this time was I explicitly defined the linux parititions as part of the installation.

I set SELINUX=permissive in /etc/system/selinux and after a reboot I can login as root.

Now I could use a good beginners tutorial on finding and fixing situations caused by selinux