Created attachment 1297141 [details]
logs

Description of problem:
GDM fails to start when SELinux default user is mapped to the guest_u user on Fedora 26. The following error message is shown in the console:
  Failed to start User Manager for UID 42 

UID 42 is gdm:
  getent passwd | grep 42
  gdm:x:42:42::/var/lib/gdm:/sbin/nologin


On Fedora 25 GDM starts without any problems when SELinux default user mapped is to guest_u user.


Version-Release number of selected component (if applicable):
gdm-3.24.2-1.fc26.x86_64

Some other component versions:
pam-1.3.0-2.fc26.x86_64
selinux-policy-targeted-3.13.1-259.fc26.noarch
systemd-pam-233-6.fc26.x86_64
systemd-libs-233-6.fc26.x86_64


How reproducible:
Always.


Steps to Reproduce:
1. Install Fedora 26 Workstation with default settings.
2. Change mapping of default SELinux user to guest_u with the following command:
  semanage login -m -s guest_u __default__
3. Reboot the computer


Actual results:
Computer boot stops in text mode. The following error message is show:
  Failed to start User Manager for UID 42 


Expected results:
Computer booting to GNOME login screen.


Additional info:
It looks like gdm is started with guest_r role and this role is picked because SELinux default user is mapped to guest_u. SELinux default user should not be used for services.

See attachment for journalctl and AVCs output.