Gnome Desktop Manager fails to start if /proc is mounted with hidepid=1.

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Environment

SUSE Linux Enterprise Server 15

Situation

Starting with SLES 12 (or Linux kernel 3.3) the kernel has two new mount options for the /proc file system.

hidepid: controls whether to hide or not the content inside /proc.
gid: defines a group that will be able to gather all processes' info.

hidepid can have three values:

0 ; (default) all users can read all world-readable /proc/PID/* files.
1 ; users may not access any /proc/<pid>/ directories, but their own.
2 ; means hidepid=1 plus all /proc/PID/ will be invisible to other users.

For security reasons, if server was configured to mount /proc with hidepid=1 ; Gnome Display Manager (GDM) may fail to startup.

With error messages similar to what is listed below :

2018-11-09T10:38:19.148175+01:00 srv01 gdm-x-session: Preparing auth file for X server
2018-11-09T10:38:19.148268+01:00 srv01 gdm-x-session: Running X server
2018-11-09T10:38:19.178031+01:00 srv01 systemd[1]: Started Process Core Dump (PID 6441/UID 0).
2018-11-09T10:38:19.207996+01:00 srv01 gdm-x-session: X server did not write display string
2018-11-09T10:38:19.208502+01:00 srv01 gdm-launch-environment]: GdmSessionWorker: child (pid:6438) done (status:70)
2018-11-09T10:38:19.208684+01:00 srv01 gdm-launch-environment]: GdmSessionWorker: uninitializing PAM
2018-11-09T10:38:19.209302+01:00 srv01 gdm-launch-environment]: pam_unix(gdm-launch-environment:session): session closed for user gdm
2018-11-09T10:38:19.209980+01:00 srv01 gdm-launch-environment]: GdmSessionWorker: jumping to VT 1
2018-11-09T10:38:19.222660+01:00 srv01 gdm-launch-environment]: GdmSessionWorker: couldn't finalize jump to VT 1: Interrupted system call
2018-11-09T10:38:19.222887+01:00 srv01 gdm-launch-environment]: GdmSessionWorker: state NONE
2018-11-09T10:38:19.223025+01:00 srv01 gdm: GdmSession: Emitting 'session-exited' signal with exit code '70'

Resolution

To fix this issue:

Create a new group
Add all users that need to read '/proc' filesystem into that group (including the 'gdm' user)
Create the file '/etc/systemd/system/systemd-logind.conf.d/hidepid.conf' with the below information (replace '<NEW_GROUP>' with the above group):

[Service]
SupplementaryGroups=<NEW_GROUP>

Remount the '/proc' file system with gid:

mount -o remount,hidepid=1,gid=<NEW_GROUP_ID> /proc

Restart GDM :

systemctl restart display-manager.service

GDM should start normally.

Cause

systemd-logind doesn't have full read access to the '/proc' file system.

Additional Information

For more information about hidepid option, please read this commit from kernel.org site.

Disclaimer

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