Bug#1125486: gdm3: /tmp/.X11-unix/ owned by gdm-greeter, causes user session to fail to start

[Ian McKellar]

Package: gdm3
Version: 49.2-3
Severity: critical
Justification: breaks the whole system
X-Debbugs-Cc: ian at mckellar.org

Dear Maintainer,

I updated my sid system and can no longer log in. I use gdm3 with the
default GNOME wayland based session. After I enter my password or used
the fingerprint sensor I got kicked right back to the login prompt.

I looked through logs and saw this kind of thing:
Jan 14 08:36:37 frau gnome-shell[3175]: Failed to start X Wayland: Wrong ownership for directory "/tmp/.X11-unix", owned by 60578 but should be same as /tmp (owned by (0)) or 1000
Jan 14 08:36:37 frau gnome-shell[3175]: == Stack trace for context 0x558653b20710 ==
Jan 14 08:36:37 frau systemd[2430]: org.gnome.Shell at wayland.service: Main process exited, code=killed, status=5/TRAP
Jan 14 08:36:37 frau systemd[2430]: org.gnome.Shell at wayland.service: Failed with result 'signal'.
Jan 14 08:36:37 frau systemd[2430]: Failed to start org.gnome.Shell at wayland.service - GNOME Shell on Wayland.

When I looked, indeed /tmp/.X11-unix is owned by the gdm greeter user
and there are two unix domain sockets owned by the gdm greeter user, and
an instance of Xwayland running as that user.

After some hackery I was able to run xlsclients against the two display
sockets and they just show:
  frau  ibus-x11
  frau  mutter-x11-frames
Using xprop I determined that they're from programs of the same name
under /usr/lib/libexec/, running as the gdm greeter user.

I can mitigate this by running the following as root from a virtual
console after I boot:
  chown root:root /tmp/.X11-unix
  chmod a+w /tmp/.X11-unix
and then everything works fine. But this isn't ideal for me, or anyone
else in the same situation.

I'm not sure what's changed - whether there's always been an Xwayland
server running in the gdm greeter session, or whether the way it's
launched as changed or what.

FYI, my xwayland package version is 2:24.1.9-1
since that might be relevant and it's not a dependency of gdm3 so not
shown below.

-- System Information:
Debian Release: forky/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'testing'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 6.18.3+deb14-amd64 (SMP w/24 CPU threads; PREEMPT)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages gdm3 depends on:
ii  accountsservice                       23.13.9-8
ii  dbus [default-dbus-system-bus]        1.16.2-2
ii  dbus-bin                              1.16.2-2
ii  dconf-cli                             0.49.0-4
ii  dconf-gsettings-backend               0.49.0-4
ii  debconf [debconf-2.0]                 1.5.91
ii  gir1.2-gdm-1.0                        49.2-3
ii  gnome-session [x-session-manager]     49.2-3
ii  gnome-session-bin                     49.2-3
ii  gnome-session-common                  49.2-3
ii  gnome-settings-daemon                 49.1-4
ii  gnome-shell                           49.2-2
ii  gnome-terminal [x-terminal-emulator]  3.58.1-2
ii  gsettings-desktop-schemas             50~alpha-1
ii  libaccountsservice0                   23.13.9-8
ii  libaudit1                             1:4.1.2-1+b1
ii  libc6                                 2.42-7
ii  libgdm1                               49.2-3
ii  libglib2.0-0t64                       2.86.3-4
ii  libglib2.0-bin                        2.86.3-4
ii  libgudev-1.0-0                        238-7
ii  libjson-glib-1.0-0                    1.10.8+ds-2
ii  libkeyutils1                          1.6.3-6
ii  libnss-systemd                        259-1
ii  libpam-modules                        1.7.0-5
ii  libpam-runtime                        1.7.0-5
ii  libpam-systemd [logind]               259-1
ii  libpam0g                              1.7.0-5
ii  librsvg2-common                       2.61.3+dfsg-3
ii  libselinux1                           3.9-2
ii  libsystemd0                           259-1
ii  libxau6                               1:1.0.11-1
ii  libxcb1                               1.17.0-2+b1
ii  polkitd                               127-1
ii  procps                                2:4.0.4-9
ii  ptyxis [x-terminal-emulator]          49.2-4
ii  systemd [systemd-sysusers]            259-1
ii  systemd-sysv                          259-1
ii  ucf                                   3.0052
ii  x11-common                            1:7.7+26
ii  x11-xserver-utils                     7.7+11
ii  xterm [x-terminal-emulator]           406-1

Versions of packages gdm3 recommends:
ii  at-spi2-core                       2.59.0-1
ii  desktop-base                       14.0.0~pre2
ii  gnome-session [x-session-manager]  49.2-3
ii  x11-xkb-utils                      7.7+9
ii  zenity                             4.1.99-1

Versions of packages gdm3 suggests:
ii  libpam-fprintd        1.94.5-2
ii  libpam-gnome-keyring  48.0-5
pn  libpam-pkcs11         <none>
pn  libpam-sss            <none>
ii  orca                  49.5-1

-- debconf information:
  gdm3/daemon_name: /usr/sbin/gdm3
* shared/default-x-display-manager: gdm3