sddm issue and patch not for inclusion

[Russell Coker <russell@coker.com.au>]

In Debian/Unstable (which will soon be frozen and become the next stable
release) the sddm X login program (the one that's generally recommended
and specifically known to generally work well with SE Linux) uses PAM to
start a session for the "greeter" (the program that asks for a password
before a new session is started).

With the policy currently in Debian that means the sddm user matches
"__default__" and gets unconfined_u:unconfined_r:unconfined_t, not what
is desirable for a program that takes input from unauthenticated users.

role xdm_r;
role xdm_r types xdm_t;
allow system_r xdm_r;
allow xdm_t xdm_tmpfs_t:file execmod;
corecmd_bin_entry_type(xdm_t)

To get this working as a test I put the above in a local policy file,
edited /etc/selinux/default/contexts/default_contexts to add a suitable
context to the system_r:xdm_t:s0 line, and run the following 2 commands:
semanage user -a -r s0 -L s0 -R xdm_r -P user xdm
semanage login -a -s xdm -r s0 sddm

I mention the above for the benefit of people who do web searches for such
things and get the list archives.

Below is the policy I'm using which will be in the next release of Debian
if no-one else has a better idea.  NB a "better idea" doesn't mean running
the greeter as unconfined_t IMHO.  Also while we can debate about whether
modifying sddm to not use PAM for the greeter session is a good idea, such
a change would potentially affect people who don't use SE Linux so I won't
even waste the time of the sddm maintainer by discussing that possibility
with them before the release.  After the release we can discuss such
things, but now we need to get things working well in the next few days in
a manner that will make users happy for the next 2 years.

Index: refpolicy-2.20210126/policy/modules/services/xserver.te
===================================================================
--- refpolicy-2.20210126.orig/policy/modules/services/xserver.te
+++ refpolicy-2.20210126/policy/modules/services/xserver.te
@@ -18,6 +18,7 @@ gen_require(`
 	class x_resource all_x_resource_perms;
 	class x_event all_x_event_perms;
 	class x_synthetic_event all_x_synthetic_event_perms;
+	role xdm_r;
 ')
 
 ########################################
@@ -152,6 +153,10 @@ init_daemon_domain(xdm_t, xdm_exec_t)
 xserver_object_types_template(xdm)
 xserver_common_x_domain_template(xdm, xdm_t)
 
+# for sddm to use pam for greeter
+role xdm_r types xdm_t;
+allow system_r xdm_r;
+
 type xdm_lock_t;
 files_lock_file(xdm_lock_t)
 
@@ -848,6 +853,11 @@ manage_files_pattern(xserver_t, xdm_tmp_
 manage_lnk_files_pattern(xserver_t, xdm_tmp_t, xdm_tmp_t)
 manage_sock_files_pattern(xserver_t, xdm_tmp_t, xdm_tmp_t)
 
+# for sddm to use pam for greeter
+corecmd_bin_entry_type(xdm_t)
+# sddm greeter needs execmod
+allow xdm_t xdm_tmpfs_t:file execmod;
+
 # Run Xorg.wrap
 can_exec(xserver_t, xserver_exec_t)
 
Index: refpolicy-2.20210126/config/appconfig-mcs/seusers
===================================================================
--- refpolicy-2.20210126.orig/config/appconfig-mcs/seusers
+++ refpolicy-2.20210126/config/appconfig-mcs/seusers
@@ -1,2 +1,3 @@
 root:unconfined_u:s0-mcs_systemhigh
 __default__:unconfined_u:s0-mcs_systemhigh
+sddm:xdm:s0
Index: refpolicy-2.20210126/policy/users
===================================================================
--- refpolicy-2.20210126.orig/policy/users
+++ refpolicy-2.20210126/policy/users
@@ -27,6 +27,7 @@ gen_user(system_u,, system_r, s0, s0 - m
 gen_user(user_u, user, user_r, s0, s0)
 gen_user(staff_u, staff, staff_r sysadm_r ifdef(`enable_mls',`secadm_r auditadm_r'), s0, s0 - mls_systemhigh, mcs_allcats)
 gen_user(sysadm_u, sysadm, sysadm_r, s0, s0 - mls_systemhigh, mcs_allcats)
+gen_user(xdm, user, xdm_r, s0, s0)
 
 # Until order dependence is fixed for users:
 ifdef(`direct_sysadm_daemon',`
Index: refpolicy-2.20210126/config/appconfig-mcs/xdm_default_contexts
===================================================================
--- /dev/null
+++ refpolicy-2.20210126/config/appconfig-mcs/xdm_default_contexts
@@ -0,0 +1 @@
+system_r:xdm_t:s0		xdm_r:xdm_t:s0
Index: refpolicy-2.20210126/policy/modules/kernel/kernel.te
===================================================================
--- refpolicy-2.20210126.orig/policy/modules/kernel/kernel.te
+++ refpolicy-2.20210126/policy/modules/kernel/kernel.te
@@ -32,6 +32,7 @@ role system_r;
 role sysadm_r;
 role staff_r;
 role user_r;
+role xdm_r;
 
 # here until order dependence is fixed:
 role unconfined_r;