Remove MAC capabilities from unconfined domains.

Linux defines two capabilities for Mandatory Access Control (MAC)
security modules, CAP_MAC_OVERRIDE (override MAC access restrictions)
and CAP_MAC_ADMIN (allow MAC configuration or state changes).
SELinux predates these capabilities and did not originally use them,
but later made use of CAP_MAC_ADMIN as a way to control the ability
to set security context values unknown to the currently loaded
SELinux policy on files.  That facility is used in Linux for e.g.
livecd creation where a file security context that is being set
on a generated filesystem is not known to the build host policy.
Internally, files with such labels are treated as having the unlabeled
security context for permission checking purposes until/unless the
context is later defined through a policy reload.

CAP_MAC_OVERRIDE is never checked by SELinux, so it never needs
to be allowed.  CAP_MAC_ADMIN is only checked if setting an
unknown security context value; the only legitimate use I can see
in Android is the recovery console, where a context may need to be set
on /system that is not defined in the recovery policy.

Remove these capabilities from unconfined domains, allow
mac_admin for the recovery domain, and add neverallow rules.

Change-Id: Ief673e12bc3caf695f3fb67cabe63e68f5f58150
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

domain.te

@@ -147,6 +147,9 @@ neverallow { domain -relabeltodomain } *:dir_file_class_set relabelto;
 ### neverallow rules
 ###
 
+neverallow domain self:capability2 mac_override;
+neverallow { domain -recovery } self:capability2 mac_admin;
+
 # Only init should be able to load SELinux policies.
 # The first load technically occurs while still in the kernel domain,
 # but this does not trigger a denial since there is no policy yet.

recovery.te

@@ -4,6 +4,8 @@ allow recovery rootfs:file entrypoint;
 unconfined_domain(recovery)
 relabelto_domain(recovery)
 
+allow recovery self:capability2 mac_admin;
+
 allow recovery {fs_type dev_type -kmem_device file_type}:dir_file_class_set relabelto;
 allow recovery unlabeled:filesystem mount;
 

unconfined.te

@@ -16,7 +16,8 @@
 # The use of this template is discouraged.
 ######################################################
 
-allow unconfineddomain self:capability_class_set *;
+allow unconfineddomain self:capability *;
+allow unconfineddomain self:capability2 ~{ mac_override mac_admin };
 allow unconfineddomain kernel:security ~{ load_policy setenforce setcheckreqprot };
 allow unconfineddomain kernel:system *;
 allow unconfineddomain domain:process ~{ execmem execstack execheap ptrace transition dyntransition };