Allow postinstall_file to be an entrypoint.

postinstall_file was an exec_type so it could be an entrypoint for the
domain_auto_trans from update_engine domain to postinstall domain. This
patch removes the exec_type from postinstall_file and exempts it from
the neverallow rule to become an entrypoint.

Bug: 28008031
TEST=postinstall_example still runs as the "postinstall" domain on edison-eng.

Change-Id: Icbf5b262c6f971ce054f1b4896c611b32a6d66b5

domain.te

@@ -201,8 +201,8 @@ neverallow { domain -init } kernel:security setsecparam;
 # Only init, ueventd and system_server should be able to access HW RNG
 neverallow { domain -init -system_server -ueventd } hw_random_device:chr_file *;
 
-# Ensure that all entrypoint executables are in exec_type.
-neverallow * { file_type -exec_type }:file entrypoint;
+# Ensure that all entrypoint executables are in exec_type or postinstall_file.
+neverallow * { file_type -exec_type -postinstall_file }:file entrypoint;
 
 # Ensure that nothing in userspace can access /dev/mem or /dev/kmem
 neverallow { domain -kernel -ueventd -init } kmem_device:chr_file *;

file.te

@@ -117,7 +117,7 @@ type storage_stub_file, file_type;
 # /postinstall: Mount point used by update_engine to run postinstall.
 type postinstall_mnt_dir, file_type;
 # Files inside the /postinstall mountpoint are all labeled as postinstall_file.
-type postinstall_file, file_type, exec_type;
+type postinstall_file, file_type;
 
 # /data/misc subdirectories
 type adb_keys_file, file_type, data_file_type;

postinstall.te

@@ -18,3 +18,7 @@ allow postinstall postinstall_file:dir r_dir_perms;
 allow postinstall shell_exec:file rx_file_perms;
 allow postinstall system_file:file rx_file_perms;
 allow postinstall toolbox_exec:file rx_file_perms;
+
+# No domain other than update_engine should transition to postinstall, as it is
+# only meant to run during the update.
+neverallow { domain -update_engine } postinstall:process { transition dyntransition };