Allow system_server to read apex_data_file.

For consistency with APKs, signature verification is performed
in the system_server. This includes checking that the signature of
an updated install matches the signature of the active package that
it updates. For this, it requires search access to /data/apex and
read access to the files under that directory.

Test: m
Change-Id: Ia073adb8892886e4767fa5529e95c110b9cbff1b

private/apexd.te

@@ -68,5 +68,5 @@ allow apexd sysfs_loop:file rw_file_perms;
 dontaudit apexd self:global_capability_class_set { dac_override dac_read_search };
 
 neverallow { domain -apexd -init } apex_data_file:dir no_w_dir_perms;
-neverallow { domain -apexd -init -kernel } apex_data_file:file no_rw_file_perms;
+neverallow { domain -apexd -init -kernel } apex_data_file:file no_w_file_perms;
 neverallow { domain -apexd } apex_mnt_dir:lnk_file no_w_file_perms;

private/system_server.te

@@ -963,6 +963,13 @@ allow system_server system_server_startup:unix_dgram_socket write;
 allow system_server apex_service:service_manager find;
 allow system_server apexd:binder call;
 
+# Allow the system server to read files under /data/apex. The system_server
+# needs these privileges to compare file signatures while processing installs.
+#
+# Only apexd is allowed to create new entries or write to any file under /data/apex.
+allow system_server apex_data_file:dir search;
+allow system_server apex_data_file:file r_file_perms;
+
 # dexoptanalyzer is currently used only for secondary dex files which
 # system_server should never access.
 neverallow system_server dexoptanalyzer_exec:file no_x_file_perms;