system_server: neverallow blk_file read/write

With the exception of the factory reset protection block device,
don't allow system_server to read or write to any other block
devices. This helps protect against a system->root escalation
when system_server has the ability to directly minipulate raw
block devices / partitions / partition tables.

This change adds a neverallow rule, which is a compile time
assertion that no SELinux policy is written which allows this
access. No new rules are added or removed.

Change-Id: I388408423097ef7cf4950197b79d4be9d666362c

neverallow_macros

 #
 # Common neverallow permissions
 define(`no_w_file_perms', `{ append create link unlink relabelfrom rename setattr write }')
+define(`no_rw_file_perms', `{ no_w_file_perms open read ioctl lock }')
 define(`no_x_file_perms', `{ execute execute_no_trans }')
 define(`no_w_dir_perms',  `{ add_name create link relabelfrom remove_name rename reparent rmdir setattr write }')

system_server.te

@@ -492,3 +492,8 @@ neverallow system_server { bluetooth_data_file nfc_data_file shell_data_file app
 # system server to dynamically load a dex file, something we do not
 # want to allow.
 neverallow system_server dex2oat_exec:file no_x_file_perms;
+
+# The only block device system_server should be accessing is
+# the frp_block_device. This helps avoid a system_server to root
+# escalation by writing to raw block devices.
+neverallow system_server { dev_type -frp_block_device }:blk_file no_rw_file_perms;