Device SELinux policy for secure persistent netd storage

This is used to persist RFC 7217 stable secrets across device reboots.
For now this is device-specific; in future releases it will be moved
to public sepolicy.

Bug: 17613910
Test: stable_secret is generated on first use and persists across reboots
Test: "adb shell getprop persist.netd.stable_secret" doesn't work after "adb unroot"
Change-Id: I0a609c724799a15b1926e62534c16810d34f2275

sepolicy/private/netd.te

@@ -0,0 +1,10 @@
+# TODO: delete this once it's in common sepolicy.
+set_prop(netd, netd_stable_secret_prop)
+
+# persist.netd.stable_secret contains RFC 7217 secret key which should never be
+# leaked to other processes. Make sure it never leaks.
+neverallow { domain -netd -init } netd_stable_secret_prop:file r_file_perms;
+
+# We want to ensure that no other process ever tries tampering with persist.netd.stable_secret,
+# the RFC 7217 secret key managed by netd. Doing so could compromise user privacy.
+neverallow { domain -netd -init } netd_stable_secret_prop:property_service set;

sepolicy/private/property.te

@@ -0,0 +1,2 @@
+# TODO: delete this once it's in common sepolicy.
+type netd_stable_secret_prop, property_type;

sepolicy/private/property_contexts

@@ -0,0 +1,2 @@
+# TODO: delete this once it's in common sepolicy.
+persist.netd.stable_secret    u:object_r:netd_stable_secret_prop:s0