system_server: replace sys_resource with sys_ptrace

Commit https://android.googlesource.com/kernel/common/+/f0ce0eee added
CAP_SYS_RESOURCE as a capability check which would allow access to
sensitive /proc/PID files. However, in an SELinux based world, allowing
this access causes CAP_SYS_RESOURCE to duplicate what CAP_SYS_PTRACE
(without :process ptrace) already provides.

Use CAP_SYS_PTRACE instead of CAP_SYS_RESOURCE.

Add a neverallow rule to prevent system_server from using this
capability to ptrace attach to any other process. This limits the
capability of system_server to only reading sensitive /proc files, but
not ptrace() access.

Test: Device boots, functionality remains identical, no sys_resource
denials from system_server.
Bug: 34951864
Change-Id: I04d745b436ad75ee1ebecf0a61c6891858022e34

private/system_server.te

@@ -68,16 +68,13 @@ allow system_server self:capability {
     net_raw
     sys_boot
     sys_nice
-    sys_resource
+    sys_ptrace
     sys_time
     sys_tty_config
 };
 
 wakelock_use(system_server)
 
-# Triggered by /proc/pid accesses, not allowed.
-dontaudit system_server self:capability sys_ptrace;
-
 # Trigger module auto-load.
 allow system_server kernel:system module_request;
 
@@ -696,3 +693,11 @@ neverallow system_server system_server_tmpfs:file execute;
 # 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;
+
+# No ptracing others
+neverallow system_server { domain -system_server }:process ptrace;
+
+# CAP_SYS_RESOURCE was traditionally needed for sensitive /proc/PID
+# file read access. However, that is now unnecessary (b/34951864)
+# This neverallow can be removed after b/34951864 is fixed.
+neverallow system_server system_server:capability sys_resource;