Restore the net.dnsX system properties.

In order to avoid app breakage, we are putting back the system
properties and using selinux to disallow access from apps
targeting the O SDK or above.

This CL is cherry-picked from internal commit
443b6c4049, but with a different
commit description.

Bug: 33308258
Bug: 33807046
Bug: 34028616
Bug: 34115651
Test: system properties are readable as root and shell
Change-Id: I8d51e8e0a620d581c4251fb2f3c1d0a813b929be

services/core/java/com/android/server/ConnectivityService.java

@@ -4564,9 +4564,28 @@ public class ConnectivityService extends IConnectivityManager.Stub
         } catch (Exception e) {
             loge("Exception in setDnsConfigurationForNetwork: " + e);
         }
+        final NetworkAgentInfo defaultNai = getDefaultNetwork();
+        if (defaultNai != null && defaultNai.network.netId == netId) {
+            setDefaultDnsSystemProperties(dnses);
+        }
         flushVmDnsCache();
     }
 
+    private void setDefaultDnsSystemProperties(Collection<InetAddress> dnses) {
+        int last = 0;
+        for (InetAddress dns : dnses) {
+            ++last;
+            String key = "net.dns" + last;
+            String value = dns.getHostAddress();
+            SystemProperties.set(key, value);
+        }
+        for (int i = last + 1; i <= mNumDnsEntries; ++i) {
+            String key = "net.dns" + i;
+            SystemProperties.set(key, "");
+        }
+        mNumDnsEntries = last;
+    }
+
     private String getNetworkPermission(NetworkCapabilities nc) {
         // TODO: make these permission strings AIDL constants instead.
         if (!nc.hasCapability(NET_CAPABILITY_NOT_RESTRICTED)) {
@@ -4783,6 +4802,7 @@ public class ConnectivityService extends IConnectivityManager.Stub
         notifyLockdownVpn(newNetwork);
         handleApplyDefaultProxy(newNetwork.linkProperties.getHttpProxy());
         updateTcpBufferSizes(newNetwork);
+        setDefaultDnsSystemProperties(newNetwork.linkProperties.getDnsServers());
     }
 
     private void processListenRequests(NetworkAgentInfo nai, boolean capabilitiesChanged) {