Don't crash if we get a DHCP packet with the wrong port.

This should only happen if we get a packet in the small time
window between binding the packet socket and programming the
BPF filter on it.

Bug: 26696823
Change-Id: I481f1bc74bbaeb9646d96e1841d2a69acdb47d62

Author: lcolitti

services/net/java/android/net/dhcp/DhcpPacket.java

@@ -804,7 +804,11 @@ public static DhcpPacket decodeFullPacket(ByteBuffer packet, int pktType) throws
             // server-to-server packets, e.g. for relays.
             if (!isPacketToOrFromClient(udpSrcPort, udpDstPort) &&
                 !isPacketServerToServer(udpSrcPort, udpDstPort)) {
-                return null;
+                // This should almost never happen because we use SO_ATTACH_FILTER on the packet
+                // socket to drop packets that don't have the right source ports. However, it's
+                // possible that a packet arrives between when the socket is bound and when the
+                // filter is set. http://b/26696823 .
+                throw new ParseException("Unexpected UDP ports %d->%d", udpSrcPort, udpDstPort);
             }
         }
 

services/tests/servicestests/src/android/net/dhcp/DhcpPacketTest.java

@@ -551,6 +551,39 @@ public void testUdpServerAnySourcePort() throws Exception {
                 "wvm.edu", "10.1.105.252", null, 86400, false, dhcpResults);
     }
 
+    @SmallTest
+    public void testUdpInvalidDstPort() throws Exception {
+        final ByteBuffer packet = ByteBuffer.wrap(HexEncoding.decode((
+            // Ethernet header.
+            "9cd917000000001c2e0000000800" +
+            // IP header.
+            "45a00148000040003d115087d18194fb0a0f7af2" +
+            // UDP header. TODO: fix invalid checksum (due to MAC address obfuscation).
+            // NOTE: The destination port is a non-DHCP port.
+            "0043aaaa01341268" +
+            // BOOTP header.
+            "02010600d628ba8200000000000000000a0f7af2000000000a0fc818" +
+            // MAC address.
+            "9cd91700000000000000000000000000" +
+            // Server name.
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            // File.
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            // Options.
+            "6382536335010236040a0169fc3304000151800104ffff000003040a0fc817060cd1818003d1819403" +
+            "d18180060f0777766d2e6564751c040a0fffffff000000"
+        ).toCharArray(), false));
+
+        try {
+            DhcpPacket.decodeFullPacket(packet, ENCAP_L2);
+            fail("Packet with invalid dst port did not throw ParseException");
+        } catch (ParseException expected) {}
+    }
+
     @SmallTest
     public void testMultipleRouters() throws Exception {
         final ByteBuffer packet = ByteBuffer.wrap(HexEncoding.decode((