Reject DHCP packets with no magic cookie

This patch adds an explicit check in the DHCP packet parser for
rejecting packets without a magic cookie, instead of relying on the
top-level try-catch-all in the parser.

This allows to add to DHCP error metrics this specific error.

It also allows to add two poor man's fuzzing tests that tries to find
additional gaps in the DHCP packet parser by
 - trying to parse all subslices of a valid offer packet.
 - trying to parse random byte arrays.

Test: covered by previously introduced malformed DHCP packet unit tests
      + additional fuzzing tests.
Bug: 31850211
Change-Id: If53c9ba9df78d7604ec018c9d67c237ae59c4833

core/java/android/net/metrics/DhcpErrorEvent.java

@@ -50,6 +50,8 @@ public final class DhcpErrorEvent implements Parcelable {
     public static final int DHCP_INVALID_OPTION_LENGTH = makeErrorCode(DHCP_ERROR, 3);
     public static final int DHCP_NO_MSG_TYPE           = makeErrorCode(DHCP_ERROR, 4);
     public static final int DHCP_UNKNOWN_MSG_TYPE      = makeErrorCode(DHCP_ERROR, 5);
+    /** {@hide} */
+    public static final int DHCP_NO_COOKIE             = makeErrorCode(DHCP_ERROR, 6);
 
     public static final int BUFFER_UNDERFLOW           = makeErrorCode(MISC_ERROR, 1);
     public static final int RECEIVE_ERROR              = makeErrorCode(MISC_ERROR, 2);

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

@@ -895,8 +895,12 @@ abstract class DhcpPacket {
                         + 64    // skip server host name (64 chars)
                         + 128); // skip boot file name (128 chars)
 
-        int dhcpMagicCookie = packet.getInt();
+        // Ensure this is a DHCP packet with a magic cookie, and not BOOTP. http://b/31850211
+        if (packet.remaining() < 4) {
+            throw new ParseException(DhcpErrorEvent.DHCP_NO_COOKIE, "not a DHCP message");
+        }
 
+        int dhcpMagicCookie = packet.getInt();
         if (dhcpMagicCookie != DHCP_MAGIC_COOKIE) {
             throw new ParseException(DhcpErrorEvent.DHCP_BAD_MAGIC_COOKIE,
                     "Bad magic cookie 0x%08x, should be 0x%08x",

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

@@ -27,6 +27,7 @@ import java.net.Inet4Address;
 import java.nio.ByteBuffer;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Random;
 import junit.framework.TestCase;
 
 import static android.net.dhcp.DhcpPacket.*;
@@ -430,7 +431,7 @@ public class DhcpPacketTest extends TestCase {
         try {
             DhcpPacket offerPacket = DhcpPacket.decodeFullPacket(packet, packet.length, ENCAP_L3);
         } catch (DhcpPacket.ParseException expected) {
-            assertDhcpErrorCodes(DhcpErrorEvent.PARSING_ERROR, expected.errorCode);
+            assertDhcpErrorCodes(DhcpErrorEvent.DHCP_NO_COOKIE, expected.errorCode);
             return;
         }
         fail("Dhcp packet parsing should have failed");
@@ -472,6 +473,55 @@ public class DhcpPacketTest extends TestCase {
         assertEquals(Integer.toHexString(expected), Integer.toHexString(got));
     }
 
+    public void testTruncatedOfferPackets() throws Exception {
+        final byte[] packet = HexDump.hexStringToByteArray(
+            // IP header.
+            "450001518d0600004011144dc0a82b01c0a82bf7" +
+            // UDP header.
+            "00430044013d9ac7" +
+            // BOOTP header.
+            "02010600dfc23d1f0002000000000000c0a82bf7c0a82b0100000000" +
+            // MAC address.
+            "30766ff2a90c00000000000000000000" +
+            // Server name.
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            // File.
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            // Options
+            "638253633501023604c0a82b01330400000e103a04000007083b0400000c4e0104ffffff00" +
+            "1c04c0a82bff0304c0a82b010604c0a82b012b0f414e44524f49445f4d455445524544ff");
+
+        for (int len = 0; len < packet.length; len++) {
+            try {
+                DhcpPacket.decodeFullPacket(packet, len, ENCAP_L3);
+            } catch (ParseException e) {
+                if (e.errorCode == DhcpErrorEvent.PARSING_ERROR) {
+                    fail(String.format("bad truncated packet of length %d", len));
+                }
+            }
+        }
+    }
+
+    public void testRandomPackets() throws Exception {
+        final int maxRandomPacketSize = 512;
+        final Random r = new Random();
+        for (int i = 0; i < 10000; i++) {
+            byte[] packet = new byte[r.nextInt(maxRandomPacketSize + 1)];
+            r.nextBytes(packet);
+            try {
+                DhcpPacket.decodeFullPacket(packet, packet.length, ENCAP_L3);
+            } catch (ParseException e) {
+                if (e.errorCode == DhcpErrorEvent.PARSING_ERROR) {
+                    fail("bad packet: " + HexDump.toHexString(packet));
+                }
+            }
+        }
+    }
+
     private byte[] mtuBytes(int mtu) {
         // 0x1a02: option 26, length 2. 0xff: no more options.
         if (mtu > Short.MAX_VALUE - Short.MIN_VALUE) {