Don't crash on (invalid) hardware address lengths > 127.

These would cause us to crash with a NegativeArraySizeException
when trying to create the clientMac array. Instead, if the length
is > 16 (invalid, because the field is only 16 bytes long), fudge
it to 6 (Ethernet / wifi).  This is a bit less liberal than the
legacy client, which doesn't check the length at all.

Bug: 23725795
Change-Id: I83f47bfc400ffa8ce85dd9d1b8eb96be5afe51a5

2 files changed, 83 insertions, 2 deletions

diff --git a/services/net/java/android/net/dhcp/DhcpPacket.java b/services/net/java/android/net/dhcp/DhcpPacket.java
index a91ddb8e4185..cbf8fc21b809 100644
--- a/services/net/java/android/net/dhcp/DhcpPacket.java
+++ b/services/net/java/android/net/dhcp/DhcpPacket.java
@@ -55,6 +55,7 @@ abstract class DhcpPacket {
     public static final int MIN_PACKET_LENGTH_L3 = MIN_PACKET_LENGTH_BOOTP + 20 + 8;
     public static final int MIN_PACKET_LENGTH_L2 = MIN_PACKET_LENGTH_L3 + 14;
 
+    public static final int HWADDR_LEN = 16;
     public static final int MAX_OPTION_LEN = 255;
     /**
      * IP layer definitions.
@@ -399,7 +400,7 @@ abstract class DhcpPacket {
         buf.put(mRelayIp.getAddress());
         buf.put(mClientMac);
         buf.position(buf.position() +
-                     (16 - mClientMac.length) // pad addr to 16 bytes
+                     (HWADDR_LEN - mClientMac.length) // pad addr to 16 bytes
                      + 64     // empty server host name (64 bytes)
                      + 128);  // empty boot file name (128 bytes)
         buf.putInt(0x63825363); // magic number
@@ -786,7 +787,7 @@ abstract class DhcpPacket {
 
         byte type = packet.get();
         byte hwType = packet.get();
-        byte addrLen = packet.get();
+        int addrLen = packet.get() & 0xff;
         byte hops = packet.get();
         transactionId = packet.getInt();
         secs = packet.getShort();
@@ -807,6 +808,16 @@ abstract class DhcpPacket {
             return null;
         }
 
+        // Some DHCP servers have been known to announce invalid client hardware address values such
+        // as 0xff. The legacy DHCP client accepted these becuause it does not check the length at
+        // all but only checks that the interface MAC address matches the first bytes of the address
+        // in the packets. We're a bit stricter: if the length is obviously invalid (i.e., bigger
+        // than the size of the field), we fudge it to 6 (Ethernet). http://b/23725795
+        // TODO: evaluate whether to make this test more liberal.
+        if (addrLen > HWADDR_LEN) {
+            addrLen = ETHER_BROADCAST.length;
+        }
+
         clientMac = new byte[addrLen];
         packet.get(clientMac);
 
diff --git a/services/tests/servicestests/src/android/net/dhcp/DhcpPacketTest.java b/services/tests/servicestests/src/android/net/dhcp/DhcpPacketTest.java
index da1df1accdb0..cd3b8bb7e0b9 100644
--- a/services/tests/servicestests/src/android/net/dhcp/DhcpPacketTest.java
+++ b/services/tests/servicestests/src/android/net/dhcp/DhcpPacketTest.java
@@ -333,6 +333,76 @@ public class DhcpPacketTest extends TestCase {
     }
 
     @SmallTest
+    public void testBadHwaddrLength() throws Exception {
+        final ByteBuffer packet = ByteBuffer.wrap(HexEncoding.decode((
+            // IP header.
+            "450001518d0600004011144dc0a82b01c0a82bf7" +
+            // UDP header.
+            "00430044013d9ac7" +
+            // BOOTP header.
+            "02010600dfc23d1f0002000000000000c0a82bf7c0a82b0100000000" +
+            // MAC address.
+            "30766ff2a90c00000000000000000000" +
+            // Server name.
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            // File.
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            "0000000000000000000000000000000000000000000000000000000000000000" +
+            // Options
+            "638253633501023604c0a82b01330400000e103a04000007083b0400000c4e0104ffffff00" +
+            "1c04c0a82bff0304c0a82b010604c0a82b012b0f414e44524f49445f4d455445524544ff"
+        ).toCharArray(), false));
+        String expectedClientMac = "30766FF2A90C";
+
+        final int hwAddrLenOffset = 20 + 8 + 2;
+        assertEquals(6, packet.get(hwAddrLenOffset));
+
+        // Expect the expected.
+        DhcpPacket offerPacket = DhcpPacket.decodeFullPacket(packet, ENCAP_L3);
+        assertNotNull(offerPacket);
+        assertEquals(6, offerPacket.getClientMac().length);
+        assertEquals(expectedClientMac, HexDump.toHexString(offerPacket.getClientMac()));
+
+        // Reduce the hardware address length and verify that it shortens the client MAC.
+        packet.flip();
+        packet.put(hwAddrLenOffset, (byte) 5);
+        offerPacket = DhcpPacket.decodeFullPacket(packet, ENCAP_L3);
+        assertNotNull(offerPacket);
+        assertEquals(5, offerPacket.getClientMac().length);
+        assertEquals(expectedClientMac.substring(0, 10),
+                HexDump.toHexString(offerPacket.getClientMac()));
+
+        packet.flip();
+        packet.put(hwAddrLenOffset, (byte) 3);
+        offerPacket = DhcpPacket.decodeFullPacket(packet, ENCAP_L3);
+        assertNotNull(offerPacket);
+        assertEquals(3, offerPacket.getClientMac().length);
+        assertEquals(expectedClientMac.substring(0, 6),
+                HexDump.toHexString(offerPacket.getClientMac()));
+
+        // Set the the hardware address length to 0xff and verify that we a) don't treat it as -1
+        // and crash, and b) hardcode it to 6.
+        packet.flip();
+        packet.put(hwAddrLenOffset, (byte) -1);
+        offerPacket = DhcpPacket.decodeFullPacket(packet, ENCAP_L3);
+        assertNotNull(offerPacket);
+        assertEquals(6, offerPacket.getClientMac().length);
+        assertEquals(expectedClientMac, HexDump.toHexString(offerPacket.getClientMac()));
+
+        // Set the the hardware address length to a positive invalid value (> 16) and verify that we
+        // hardcode it to 6.
+        packet.flip();
+        packet.put(hwAddrLenOffset, (byte) 17);
+        offerPacket = DhcpPacket.decodeFullPacket(packet, ENCAP_L3);
+        assertNotNull(offerPacket);
+        assertEquals(6, offerPacket.getClientMac().length);
+        assertEquals(expectedClientMac, HexDump.toHexString(offerPacket.getClientMac()));
+    }
+
+    @SmallTest
     public void testPadAndOverloadedOptionsOffer() throws Exception {
         // A packet observed in the real world that is interesting for two reasons:
         //