Make IPv4 check the IP header checksum

The IPv4 header checksum has not been checked, at least in recent times,
so add code to do so. Fix all the tests that fail because they never
needed to set the checksum.

Fixes #4484

PiperOrigin-RevId: 337556243

pkg/tcpip/network/ip_test.go

@@ -322,6 +322,7 @@ func TestSourceAddressValidation(t *testing.T) {
 			SrcAddr:     src,
 			DstAddr:     localIPv4Addr,
 		})
+		ip.SetChecksum(^ip.CalculateChecksum())
 
 		e.InjectInbound(header.IPv4ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
 			Data: hdr.View().ToVectorisedView(),
@@ -344,7 +345,6 @@ func TestSourceAddressValidation(t *testing.T) {
 			SrcAddr:       src,
 			DstAddr:       localIPv6Addr,
 		})
-
 		e.InjectInbound(header.IPv6ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
 			Data: hdr.View().ToVectorisedView(),
 		}))
@@ -581,6 +581,7 @@ func TestIPv4Receive(t *testing.T) {
 		SrcAddr:     remoteIPv4Addr,
 		DstAddr:     localIPv4Addr,
 	})
+	ip.SetChecksum(^ip.CalculateChecksum())
 
 	// Make payload be non-zero.
 	for i := header.IPv4MinimumSize; i < totalLen; i++ {
@@ -662,6 +663,7 @@ func TestIPv4ReceiveControl(t *testing.T) {
 				SrcAddr:     "\x0a\x00\x00\xbb",
 				DstAddr:     localIPv4Addr,
 			})
+			ip.SetChecksum(^ip.CalculateChecksum())
 
 			// Create the ICMP header.
 			icmp := header.ICMPv4(view[header.IPv4MinimumSize:])
@@ -681,6 +683,7 @@ func TestIPv4ReceiveControl(t *testing.T) {
 				SrcAddr:        localIPv4Addr,
 				DstAddr:        remoteIPv4Addr,
 			})
+			ip.SetChecksum(^ip.CalculateChecksum())
 
 			// Make payload be non-zero.
 			for i := dataOffset; i < len(view); i++ {
@@ -734,6 +737,8 @@ func TestIPv4FragmentationReceive(t *testing.T) {
 		SrcAddr:        remoteIPv4Addr,
 		DstAddr:        localIPv4Addr,
 	})
+	ip1.SetChecksum(^ip1.CalculateChecksum())
+
 	// Make payload be non-zero.
 	for i := header.IPv4MinimumSize; i < totalLen; i++ {
 		frag1[i] = uint8(i)
@@ -750,6 +755,8 @@ func TestIPv4FragmentationReceive(t *testing.T) {
 		SrcAddr:        remoteIPv4Addr,
 		DstAddr:        localIPv4Addr,
 	})
+	ip2.SetChecksum(^ip2.CalculateChecksum())
+
 	// Make payload be non-zero.
 	for i := header.IPv4MinimumSize; i < totalLen; i++ {
 		frag2[i] = uint8(i)

pkg/tcpip/network/ipv4/ipv4.go

@@ -440,6 +440,32 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt *stack.PacketBuffer) {
 		return
 	}
 
+	// There has been some confusion regarding verifying checksums. We need
+	// just look for negative 0 (0xffff) as the checksum, as it's not possible to
+	// get positive 0 (0) for the checksum. Some bad implementations could get it
+	// when doing entry replacement in the early days of the Internet,
+	// however the lore that one needs to check for both persists.
+	//
+	// RFC 1624 section 1 describes the source of this confusion as:
+	//     [the partial recalculation method described in RFC 1071] computes a
+	//     result for certain cases that differs from the one obtained from
+	//     scratch (one's complement of one's complement sum of the original
+	//     fields).
+	//
+	// However RFC 1624 section 5 clarifies that if using the verification method
+	// "recommended by RFC 1071, it does not matter if an intermediate system
+	// generated a -0 instead of +0".
+	//
+	// RFC1071 page 1 specifies the verification method as:
+	//	  (3)  To check a checksum, the 1's complement sum is computed over the
+	//        same set of octets, including the checksum field.  If the result
+	//        is all 1 bits (-0 in 1's complement arithmetic), the check
+	//        succeeds.
+	if h.CalculateChecksum() != 0xffff {
+		r.Stats().IP.MalformedPacketsReceived.Increment()
+		return
+	}
+
 	// As per RFC 1122 section 3.2.1.3:
 	//   When a host sends any datagram, the IP source address MUST
 	//   be one of its own IP addresses (but not a broadcast or

pkg/tcpip/network/ipv4/ipv4_test.go

@@ -120,6 +120,7 @@ func TestIPv4Sanity(t *testing.T) {
 	tests := []struct {
 		name              string
 		headerLength      uint8 // value of 0 means "use correct size"
+		badHeaderChecksum bool
 		maxTotalLength    uint16
 		transportProtocol uint8
 		TTL               uint8
@@ -135,6 +136,14 @@ func TestIPv4Sanity(t *testing.T) {
 			transportProtocol: uint8(header.ICMPv4ProtocolNumber),
 			TTL:               ttl,
 		},
+		{
+			name:              "bad header checksum",
+			maxTotalLength:    defaultMTU,
+			transportProtocol: uint8(header.ICMPv4ProtocolNumber),
+			TTL:               ttl,
+			badHeaderChecksum: true,
+			shouldFail:        true,
+		},
 		// The TTL tests check that we are not rejecting an incoming packet
 		// with a zero or one TTL, which has been a point of confusion in the
 		// past as RFC 791 says: "If this field contains the value zero, then the
@@ -290,6 +299,12 @@ func TestIPv4Sanity(t *testing.T) {
 			if test.headerLength != 0 {
 				ip.SetHeaderLength(test.headerLength)
 			}
+			ip.SetChecksum(0)
+			ipHeaderChecksum := ip.CalculateChecksum()
+			if test.badHeaderChecksum {
+				ipHeaderChecksum += 42
+			}
+			ip.SetChecksum(^ipHeaderChecksum)
 			requestPkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
 				Data: hdr.View().ToVectorisedView(),
 			})
@@ -1427,6 +1442,7 @@ func TestReceiveFragments(t *testing.T) {
 					SrcAddr:        frag.srcAddr,
 					DstAddr:        frag.dstAddr,
 				})
+				ip.SetChecksum(^ip.CalculateChecksum())
 
 				vv := hdr.View().ToVectorisedView()
 				vv.AppendView(frag.payload)
@@ -1695,6 +1711,7 @@ func TestPacketQueing(t *testing.T) {
 					SrcAddr:     host2IPv4Addr.AddressWithPrefix.Address,
 					DstAddr:     host1IPv4Addr.AddressWithPrefix.Address,
 				})
+				ip.SetChecksum(^ip.CalculateChecksum())
 				e.InjectInbound(ipv4.ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
 					Data: hdr.View().ToVectorisedView(),
 				}))
@@ -1738,6 +1755,7 @@ func TestPacketQueing(t *testing.T) {
 					SrcAddr:     host2IPv4Addr.AddressWithPrefix.Address,
 					DstAddr:     host1IPv4Addr.AddressWithPrefix.Address,
 				})
+				ip.SetChecksum(^ip.CalculateChecksum())
 				e.InjectInbound(header.IPv4ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
 					Data: hdr.View().ToVectorisedView(),
 				}))

pkg/tcpip/tests/integration/multicast_broadcast_test.go

@@ -80,6 +80,7 @@ func TestPingMulticastBroadcast(t *testing.T) {
 			SrcAddr:     remoteIPv4Addr,
 			DstAddr:     dst,
 		})
+		ip.SetChecksum(^ip.CalculateChecksum())
 
 		e.InjectInbound(header.IPv4ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
 			Data: hdr.View().ToVectorisedView(),
@@ -250,6 +251,7 @@ func TestIncomingMulticastAndBroadcast(t *testing.T) {
 			SrcAddr:     remoteIPv4Addr,
 			DstAddr:     dst,
 		})
+		ip.SetChecksum(^ip.CalculateChecksum())
 
 		e.InjectInbound(header.IPv4ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
 			Data: hdr.View().ToVectorisedView(),