Implement UDP cheksum verification.

Test:
 - TestIncrementChecksumErrors

Fixes #2943

PiperOrigin-RevId: 317348158

pkg/sentry/socket/netstack/netstack.go

@@ -191,6 +191,7 @@ var Metrics = tcpip.Stats{
 		MalformedPacketsReceived: mustCreateMetric("/netstack/udp/malformed_packets_received", "Number of incoming UDP datagrams dropped due to the UDP header being in a malformed state."),
 		PacketsSent:              mustCreateMetric("/netstack/udp/packets_sent", "Number of UDP datagrams sent."),
 		PacketSendErrors:         mustCreateMetric("/netstack/udp/packet_send_errors", "Number of UDP datagrams failed to be sent."),
+		ChecksumErrors:           mustCreateMetric("/netstack/udp/checksum_errors", "Number of UDP datagrams dropped due to bad checksums."),
 	},
 }
 

pkg/sentry/socket/netstack/stack.go

@@ -313,7 +313,7 @@ func (s *Stack) Statistics(stat interface{}, arg string) error {
 			udp.PacketsSent.Value(),         // OutDatagrams.
 			udp.ReceiveBufferErrors.Value(), // RcvbufErrors.
 			0,                               // Udp/SndbufErrors.
-			0,                               // Udp/InCsumErrors.
+			udp.ChecksumErrors.Value(),      // Udp/InCsumErrors.
 			0,                               // Udp/IgnoredMulti.
 		}
 	default:

pkg/tcpip/tcpip.go

@@ -1224,6 +1224,9 @@ type UDPStats struct {
 
 	// PacketSendErrors is the number of datagrams failed to be sent.
 	PacketSendErrors *StatCounter
+
+	// ChecksumErrors is the number of datagrams dropped due to bad checksums.
+	ChecksumErrors *StatCounter
 }
 
 // Stats holds statistics about the networking stack.
@@ -1267,6 +1270,9 @@ type ReceiveErrors struct {
 	// ClosedReceiver is the number of received packets dropped because
 	// of receiving endpoint state being closed.
 	ClosedReceiver StatCounter
+
+	// ChecksumErrors is the number of packets dropped due to bad checksums.
+	ChecksumErrors StatCounter
 }
 
 // SendErrors collects packet send errors within the transport layer for

pkg/tcpip/transport/udp/endpoint.go

@@ -1350,6 +1350,24 @@ func (e *endpoint) HandlePacket(r *stack.Route, id stack.TransportEndpointID, pk
 		return
 	}
 
+	// Verify checksum unless RX checksum offload is enabled.
+	// On IPv4, UDP checksum is optional, and a zero value means
+	// the transmitter omitted the checksum generation (RFC768).
+	// On IPv6, UDP checksum is not optional (RFC2460 Section 8.1).
+	if r.Capabilities()&stack.CapabilityRXChecksumOffload == 0 &&
+		(hdr.Checksum() != 0 || r.NetProto == header.IPv6ProtocolNumber) {
+		xsum := r.PseudoHeaderChecksum(ProtocolNumber, hdr.Length())
+		for _, v := range pkt.Data.Views() {
+			xsum = header.Checksum(v, xsum)
+		}
+		if hdr.CalculateChecksum(xsum) != 0xffff {
+			// Checksum Error.
+			e.stack.Stats().UDP.ChecksumErrors.Increment()
+			e.stats.ReceiveErrors.ChecksumErrors.Increment()
+			return
+		}
+	}
+
 	e.rcvMu.Lock()
 	e.stack.Stats().UDP.PacketsReceived.Increment()
 	e.stats.PacketsReceived.Increment()