isValid() method

dhcpv4/nclient4/conn_unix.go

@@ -99,24 +99,16 @@ func (upc *BroadcastRawUDPConn) ReadFrom(b []byte) (int, net.Addr, error) {
 		pkt = pkt[:n]
 		buf := uio.NewBigEndianBuffer(pkt)
 
-		// To read the header length, access data directly.
-		if !buf.Has(ipv4MinimumSize) {
-			continue
-		}
-
 		ipHdr := ipv4(buf.Data())
-		headerLength := ipHdr.headerLength()
 
-		if !buf.Has(int(headerLength)) {
+		if !ipHdr.isValid(n) {
 			continue
 		}
 
-		ipHdr = ipv4(buf.Consume(int(headerLength)))
+		ipHdr = ipv4(buf.Consume(int(ipHdr.headerLength())))
 
-		if headerLength > protocol {
-			if ipHdr.transportProtocol() != udpProtocolNumber {
-				continue
-			}
+		if ipHdr.transportProtocol() != udpProtocolNumber {
+			continue
 		}
 
 		if !buf.Has(udpHdrLen) {

dhcpv4/nclient4/ipv4.go

@@ -14,6 +14,7 @@
 //
 // This file contains code taken from gVisor.
 
+//go:build go1.12
 // +build go1.12
 
 package nclient4
@@ -26,16 +27,17 @@ import (
 )
 
 const (
-	versIHL     = 0
-	tos         = 1
-	totalLen    = 2
-	id          = 4
-	flagsFO     = 6
-	ttl         = 8
-	protocol    = 9
-	checksumOff = 10
-	srcAddr     = 12
-	dstAddr     = 16
+	versIHL        = 0
+	tos            = 1
+	totalLen       = 2
+	id             = 4
+	flagsFO        = 6
+	ttl            = 8
+	protocol       = 9
+	checksumOff    = 10
+	srcAddr        = 12
+	dstAddr        = 16
+	ipVersionShift = 4
 )
 
 // transportProtocolNumber is the number of a transport protocol.
@@ -95,8 +97,40 @@ const (
 
 	// ipv4AddressSize is the size, in bytes, of an IPv4 address.
 	ipv4AddressSize = 4
+
+	// IPv4Version is the version of the IPv4 protocol.
+	ipv4Version = 4
 )
 
+// IPVersion returns the version of IP used in the given packet. It returns -1
+// if the packet is not large enough to contain the version field.
+func ipVersion(b []byte) int {
+	// Length must be at least offset+length of version field.
+	if len(b) < versIHL+1 {
+		return -1
+	}
+	return int(b[versIHL] >> ipVersionShift)
+}
+
+// IsValid performs basic validation on the packet.
+func (b ipv4) isValid(pktSize int) bool {
+	if len(b) < ipv4MinimumSize {
+		return false
+	}
+
+	hlen := int(b.headerLength())
+	tlen := int(b.totalLength())
+	if hlen < ipv4MinimumSize || hlen > tlen || tlen > pktSize {
+		return false
+	}
+
+	if ipVersion(b) != ipv4Version {
+		return false
+	}
+
+	return true
+}
+
 // headerLength returns the value of the "header length" field of the ipv4
 // header.
 func (b ipv4) headerLength() uint8 {