packDataOpt: fix overflow in packing opt (#966)

An option needs a minimum of 4 bytes that contains OPTION-CODE and OPTION-LENGTH
The code was checkign off + 3 > len(msg) instead of off + 4

fuzz_test.go

@@ -0,0 +1,73 @@
+package dns
+
+import (
+	"net"
+	"testing"
+)
+
+// TestPackDataOpt tests generated using fuzz.go and with a message pack
+// containing the following bytes:
+// "0000\x00\x00000000\x00\x00/00000" +
+// "0\x00\v\x00#\b00000000\x00\x00)000" +
+// "000\x00\x1c00\x00\x0000\x00\x01000\x00\x00\x00\b" +
+// "\x00\v\x00\x02\x0000000000"
+// That bytes sequence created the overflow error.
+func TestPackDataOpt(t *testing.T) {
+	type args struct {
+		option []EDNS0
+		msg    []byte
+		off    int
+	}
+	tests := []struct {
+		name       string
+		args       args
+		want       int
+		wantErr    bool
+		wantErrMsg string
+	}{
+		{
+			name: "overflow",
+			args: args{
+				option: []EDNS0{
+					&EDNS0_LOCAL{Code: 0x3030, Data: []uint8{}},
+					&EDNS0_LOCAL{Code: 0x3030, Data: []uint8{0x30}},
+					&EDNS0_LOCAL{Code: 0x3030, Data: []uint8{}},
+					&EDNS0_SUBNET{
+						Code: 0x0, Family: 0x2,
+						SourceNetmask: 0x0, SourceScope: 0x30,
+						Address: net.IP{0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}},
+				},
+				msg: []byte{
+					0x30, 0x30, 0x30, 0x30, 0x00, 0x00, 0x00, 0x2,
+					0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x2f, 0x30,
+					0x30, 0x30, 0x30, 0x30, 0x30, 0x00, 0x0b, 0x00,
+					0x23, 0x08, 0x30, 0x30, 0x30, 0x30, 0x30, 0x30,
+					0x30, 0x30, 0x00, 0x00, 0x29, 0x30, 0x30, 0x30,
+					0x30, 0x30, 0x30, 0x00, 0x00, 0x30, 0x30, 0x00,
+					0x00, 0x30, 0x30, 0x00, 0x01, 0x30, 0x00, 0x00,
+					0x00,
+				},
+				off: 54,
+			},
+			wantErr:    true,
+			wantErrMsg: "dns: overflow packing opt",
+			want:       57,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := packDataOpt(tt.args.option, tt.args.msg, tt.args.off)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("packDataOpt() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if err != nil && tt.wantErrMsg != err.Error() {
+				t.Errorf("packDataOpt() error msg = %v, wantErrMsg %v", err.Error(), tt.wantErrMsg)
+				return
+			}
+			if got != tt.want {
+				t.Errorf("packDataOpt() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

msg_helpers.go

@@ -495,7 +495,7 @@ Option:
 func packDataOpt(options []EDNS0, msg []byte, off int) (int, error) {
 	for _, el := range options {
 		b, err := el.pack()
-		if err != nil || off+3 > len(msg) {
+		if err != nil || off+4 > len(msg) {
 			return len(msg), &Error{err: "overflow packing opt"}
 		}
 		binary.BigEndian.PutUint16(msg[off:], el.Option())      // Option code