paths: Add check for hopfield count <= 64 when deserializing a scion path

pkg/slayers/path/scion/BUILD.bazel

@@ -24,6 +24,7 @@ go_test(
     ],
     deps = [
         ":go_default_library",
+        "//pkg/private/serrors:go_default_library",
         "//pkg/slayers/path:go_default_library",
         "@com_github_stretchr_testify//assert:go_default_library",
         "@com_github_stretchr_testify//require:go_default_library",

pkg/slayers/path/scion/base.go

@@ -22,10 +22,17 @@ import (
 	"github.com/scionproto/scion/pkg/slayers/path"
 )
 
-// MetaLen is the length of the PathMetaHeader.
-const MetaLen = 4
+const (
+	// MaxINFs is the maximum number of info fields in a SCION path.
+	MaxINFs = 3
+	// MaxHops is the maximum number of hop fields in a SCION path.
+	MaxHops = 64
 
-const PathType path.Type = 1
+	// MetaLen is the length of the PathMetaHeader.
+	MetaLen = 4
+
+	PathType path.Type = 1
+)
 
 func RegisterPath() {
 	path.RegisterPath(path.Metadata{
@@ -66,6 +73,14 @@ func (s *Base) DecodeFromBytes(data []byte) error {
 		}
 		s.NumHops += int(s.PathMeta.SegLen[i])
 	}
+
+	// We must check the validity of NumHops. It is possible to fit more than 64 hops in
+	// the length of a scion header. Yet a path of more than 64 hops cannot be followed to
+	// the end because CurrHF is only 6 bits long.
+	if s.NumHops > MaxHops {
+		return serrors.New("NumHops too large", "NumHops", s.NumHops, "Maximum", MaxHops)
+	}
+
 	return nil
 }
 
@@ -113,7 +128,9 @@ func (s *Base) infIndexForHF(hf uint8) uint8 {
 	}
 }
 
-// Len returns the length of the path in bytes.
+// Len returns the length of the path in bytes. That is, the number of byte required to
+// store it, based on the metadata. The actual number of bytes available to contain it
+// can be inferred from the common header field HdrLen. It may or may not be consistent.
 func (s *Base) Len() int {
 	return MetaLen + s.NumINF*path.InfoLen + s.NumHops*path.HopLen
 }

pkg/slayers/path/scion/decoded.go

@@ -19,13 +19,6 @@ import (
 	"github.com/scionproto/scion/pkg/slayers/path"
 )
 
-const (
-	// MaxINFs is the maximum number of info fields in a SCION path.
-	MaxINFs = 3
-	// MaxHops is the maximum number of hop fields in a SCION path.
-	MaxHops = 64
-)
-
 // Decoded implements the SCION (data-plane) path type. Decoded is intended to be used in
 // non-performance critical code paths, where the convenience of having a fully parsed path trumps
 // the loss of performance.

pkg/slayers/path/scion/raw_test.go

@@ -21,6 +21,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/scionproto/scion/pkg/private/serrors"
 	"github.com/scionproto/scion/pkg/slayers/path"
 	"github.com/scionproto/scion/pkg/slayers/path/scion"
 )
@@ -51,6 +52,19 @@ var emptyRawTestPath = &scion.Raw{
 	Raw: make([]byte, scion.MetaLen),
 }
 
+var overlongPath = &scion.Raw{
+	Base: scion.Base{
+		PathMeta: scion.MetaHdr{
+			CurrINF: 0,
+			CurrHF:  0,
+			SegLen:  [3]uint8{24, 24, 17},
+		},
+		NumINF:  3,
+		NumHops: 65,
+	},
+	Raw: rawPath,
+}
+
 func TestRawSerialize(t *testing.T) {
 	b := make([]byte, rawTestPath.Len())
 	assert.NoError(t, rawTestPath.SerializeTo(b))
@@ -63,14 +77,23 @@ func TestRawDecodeFromBytes(t *testing.T) {
 	assert.Equal(t, rawTestPath, s)
 }
 
-func TestRawSerliazeDecode(t *testing.T) {
+func TestRawSerializeDecode(t *testing.T) {
 	b := make([]byte, rawTestPath.Len())
 	assert.NoError(t, rawTestPath.SerializeTo(b))
 	s := &scion.Raw{}
 	assert.NoError(t, s.DecodeFromBytes(b))
 	assert.Equal(t, rawTestPath, s)
 }
 
+func TestOverlongSerializeDecode(t *testing.T) {
+	b := make([]byte, overlongPath.Len())
+	assert.NoError(t, overlongPath.SerializeTo(b)) // permitted, if only to enable this test.
+	s := &scion.Raw{}
+	expected := serrors.New("NumHops too large", "NumHops", 65, "Maximum", scion.MaxHops)
+	err := s.DecodeFromBytes(b)
+	assert.Equal(t, expected.Error(), err.Error())
+}
+
 func TestRawReverse(t *testing.T) {
 	for name, tc := range pathReverseCases {
 		name, tc := name, tc

router/dataplane_test.go

@@ -34,6 +34,7 @@ import (
 
 	"github.com/scionproto/scion/pkg/addr"
 	libepic "github.com/scionproto/scion/pkg/experimental/epic"
+	"github.com/scionproto/scion/pkg/private/serrors"
 	"github.com/scionproto/scion/pkg/private/util"
 	"github.com/scionproto/scion/pkg/private/xtest"
 	"github.com/scionproto/scion/pkg/scrypto"
@@ -620,6 +621,55 @@ func TestProcessPkt(t *testing.T) {
 			egressInterface: 0,
 			assertFunc:      assert.NoError,
 		},
+		"inbound_longpath": {
+			prepareDP: func(ctrl *gomock.Controller) *router.DataPlane {
+				return router.NewDP(fakeExternalInterfaces,
+					nil, mock_router.NewMockBatchConn(ctrl),
+					fakeInternalNextHops, nil,
+					xtest.MustParseIA("1-ff00:0:110"), nil, key)
+			},
+			mockMsg: func(afterProcessing bool) *ipv4.Message {
+				spkt, dpath := prepBaseMsg(now)
+				spkt.DstIA = xtest.MustParseIA("1-ff00:0:110")
+				dst := addr.MustParseHost("10.0.100.100")
+				_ = spkt.SetDstAddr(dst)
+				dpath.HopFields = []path.HopField{
+					{ConsIngress: 41, ConsEgress: 40},
+					{ConsIngress: 31, ConsEgress: 30},
+					{ConsIngress: 1, ConsEgress: 0},
+				}
+
+				// Everything is the same a in the inbound test, except that we tossed in
+				// 64 extra hops and two extra segments.
+				dpath.Base.PathMeta.CurrHF = 2
+				dpath.Base.PathMeta.SegLen = [3]uint8{24, 24, 17}
+				dpath.InfoFields = append(
+					dpath.InfoFields,
+					path.InfoField{SegID: 0x112, ConsDir: true, Timestamp: util.TimeToSecs(now)},
+					path.InfoField{SegID: 0x113, ConsDir: true, Timestamp: util.TimeToSecs(now)},
+				)
+				dpath.Base.NumINF = 3
+				dpath.Base.NumHops = 65
+
+				dpath.HopFields[2].Mac = computeMAC(t, key, dpath.InfoFields[0], dpath.HopFields[2])
+				ret := toMsg(t, spkt, dpath)
+				if afterProcessing {
+					ret.Addr = &net.UDPAddr{IP: dst.IP().AsSlice(), Port: topology.EndhostPort}
+					ret.Flags, ret.NN, ret.N, ret.OOB = 0, 0, 0, nil
+				}
+				return ret
+			},
+			srcInterface:    1,
+			egressInterface: 0,
+			assertFunc: func(t assert.TestingT, err error, _ ...interface{}) bool {
+				if !assert.Error(t, err) {
+					return false
+				}
+				expected := serrors.New("NumHops too large",
+					"NumHops", 65, "Maximum", scion.MaxHops)
+				return assert.Equal(t, expected.Error(), err.Error())
+			},
+		},
 		"outbound": {
 			prepareDP: func(ctrl *gomock.Controller) *router.DataPlane {
 				return router.NewDP(