Adding extra data check on DecodeBlockMsg

fix: types: error out on decoding BlockMsg with extraneous data
Fixes OSS-fuzz issue 48208: lotus:fuzz_block_msg

Signed-off-by: Yolan Romailler <anomalroil@users.noreply.github.com>

chain/types/blockmsg.go

@@ -2,6 +2,7 @@ package types
 
 import (
 	"bytes"
+	"fmt"
 
 	"github.com/ipfs/go-cid"
 )
@@ -14,10 +15,13 @@ type BlockMsg struct {
 
 func DecodeBlockMsg(b []byte) (*BlockMsg, error) {
 	var bm BlockMsg
-	if err := bm.UnmarshalCBOR(bytes.NewReader(b)); err != nil {
+	data := bytes.NewReader(b)
+	if err := bm.UnmarshalCBOR(data); err != nil {
 		return nil, err
 	}
-
+	if l := data.Len(); l != 0 {
+		return nil, fmt.Errorf("extraneous data in BlockMsg CBOR encoding: got %d unexpected bytes", l)
+	}
 	return &bm, nil
 }
 

chain/types/blockmsg_test.go

@@ -0,0 +1,40 @@
+package types
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDecodeBlockMsg(t *testing.T) {
+	type args struct {
+		b []byte
+	}
+	tests := []struct {
+		name    string
+		data    []byte
+		want    *BlockMsg
+		wantErr bool
+	}{
+		{"decode empty BlockMsg with extra data at the end", []byte{0x83, 0xf6, 0x80, 0x80, 0x20}, new(BlockMsg), true},
+		{"decode valid empty BlockMsg", []byte{0x83, 0xf6, 0x80, 0x80}, new(BlockMsg), false},
+		{"decode invalid cbor", []byte{0x83, 0xf6, 0x80}, nil, true},
+	}
+	for _, tt := range tests {
+		data := tt.data
+		want := tt.want
+		wantErr := tt.wantErr
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := DecodeBlockMsg(data)
+			if wantErr {
+				assert.Errorf(t, err, "DecodeBlockMsg(%v)", data)
+				return
+			}
+			assert.NoErrorf(t, err, "DecodeBlockMsg(%v)", data)
+			assert.Equalf(t, want, got, "DecodeBlockMsg(%v)", data)
+			serialized, err := got.Serialize()
+			assert.NoErrorf(t, err, "DecodeBlockMsg(%v)", data)
+			assert.Equalf(t, serialized, data, "DecodeBlockMsg(%v)", data)
+		})
+	}
+}