psbt: define MaxPsbtKeyLength and check against it when decoding

This commit defines MaxPsbtKeyLength and checks that decoding a
key from a PSBT blob doesn't attempt to allocate too much memory.

psbt/psbt.go

@@ -33,6 +33,10 @@ var (
 //less than 4M.
 const MaxPsbtValueLength = 4000000
 
+// MaxPsbtKeyLength is the length of the largest key that we'll successfully
+// deserialize from the wire. Anything more will return ErrInvalidKeydata.
+const MaxPsbtKeyLength = 10000
+
 var (
 
 	// ErrInvalidPsbtFormat is a generic error for any situation in which a

psbt/utils.go

@@ -237,6 +237,11 @@ func getKey(r io.Reader) (int, []byte, error) {
 		return -1, nil, nil
 	}
 
+	// Check that we don't attempt to decode a dangerously large key.
+	if count > MaxPsbtKeyLength {
+		return -1, nil, ErrInvalidKeydata
+	}
+
 	// Next, we ready out the designated number of bytes, which may include
 	// a type, key, and optional data.
 	keyTypeAndData := make([]byte, count)