Read length after args in String#setbyte

Prevents RCE
Reported by https://hackerone.com/raydot

mrbgems/mruby-string-ext/src/string.c

@@ -23,10 +23,11 @@ static mrb_value
 mrb_str_setbyte(mrb_state *mrb, mrb_value str)
 {
   mrb_int pos, byte;
-  long len = RSTRING_LEN(str);
+  long len;
 
   mrb_get_args(mrb, "ii", &pos, &byte);
 
+  len = RSTRING_LEN(str);
   if (pos < -len || len <= pos)
     mrb_raisef(mrb, E_INDEX_ERROR, "index %S is out of array", mrb_fixnum_value(pos));
   if (pos < 0)

mrbgems/mruby-string-ext/test/string.rb

@@ -30,6 +30,18 @@ assert('String#setbyte') do
   assert_equal("Hello", str1)
 end
 
+assert("String#setbyte raises IndexError if arg conversion resizes String") do
+  $s = "01234\n"
+  class Tmp
+      def to_i
+          $s.chomp! ''
+          95
+      end
+  end
+  tmp = Tmp.new
+  assert_raise(IndexError) { $s.setbyte(5, tmp) }
+end
+
 assert('String#byteslice') do
   str1 = "hello"
   assert_equal("e", str1.byteslice(1))