Fix heap-use-after-free in Parser error handling

Fixes #2643

Author: glebm

src/error_handling.cpp

@@ -15,8 +15,8 @@ namespace Sass {
       prefix("Error"), pstate(pstate), traces(traces)
     { }
 
-    InvalidSass::InvalidSass(ParserState pstate, Backtraces traces, std::string msg)
-    : Base(pstate, msg, traces)
+    InvalidSass::InvalidSass(ParserState pstate, Backtraces traces, std::string msg, char* owned_src)
+    : Base(pstate, msg, traces), owned_src(owned_src)
     { }
 
 

src/error_handling.hpp

@@ -37,8 +37,9 @@ namespace Sass {
 
     class InvalidSass : public Base {
       public:
-        InvalidSass(ParserState pstate, Backtraces traces, std::string msg);
-        virtual ~InvalidSass() throw() {};
+        InvalidSass(ParserState pstate, Backtraces traces, std::string msg, char* owned_src = nullptr);
+        virtual ~InvalidSass() throw() { sass_free_memory(owned_src); };
+        char *owned_src;
     };
 
     class InvalidParent : public Base {

src/parser.cpp

@@ -3054,8 +3054,11 @@ namespace Sass {
   {
     Position p(pos.line ? pos : before_token);
     ParserState pstate(path, source, p, Offset(0, 0));
+    // `pstate.src` may not outlive stack unwind so we must copy it.
+    char *src_copy = sass_copy_c_string(pstate.src);
+    pstate.src = src_copy;
     traces.push_back(Backtrace(pstate));
-    throw Exception::InvalidSass(pstate, traces, msg);
+    throw Exception::InvalidSass(pstate, traces, msg, src_copy);
   }
 
   void Parser::error(std::string msg)