script: Avoid a use-after-free when redefining a function during execution

Defining a new function with the same name as a previously defined
function causes the grub_script and associated resources for the
previous function to be freed. If the previous function is currently
executing when a function with the same name is defined, this results
in use-after-frees when processing subsequent commands in the original
function.

Instead, reject a new function definition if it has the same name as
a previously defined function, and that function is currently being
executed. Although a behavioural change, this should be backwards
compatible with existing configurations because they can't be
dependent on the current behaviour without being broken.

Fixes: CVE-2020-15706

Signed-off-by: Chris Coulson <chris.coulson@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/script/execute.c

@@ -838,7 +838,9 @@ grub_script_function_call (grub_script_function_t func, int argc, char **args)
   old_scope = scope;
   scope = &new_scope;
 
+  func->executing++;
   ret = grub_script_execute (func->func);
+  func->executing--;
 
   function_return = 0;
   active_loops = loops;

grub-core/script/function.c

@@ -34,6 +34,7 @@ grub_script_function_create (struct grub_script_arg *functionname_arg,
   func = (grub_script_function_t) grub_malloc (sizeof (*func));
   if (! func)
     return 0;
+  func->executing = 0;
 
   func->name = grub_strdup (functionname_arg->str);
   if (! func->name)
@@ -60,11 +61,20 @@ grub_script_function_create (struct grub_script_arg *functionname_arg,
       grub_script_function_t q;
 
       q = *p;
+      grub_free (func);
+      if (q->executing > 0)
+        {
+          grub_error (GRUB_ERR_BAD_ARGUMENT,
+		      N_("attempt to redefine a function being executed"));
+          func = NULL;
+        }
+      else
+        {
           grub_script_free (q->func);
           q->func = cmd;
-      grub_free (func);
           func = q;
         }
+    }
   else
     {
       func->next = *p;

grub-core/script/parser.y

@@ -289,7 +289,8 @@ function: "function" "name"
 	      grub_script_mem_free (state->func_mem);
 	    else {
 	      script->children = state->scripts;
-	      grub_script_function_create ($2, script);
+	      if (!grub_script_function_create ($2, script))
+		grub_script_free (script);
 	    }
 
 	    state->scripts = $<scripts>3;

include/grub/script_sh.h

@@ -361,6 +361,8 @@ struct grub_script_function
 
   /* The next element.  */
   struct grub_script_function *next;
+
+  unsigned executing;
 };
 typedef struct grub_script_function *grub_script_function_t;