[SECURITY PATCH 19/28] script: Avoid a use-after-free when redefining a function during execution

[Daniel Kiper]

From: Chris Coulson <chris.coulson@canonical.com>

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  |  2 ++
 grub-core/script/function.c | 16 +++++++++++++---
 grub-core/script/parser.y   |  3 ++-
 include/grub/script_sh.h    |  2 ++
 4 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/grub-core/script/execute.c b/grub-core/script/execute.c
index 8a9161cc8..ce83edd4b 100644
--- a/grub-core/script/execute.c
+++ b/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;
diff --git a/grub-core/script/function.c b/grub-core/script/function.c
index d36655e51..3aad04bf9 100644
--- a/grub-core/script/function.c
+++ b/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)
@@ -60,10 +61,19 @@ grub_script_function_create (struct grub_script_arg 
*functionname_arg,
       grub_script_function_t q;
 
       q = *p;
-      grub_script_free (q->func);
-      q->func = cmd;
       grub_free (func);
-      func = q;
+      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;
+          func = q;
+        }
     }
   else
     {
diff --git a/grub-core/script/parser.y b/grub-core/script/parser.y
index 4f0ab8319..f80b86b6f 100644
--- a/grub-core/script/parser.y
+++ b/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;
diff --git a/include/grub/script_sh.h b/include/grub/script_sh.h
index b382bcf09..6c48e0751 100644
--- a/include/grub/script_sh.h
+++ b/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;
 
-- 
2.11.0