From d8a937ccae5c6d86dc4375698afca5cefdcd01e1 Mon Sep 17 00:00:00 2001
From: B Horn <b@horn.uk>
Date: Thu, 18 Apr 2024 19:04:13 +0100
Subject: [PATCH 27/73] script/execute: Limit the recursion depth

If unbounded recursion is allowed it becomes possible to collide the
stack with the heap. As UEFI firmware often lacks guard pages this
becomes an exploitable issue as it is possible in some cases to do
a controlled overwrite of a section of this heap region with
arbitrary data.

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/script/execute.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/grub-core/script/execute.c b/grub-core/script/execute.c
index 14ff09094..e1450f45d 100644
--- a/grub-core/script/execute.c
+++ b/grub-core/script/execute.c
@@ -33,10 +33,18 @@
    is sizeof (int) * 3, and one extra for a possible -ve sign.  */
 #define ERRNO_DIGITS_MAX  (sizeof (int) * 3 + 1)
 
+/*
+ * A limit on recursion, to avoid colliding with the heap. UEFI defines a baseline
+ * stack size of 128 KiB. So, assuming at most 1-2 KiB per iteration this should
+ * keep us safe.
+ */
+#define MAX_RECURSION_DEPTH 64
+
 static unsigned long is_continue;
 static unsigned long active_loops;
 static unsigned long active_breaks;
 static unsigned long function_return;
+static unsigned long recursion_depth;
 
 #define GRUB_SCRIPT_SCOPE_MALLOCED      1
 #define GRUB_SCRIPT_SCOPE_ARGS_MALLOCED 2
@@ -816,7 +824,13 @@ grub_script_execute_cmd (struct grub_script_cmd *cmd)
   if (cmd == 0)
     return 0;
 
+  recursion_depth++;
+
+  if (recursion_depth >= MAX_RECURSION_DEPTH)
+    return grub_error (GRUB_ERR_RECURSION_DEPTH, N_("maximum recursion depth exceeded"));
+
   ret = cmd->exec (cmd);
+  recursion_depth--;
 
   grub_snprintf (errnobuf, sizeof (errnobuf), "%d", ret);
   grub_env_set ("?", errnobuf);
-- 
2.33.0

