From a8d6b06331a75d75b46f3dd6cc6fcd40dcf604b7 Mon Sep 17 00:00:00 2001
From: Lidong Chen <lidong.chen@oracle.com>
Date: Thu, 6 Feb 2025 18:16:57 +0000
Subject: [PATCH 71/73] kern/misc: Add sanity check after grub_strtoul() call

When the format string, fmt0, includes a positional argument
grub_strtoul() or grub_strtoull() is called to extract the argument
position. However, the returned argument position isn't fully validated.
If the format is something like "%0$x" then these functions return
0 which leads to an underflow in the calculation of the args index, curn.
The fix is to add a check to ensure the extracted argument position is
greater than 0 before computing curn. Additionally, replace one
grub_strtoull() with grub_strtoul() and change curn type to make code
more correct.

Fixes: CID 473841

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/kern/misc.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/grub-core/kern/misc.c b/grub-core/kern/misc.c
index 7cee5d75c..2b7922393 100644
--- a/grub-core/kern/misc.c
+++ b/grub-core/kern/misc.c
@@ -830,7 +830,7 @@ parse_printf_arg_fmt (const char *fmt0, struct printf_args *args,
   while ((c = *fmt++) != 0)
     {
       int longfmt = 0;
-      grub_size_t curn;
+      unsigned long curn;
       const char *p;
 
       if (c != '%')
@@ -848,7 +848,10 @@ parse_printf_arg_fmt (const char *fmt0, struct printf_args *args,
 
       if (*fmt == '$')
 	{
-	  curn = grub_strtoull (p, 0, 10) - 1;
+	  curn = grub_strtoul (p, 0, 10);
+	  if (curn == 0)
+	    continue;
+	  curn--;
 	  fmt++;
 	}
 
@@ -1034,6 +1037,8 @@ grub_vsnprintf_real (char *str, grub_size_t max_len, const char *fmt0,
 
       if (*fmt == '$')
 	{
+	  if (format1 == 0)
+	    continue;
 	  curn = format1 - 1;
 	  fmt++;
 	  format1 = 0;
-- 
2.33.0

