buildroot/boot/grub2/0074-Constant-time-grub_crypto_memcmp.patch

From 4bbd6ae38efc7c0ae9ffd343157b7f3f37bd729c Mon Sep 17 00:00:00 2001
From: Gary Lin <glin@suse.com>
Date: Fri, 25 Jul 2025 13:50:23 +0800
Subject: [PATCH] Constant-time grub_crypto_memcmp()

Use the constant-time algorithm to compare the given memory blocks.
The code is extracted from the upstream commit:
0739d24cd1648531d0708d1079ff6bbfa6140268

Fix: bsc#1234959

Signed-off-by: Gary Lin <glin@suse.com>
Upstream: not submitted upstream, as upstream has switched to gcrypt
Taken-from: https://build.opensuse.org/projects/SUSE:SLE-15-SP5:Update/packages/grub2.39923/files/grub2-constant-time-grub_crypto_memcmp.patch?expand=0
Fixes: https://www.cve.org/CVERecord?id=CVE-2024-56738
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
---
 grub-core/lib/crypto.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/grub-core/lib/crypto.c b/grub-core/lib/crypto.c
index 396f76410..19db7870a 100644
--- a/grub-core/lib/crypto.c
+++ b/grub-core/lib/crypto.c
@@ -433,19 +433,28 @@ grub_crypto_gcry_error (gcry_err_code_t in)
   return GRUB_ACCESS_DENIED;
 }
 
+/*
+ * Compare byte arrays of length LEN, return 1 if it's not same,
+ * 0, otherwise.
+ */
 int
-grub_crypto_memcmp (const void *a, const void *b, grub_size_t n)
+grub_crypto_memcmp (const void *b1, const void *b2, grub_size_t len)
 {
-  register grub_size_t counter = 0;
-  const grub_uint8_t *pa, *pb;
+  const grub_uint8_t *a = b1;
+  const grub_uint8_t *b = b2;
+  int ab, ba;
+  grub_size_t i;
 
-  for (pa = a, pb = b; n; pa++, pb++, n--)
+  /* Constant-time compare. */
+  for (i = 0, ab = 0, ba = 0; i < len; i++)
     {
-      if (*pa != *pb)
-	counter++;
+      /* If a[i] != b[i], either ab or ba will be negative. */
+      ab |= a[i] - b[i];
+      ba |= b[i] - a[i];
     }
 
-  return !!counter;
+  /* 'ab | ba' is negative when buffers are not equal, extract sign bit.  */
+  return ((unsigned int)(ab | ba) >> (sizeof(unsigned int) * 8 - 1)) & 1;
 }
 
 #ifndef GRUB_UTIL
-- 
2.50.1