grub2: fix CVE-2024-56738

Backport an algorithmic change to grub_crypto_memcmp() so that it
completes in constant time and thus isn't susceptible to side-channel
attacks.

(From OE-Core rev: 30a1cc225a2bd5d044bf608d863a67df3f9c03be)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 75 insertions, 0 deletions

diff --git a/meta/recipes-bsp/grub/files/CVE-2024-56738.patch b/meta/recipes-bsp/grub/files/CVE-2024-56738.patch
new file mode 100644
index 0000000000..c7b64aa6ed
--- /dev/null
+++ b/meta/recipes-bsp/grub/files/CVE-2024-56738.patch
@@ -0,0 +1,75 @@
+From 4cef2fc7308b2132317ad166939994f098b41561 Mon Sep 17 00:00:00 2001
+From: Ross Burton <ross.burton@arm.com>
+Date: Tue, 9 Sep 2025 14:23:14 +0100
+Subject: [PATCH] CVE-2024-56738
+
+Backport an algorithmic change to grub_crypto_memcmp() so that it completes in
+constant time and thus isn't susceptible to side-channel attacks.
+
+This is a partial backport of grub 0739d24cd
+("libgcrypt: Adjust import script, definitions and API users for libgcrypt 1.11")
+
+CVE: CVE-2024-56738
+Upstream-Status: Backport [0739d24cd]
+Signed-off-by: Ross Burton <ross.burton@arm.com>
+---
+ grub-core/lib/crypto.c | 23 ++++++++++++++++-------
+ include/grub/crypto.h  |  2 +-
+ 2 files changed, 17 insertions(+), 8 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
+diff --git a/include/grub/crypto.h b/include/grub/crypto.h
+index 31c87c302..20ad4c5f7 100644
+--- a/include/grub/crypto.h
++++ b/include/grub/crypto.h
+@@ -393,7 +393,7 @@ grub_crypto_pbkdf2 (const struct gcry_md_spec *md,
+                    grub_uint8_t *DK, grub_size_t dkLen);
+ 
+ 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);
+ 
+ int
+ grub_password_get (char buf[], unsigned buf_size);
+-- 
+2.43.0
+