[bug #66603] grub_crypto_memcmp side-channel

[Jonathan Bar Or ("JBO")]

URL:
  <https://savannah.gnu.org/bugs/?66603>

                 Summary: grub_crypto_memcmp side-channel
                   Group: GNU GRUB
               Submitter: yo_yo_yo_jbo
               Submitted: Fri 27 Dec 2024 04:22:41 PM UTC
                Category: Security
                Severity: Major
                Priority: 5 - Normal
              Item Group: Software Error
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name:
        Originator Email:
             Open/Closed: Open
         Discussion Lock: Any
                 Release: other
                 Release:
         Reproducibility: None
         Planned Release: None


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Follow-up Comments:


-------------------------------------------------------
Date: Fri 27 Dec 2024 04:22:41 PM UTC By: Jonathan Bar Or ("JBO")
<yo_yo_yo_jbo>
The cryptographic comparison function "grub_crypto_memcmp" is susceptible to
side-channel attack due to the "if" condition when comparing bytes:

if (*pa != *pb)
        counter++;

The recommended way would be the following:

int
grub_crypto_memcmp (const void *a, const void *b, grub_size_t n)
{
  register grub_uint8_t indicator = 0;
  grub_uint8_t* pa = a, *b = b;
  grub_size_t i;
  
  for (i=0; i<n;i++)
    indicator |= (pa[i] ^ pb[i]);

  return !!indicator;
}








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