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Dear Security team,</div>
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I am submitting a fix for CVE-2024-56738 affecting the GRUB2 package in Debian.</div>
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Description of the vulnerability:</div>
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GNU GRUB (aka GRUB2) through 2.12 does not use a constant-time algorithm for grub_crypto_memcmp and thus allows side-channel attacks. The current implementation returns early when a difference is found, which can lead to timing attacks that reveal information
about the compared data.</div>
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Affected Debian versions:</div>
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- bookworm</div>
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- bullseye</div>
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- trixie/sid</div>
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The fix implements a constant-time comparison algorithm that:</div>
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1. Uses bitwise operations (XOR and OR) instead of conditional branching</div>
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2. Always processes all bytes regardless of whether differences are found</div>
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3. Uses volatile to prevent compiler optimizations that could reintroduce timing issues</div>
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I've verified that the patch is syntactically correct and implements proper constant-time comparison according to cryptographic best practices.</div>
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I've attached the patch file to this email.</div>
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Thanks,</div>
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Mostafa Amin</div>
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