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2024-05-10 37f49e37ab4cb5d0bc4c60eb5c6d4dd57db767bb

 kernel/fs/crypto/hkdf.c
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@@ -16,9 +16,14 @@
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 /*
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  * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
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- * SHA-512 because it is reasonably secure and efficient; and since it produces
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- * a 64-byte digest, deriving an AES-256-XTS key preserves all 64 bytes of
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- * entropy from the master key and requires only one iteration of HKDF-Expand.
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+ * SHA-512 because it is well-established, secure, and reasonably efficient.
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+ *
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+ * HKDF-SHA256 was also considered, as its 256-bit security strength would be
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+ * sufficient here.  A 512-bit security strength is "nice to have", though.
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+ * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256.  In the
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+ * common case of deriving an AES-256-XTS key (512 bits), that can result in
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+ * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
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+ * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
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  */
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 #define HKDF_HMAC_ALG		"hmac(sha512)"
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 #define HKDF_HASHLEN		SHA512_DIGEST_SIZE
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@@ -44,18 +49,13 @@
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 			unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
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 {
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 	static const u8 default_salt[HKDF_HASHLEN];
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-	SHASH_DESC_ON_STACK(desc, hmac_tfm);
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 	int err;
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 	err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
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 	if (err)
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 		return err;
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-	desc->tfm = hmac_tfm;
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-	desc->flags = 0;
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-	err = crypto_shash_digest(desc, ikm, ikmlen, prk);
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-	shash_desc_zero(desc);
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-	return err;
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+	return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
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 }
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 /*
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@@ -129,7 +129,6 @@
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 		return -EINVAL;
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 	desc->tfm = hkdf->hmac_tfm;
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-	desc->flags = 0;
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 	memcpy(prefix, "fscrypt\0", 8);
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 	prefix[8] = context;