linux-kernel - Re: [PATCH RFC] crypto/hkdf: Fix salt length short issue in FIPS mode

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Message-ID: <20251126174158.GA71370@quark>
Date: Wed, 26 Nov 2025 09:41:58 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: Li Tian <litian@...hat.com>
Cc: linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fscrypt@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>,
	"Theodore Y . Ts'o" <tytso@....edu>,
	Jaegeuk Kim <jaegeuk@...nel.org>
Subject: Re: [PATCH RFC] crypto/hkdf: Fix salt length short issue in FIPS mode

On Wed, Nov 26, 2025 at 09:42:22PM +0800, Li Tian wrote:
> Under FIPS mode, the hkdf test fails because salt is required
> to be at least 32 bytes long. Pad salt with 0's.
> 
> Signed-off-by: Li Tian <litian@...hat.com>
> ---
>  crypto/hkdf.c         | 11 ++++++++++-
>  fs/crypto/hkdf.c      | 13 -------------
>  include/crypto/hkdf.h | 13 +++++++++++++
>  3 files changed, 23 insertions(+), 14 deletions(-)
> 
> diff --git a/crypto/hkdf.c b/crypto/hkdf.c
> index 82d1b32ca6ce..9af0ef4dfb35 100644
> --- a/crypto/hkdf.c
> +++ b/crypto/hkdf.c
> @@ -46,6 +46,15 @@ int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
>  		 u8 *prk)
>  {
>  	int err;
> +	u8 tmp_salt[HKDF_HASHLEN];
> +
> +	if (saltlen < HKDF_HASHLEN) {
> +		/* Copy salt and pad with zeros to HashLen */
> +		memcpy(tmp_salt, salt, saltlen);
> +		memset(tmp_salt + saltlen, 0, HKDF_HASHLEN - saltlen);
> +		salt = tmp_salt;
> +		saltlen = HKDF_HASHLEN;
> +	}
>  
>  	err = crypto_shash_setkey(hmac_tfm, salt, saltlen);
>  	if (!err)
> @@ -151,7 +160,7 @@ struct hkdf_testvec {
>   */
>  static const struct hkdf_testvec hkdf_sha256_tv[] = {
>  	{
> -		.test = "basic hdkf test",
> +		.test = "basic hkdf test",
>  		.ikm  = "\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b"
>  			"\x0b\x0b\x0b\x0b\x0b\x0b",
>  		.ikm_size = 22,
> diff --git a/fs/crypto/hkdf.c b/fs/crypto/hkdf.c
> index 706f56d0076e..5e4844c1d3d7 100644
> --- a/fs/crypto/hkdf.c
> +++ b/fs/crypto/hkdf.c
> @@ -13,19 +13,6 @@
>  
>  #include "fscrypt_private.h"
>  
> -/*
> - * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
> - * SHA-512 because it is well-established, secure, and reasonably efficient.
> - *
> - * HKDF-SHA256 was also considered, as its 256-bit security strength would be
> - * sufficient here.  A 512-bit security strength is "nice to have", though.
> - * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256.  In the
> - * common case of deriving an AES-256-XTS key (512 bits), that can result in
> - * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
> - * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
> - */
> -#define HKDF_HASHLEN		SHA512_DIGEST_SIZE
> -
>  /*
>   * HKDF consists of two steps:
>   *
> diff --git a/include/crypto/hkdf.h b/include/crypto/hkdf.h
> index 6a9678f508f5..7ef55ce875e2 100644
> --- a/include/crypto/hkdf.h
> +++ b/include/crypto/hkdf.h
> @@ -11,6 +11,19 @@
>  
>  #include <crypto/hash.h>
>  
> +/*
> + * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
> + * SHA-512 because it is well-established, secure, and reasonably efficient.
> + *
> + * HKDF-SHA256 was also considered, as its 256-bit security strength would be
> + * sufficient here.  A 512-bit security strength is "nice to have", though.
> + * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256.  In the
> + * common case of deriving an AES-256-XTS key (512 bits), that can result in
> + * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
> + * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
> + */
> +#define HKDF_HASHLEN            SHA512_DIGEST_SIZE
> +
>  int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
>  		 unsigned int ikmlen, const u8 *salt, unsigned int saltlen,
>  		 u8 *prk);

It seems you're trying to pad all the salts to 64 bytes?  That doesn't
make sense.  Just skip the salt_size == 0 test vector when fips_enabled.  

And either way, no need to mess with the code in fs/crypto/.

- Eric