diff mbox series

[5/5] fs: use HKDF implementation from kernel crypto API

Message ID 7857050.T7Z3S40VBb@positron.chronox.de (mailing list archive)
State Superseded
Headers show
Series Add KDF implementations to crypto API | expand

Commit Message

Stephan Müller Jan. 4, 2021, 9:50 p.m. UTC
As the kernel crypto API implements HKDF, replace the
file-system-specific HKDF implementation with the generic HKDF
implementation.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
 fs/crypto/Kconfig           |   2 +-
 fs/crypto/fscrypt_private.h |   4 +-
 fs/crypto/hkdf.c            | 108 +++++++++---------------------------
 3 files changed, 30 insertions(+), 84 deletions(-)

Comments

Eric Biggers Jan. 7, 2021, 7:19 a.m. UTC | #1
On Mon, Jan 04, 2021 at 10:50:49PM +0100, Stephan Müller wrote:
> As the kernel crypto API implements HKDF, replace the
> file-system-specific HKDF implementation with the generic HKDF
> implementation.
> 
> Signed-off-by: Stephan Mueller <smueller@chronox.de>
> ---
>  fs/crypto/Kconfig           |   2 +-
>  fs/crypto/fscrypt_private.h |   4 +-
>  fs/crypto/hkdf.c            | 108 +++++++++---------------------------
>  3 files changed, 30 insertions(+), 84 deletions(-)
> 
> diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig
> index a5f5c30368a2..9450e958f1d1 100644
> --- a/fs/crypto/Kconfig
> +++ b/fs/crypto/Kconfig
> @@ -2,7 +2,7 @@
>  config FS_ENCRYPTION
>  	bool "FS Encryption (Per-file encryption)"
>  	select CRYPTO
> -	select CRYPTO_HASH
> +	select CRYPTO_HKDF
>  	select CRYPTO_SKCIPHER
>  	select CRYPTO_LIB_SHA256
>  	select KEYS
> diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
> index 3fa965eb3336..0d6871838099 100644
> --- a/fs/crypto/fscrypt_private.h
> +++ b/fs/crypto/fscrypt_private.h
> @@ -304,7 +304,7 @@ struct fscrypt_hkdf {
>  	struct crypto_shash *hmac_tfm;
>  };
>  
> -int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
> +int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
>  		      unsigned int master_key_size);

It shouldn't be necessary to remove const here.

>  
>  /*
> @@ -323,7 +323,7 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
>  #define HKDF_CONTEXT_INODE_HASH_KEY	7 /* info=<empty>		*/
>  
>  int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
> -			const u8 *info, unsigned int infolen,
> +			u8 *info, unsigned int infolen,
>  			u8 *okm, unsigned int okmlen);

Likewise.  In fact some callers rely on 'info' not being modified.

> -/*
> + *
>   * Compute HKDF-Extract using the given master key as the input keying material,
>   * and prepare an HMAC transform object keyed by the resulting pseudorandom key.
>   *
>   * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
>   * times without having to recompute HKDF-Extract each time.
>   */
> -int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
> +int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
>  		      unsigned int master_key_size)
>  {
> +	/* HKDF-Extract (RFC 5869 section 2.2), unsalted */
> +	const struct kvec seed[] = { {
> +		.iov_base = NULL,
> +		.iov_len = 0
> +	}, {
> +		.iov_base = master_key,
> +		.iov_len = master_key_size
> +	} };
>  	struct crypto_shash *hmac_tfm;
> -	u8 prk[HKDF_HASHLEN];
>  	int err;
>  
>  	hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
> @@ -74,16 +65,12 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
>  		return PTR_ERR(hmac_tfm);
>  	}
>  
> -	if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
> +	if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != HKDF_HASHLEN)) {
>  		err = -EINVAL;
>  		goto err_free_tfm;
>  	}
>  
> -	err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
> -	if (err)
> -		goto err_free_tfm;
> -
> -	err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
> +	err = crypto_hkdf_setkey(hmac_tfm, seed, ARRAY_SIZE(seed));
>  	if (err)
>  		goto err_free_tfm;

It's weird that the salt and key have to be passed in a kvec.
Why not just have normal function parameters like:

	int crypto_hkdf_setkey(struct crypto_shash *hmac_tfm,
			       const u8 *key, size_t keysize,
			       const u8 *salt, size_t saltsize);

>  int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
> -			const u8 *info, unsigned int infolen,
> +			u8 *info, unsigned int infolen,
>  			u8 *okm, unsigned int okmlen)
>  {
> -	SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
> -	u8 prefix[9];
> -	unsigned int i;
> -	int err;
> -	const u8 *prev = NULL;
> -	u8 counter = 1;
> -	u8 tmp[HKDF_HASHLEN];
> -
> -	if (WARN_ON(okmlen > 255 * HKDF_HASHLEN))
> -		return -EINVAL;
> -
> -	desc->tfm = hkdf->hmac_tfm;
> -
> -	memcpy(prefix, "fscrypt\0", 8);
> -	prefix[8] = context;
> -
> -	for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
> +	const struct kvec info_iov[] = { {
> +		.iov_base = "fscrypt\0",
> +		.iov_len = 8,
> +	}, {
> +		.iov_base = &context,
> +		.iov_len = 1,
> +	}, {
> +		.iov_base = info,
> +		.iov_len = infolen,
> +	} };
> +	int err = crypto_hkdf_generate(hkdf->hmac_tfm,
> +				       info_iov, ARRAY_SIZE(info_iov),
> +				       okm, okmlen);
>  
> -		err = crypto_shash_init(desc);
> -		if (err)
> -			goto out;
> -
> -		if (prev) {
> -			err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
> -			if (err)
> -				goto out;
> -		}
> -
> -		err = crypto_shash_update(desc, prefix, sizeof(prefix));
> -		if (err)
> -			goto out;
> -
> -		err = crypto_shash_update(desc, info, infolen);
> -		if (err)
> -			goto out;
> -
> -		BUILD_BUG_ON(sizeof(counter) != 1);
> -		if (okmlen - i < HKDF_HASHLEN) {
> -			err = crypto_shash_finup(desc, &counter, 1, tmp);
> -			if (err)
> -				goto out;
> -			memcpy(&okm[i], tmp, okmlen - i);
> -			memzero_explicit(tmp, sizeof(tmp));
> -		} else {
> -			err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
> -			if (err)
> -				goto out;
> -		}
> -		counter++;
> -		prev = &okm[i];
> -	}
> -	err = 0;
> -out:
>  	if (unlikely(err))
>  		memzero_explicit(okm, okmlen); /* so caller doesn't need to */
> -	shash_desc_zero(desc);

Shouldn't crypto_hkdf_generate() handle the above memzero_explicit() of the
output buffer on error, so that all callers don't need to do it?

- Eric
Stephan Müller Jan. 7, 2021, 7:49 a.m. UTC | #2
Am Mittwoch, dem 06.01.2021 um 23:19 -0800 schrieb Eric Biggers:
> On Mon, Jan 04, 2021 at 10:50:49PM +0100, Stephan Müller wrote:
> > As the kernel crypto API implements HKDF, replace the
> > file-system-specific HKDF implementation with the generic HKDF
> > implementation.
> > 
> > Signed-off-by: Stephan Mueller <smueller@chronox.de>
> > ---
> >  fs/crypto/Kconfig           |   2 +-
> >  fs/crypto/fscrypt_private.h |   4 +-
> >  fs/crypto/hkdf.c            | 108 +++++++++---------------------------
> >  3 files changed, 30 insertions(+), 84 deletions(-)
> > 
> > diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig
> > index a5f5c30368a2..9450e958f1d1 100644
> > --- a/fs/crypto/Kconfig
> > +++ b/fs/crypto/Kconfig
> > @@ -2,7 +2,7 @@
> >  config FS_ENCRYPTION
> >         bool "FS Encryption (Per-file encryption)"
> >         select CRYPTO
> > -       select CRYPTO_HASH
> > +       select CRYPTO_HKDF
> >         select CRYPTO_SKCIPHER
> >         select CRYPTO_LIB_SHA256
> >         select KEYS
> > diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
> > index 3fa965eb3336..0d6871838099 100644
> > --- a/fs/crypto/fscrypt_private.h
> > +++ b/fs/crypto/fscrypt_private.h
> > @@ -304,7 +304,7 @@ struct fscrypt_hkdf {
> >         struct crypto_shash *hmac_tfm;
> >  };
> >  
> > -int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
> > +int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
> >                       unsigned int master_key_size);
> 
> It shouldn't be necessary to remove const here.

Unfortunately it is when adding the pointer to struct kvec
> 
> >  
> >  /*
> > @@ -323,7 +323,7 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const
> > u8 *master_key,
> >  #define HKDF_CONTEXT_INODE_HASH_KEY    7 /* info=<empty>               */
> >  
> >  int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
> > -                       const u8 *info, unsigned int infolen,
> > +                       u8 *info, unsigned int infolen,
> >                         u8 *okm, unsigned int okmlen);
> 
> Likewise.  In fact some callers rely on 'info' not being modified.

Same here.
> 
> > -/*
> > + *
> >   * Compute HKDF-Extract using the given master key as the input keying
> > material,
> >   * and prepare an HMAC transform object keyed by the resulting
> > pseudorandom key.
> >   *
> >   * Afterwards, the keyed HMAC transform object can be used for HKDF-
> > Expand many
> >   * times without having to recompute HKDF-Extract each time.
> >   */
> > -int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
> > +int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
> >                       unsigned int master_key_size)
> >  {
> > +       /* HKDF-Extract (RFC 5869 section 2.2), unsalted */
> > +       const struct kvec seed[] = { {
> > +               .iov_base = NULL,
> > +               .iov_len = 0
> > +       }, {
> > +               .iov_base = master_key,
> > +               .iov_len = master_key_size
> > +       } };
> >         struct crypto_shash *hmac_tfm;
> > -       u8 prk[HKDF_HASHLEN];
> >         int err;
> >  
> >         hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
> > @@ -74,16 +65,12 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const
> > u8 *master_key,
> >                 return PTR_ERR(hmac_tfm);
> >         }
> >  
> > -       if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
> > +       if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != HKDF_HASHLEN)) {
> >                 err = -EINVAL;
> >                 goto err_free_tfm;
> >         }
> >  
> > -       err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
> > -       if (err)
> > -               goto err_free_tfm;
> > -
> > -       err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
> > +       err = crypto_hkdf_setkey(hmac_tfm, seed, ARRAY_SIZE(seed));
> >         if (err)
> >                 goto err_free_tfm;
> 
> It's weird that the salt and key have to be passed in a kvec.
> Why not just have normal function parameters like:
> 
>         int crypto_hkdf_setkey(struct crypto_shash *hmac_tfm,
>                                const u8 *key, size_t keysize,
>                                const u8 *salt, size_t saltsize);

I wanted to have an identical interface for all types of KDFs to allow turning
them into a template eventually. For example, SP800-108 KDFs only have one
parameter. Hence the use of a kvec.

> 
> >  int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
> > -                       const u8 *info, unsigned int infolen,
> > +                       u8 *info, unsigned int infolen,
> >                         u8 *okm, unsigned int okmlen)
> >  {
> > -       SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
> > -       u8 prefix[9];
> > -       unsigned int i;
> > -       int err;
> > -       const u8 *prev = NULL;
> > -       u8 counter = 1;
> > -       u8 tmp[HKDF_HASHLEN];
> > -
> > -       if (WARN_ON(okmlen > 255 * HKDF_HASHLEN))
> > -               return -EINVAL;
> > -
> > -       desc->tfm = hkdf->hmac_tfm;
> > -
> > -       memcpy(prefix, "fscrypt\0", 8);
> > -       prefix[8] = context;
> > -
> > -       for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
> > +       const struct kvec info_iov[] = { {
> > +               .iov_base = "fscrypt\0",
> > +               .iov_len = 8,
> > +       }, {
> > +               .iov_base = &context,
> > +               .iov_len = 1,
> > +       }, {
> > +               .iov_base = info,
> > +               .iov_len = infolen,
> > +       } };
> > +       int err = crypto_hkdf_generate(hkdf->hmac_tfm,
> > +                                      info_iov, ARRAY_SIZE(info_iov),
> > +                                      okm, okmlen);
> >  
> > -               err = crypto_shash_init(desc);
> > -               if (err)
> > -                       goto out;
> > -
> > -               if (prev) {
> > -                       err = crypto_shash_update(desc, prev,
> > HKDF_HASHLEN);
> > -                       if (err)
> > -                               goto out;
> > -               }
> > -
> > -               err = crypto_shash_update(desc, prefix, sizeof(prefix));
> > -               if (err)
> > -                       goto out;
> > -
> > -               err = crypto_shash_update(desc, info, infolen);
> > -               if (err)
> > -                       goto out;
> > -
> > -               BUILD_BUG_ON(sizeof(counter) != 1);
> > -               if (okmlen - i < HKDF_HASHLEN) {
> > -                       err = crypto_shash_finup(desc, &counter, 1, tmp);
> > -                       if (err)
> > -                               goto out;
> > -                       memcpy(&okm[i], tmp, okmlen - i);
> > -                       memzero_explicit(tmp, sizeof(tmp));
> > -               } else {
> > -                       err = crypto_shash_finup(desc, &counter, 1,
> > &okm[i]);
> > -                       if (err)
> > -                               goto out;
> > -               }
> > -               counter++;
> > -               prev = &okm[i];
> > -       }
> > -       err = 0;
> > -out:
> >         if (unlikely(err))
> >                 memzero_explicit(okm, okmlen); /* so caller doesn't need
> > to */
> > -       shash_desc_zero(desc);
> 
> Shouldn't crypto_hkdf_generate() handle the above memzero_explicit() of the
> output buffer on error, so that all callers don't need to do it?

Yes, I will move it to HKDF (and the SP800-108 KDF as well).

Thanks for the review
Stephan
> 
> - Eric
Eric Biggers Jan. 7, 2021, 6:47 p.m. UTC | #3
On Thu, Jan 07, 2021 at 08:49:52AM +0100, Stephan Mueller wrote:
> > > -int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
> > > +int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
> > >                       unsigned int master_key_size);
> > 
> > It shouldn't be necessary to remove const here.
> 
> Unfortunately it is when adding the pointer to struct kvec
> > 
> > >  
> > >  /*
> > > @@ -323,7 +323,7 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const
> > > u8 *master_key,
> > >  #define HKDF_CONTEXT_INODE_HASH_KEY    7 /* info=<empty>               */
> > >  
> > >  int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
> > > -                       const u8 *info, unsigned int infolen,
> > > +                       u8 *info, unsigned int infolen,
> > >                         u8 *okm, unsigned int okmlen);
> > 
> > Likewise.  In fact some callers rely on 'info' not being modified.
> 
> Same here.

If the HKDF API will have a quirk like this, it's better not to "leak" it into
the prototypes of these fscrypt functions.  Just add the needed casts in
fscrypt_init_hkdf() and fscrypt_hkdf_expand().

> > > -       err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
> > > +       err = crypto_hkdf_setkey(hmac_tfm, seed, ARRAY_SIZE(seed));
> > >         if (err)
> > >                 goto err_free_tfm;
> > 
> > It's weird that the salt and key have to be passed in a kvec.
> > Why not just have normal function parameters like:
> > 
> >         int crypto_hkdf_setkey(struct crypto_shash *hmac_tfm,
> >                                const u8 *key, size_t keysize,
> >                                const u8 *salt, size_t saltsize);
> 
> I wanted to have an identical interface for all types of KDFs to allow turning
> them into a template eventually. For example, SP800-108 KDFs only have one
> parameter. Hence the use of a kvec.

But the API being provided is a library function specifically for HKDF.
So there's no need to make it conform to some other API.

- Eric
diff mbox series

Patch

diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig
index a5f5c30368a2..9450e958f1d1 100644
--- a/fs/crypto/Kconfig
+++ b/fs/crypto/Kconfig
@@ -2,7 +2,7 @@ 
 config FS_ENCRYPTION
 	bool "FS Encryption (Per-file encryption)"
 	select CRYPTO
-	select CRYPTO_HASH
+	select CRYPTO_HKDF
 	select CRYPTO_SKCIPHER
 	select CRYPTO_LIB_SHA256
 	select KEYS
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 3fa965eb3336..0d6871838099 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -304,7 +304,7 @@  struct fscrypt_hkdf {
 	struct crypto_shash *hmac_tfm;
 };
 
-int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
+int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
 		      unsigned int master_key_size);
 
 /*
@@ -323,7 +323,7 @@  int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
 #define HKDF_CONTEXT_INODE_HASH_KEY	7 /* info=<empty>		*/
 
 int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
-			const u8 *info, unsigned int infolen,
+			u8 *info, unsigned int infolen,
 			u8 *okm, unsigned int okmlen);
 
 void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf);
diff --git a/fs/crypto/hkdf.c b/fs/crypto/hkdf.c
index e0ec21055505..f837cb8ec0a5 100644
--- a/fs/crypto/hkdf.c
+++ b/fs/crypto/hkdf.c
@@ -9,7 +9,7 @@ 
  * Copyright 2019 Google LLC
  */
 
-#include <crypto/hash.h>
+#include <crypto/hkdf.h>
 #include <crypto/sha2.h>
 
 #include "fscrypt_private.h"
@@ -37,34 +37,25 @@ 
  * unnecessarily long master keys.  Thus fscrypt still does HKDF-Extract.  No
  * salt is used, since fscrypt master keys should already be pseudorandom and
  * there's no way to persist a random salt per master key from kernel mode.
- */
-
-/* HKDF-Extract (RFC 5869 section 2.2), unsalted */
-static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
-			unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
-{
-	static const u8 default_salt[HKDF_HASHLEN];
-	int err;
-
-	err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
-	if (err)
-		return err;
-
-	return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
-}
-
-/*
+ *
  * Compute HKDF-Extract using the given master key as the input keying material,
  * and prepare an HMAC transform object keyed by the resulting pseudorandom key.
  *
  * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
  * times without having to recompute HKDF-Extract each time.
  */
-int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
+int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
 		      unsigned int master_key_size)
 {
+	/* HKDF-Extract (RFC 5869 section 2.2), unsalted */
+	const struct kvec seed[] = { {
+		.iov_base = NULL,
+		.iov_len = 0
+	}, {
+		.iov_base = master_key,
+		.iov_len = master_key_size
+	} };
 	struct crypto_shash *hmac_tfm;
-	u8 prk[HKDF_HASHLEN];
 	int err;
 
 	hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
@@ -74,16 +65,12 @@  int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
 		return PTR_ERR(hmac_tfm);
 	}
 
-	if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
+	if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != HKDF_HASHLEN)) {
 		err = -EINVAL;
 		goto err_free_tfm;
 	}
 
-	err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
-	if (err)
-		goto err_free_tfm;
-
-	err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
+	err = crypto_hkdf_setkey(hmac_tfm, seed, ARRAY_SIZE(seed));
 	if (err)
 		goto err_free_tfm;
 
@@ -93,7 +80,6 @@  int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
 err_free_tfm:
 	crypto_free_shash(hmac_tfm);
 out:
-	memzero_explicit(prk, sizeof(prk));
 	return err;
 }
 
@@ -109,65 +95,25 @@  int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
  * accidentally repeat an info string when using HKDF for different purposes.)
  */
 int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
-			const u8 *info, unsigned int infolen,
+			u8 *info, unsigned int infolen,
 			u8 *okm, unsigned int okmlen)
 {
-	SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
-	u8 prefix[9];
-	unsigned int i;
-	int err;
-	const u8 *prev = NULL;
-	u8 counter = 1;
-	u8 tmp[HKDF_HASHLEN];
-
-	if (WARN_ON(okmlen > 255 * HKDF_HASHLEN))
-		return -EINVAL;
-
-	desc->tfm = hkdf->hmac_tfm;
-
-	memcpy(prefix, "fscrypt\0", 8);
-	prefix[8] = context;
-
-	for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
+	const struct kvec info_iov[] = { {
+		.iov_base = "fscrypt\0",
+		.iov_len = 8,
+	}, {
+		.iov_base = &context,
+		.iov_len = 1,
+	}, {
+		.iov_base = info,
+		.iov_len = infolen,
+	} };
+	int err = crypto_hkdf_generate(hkdf->hmac_tfm,
+				       info_iov, ARRAY_SIZE(info_iov),
+				       okm, okmlen);
 
-		err = crypto_shash_init(desc);
-		if (err)
-			goto out;
-
-		if (prev) {
-			err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
-			if (err)
-				goto out;
-		}
-
-		err = crypto_shash_update(desc, prefix, sizeof(prefix));
-		if (err)
-			goto out;
-
-		err = crypto_shash_update(desc, info, infolen);
-		if (err)
-			goto out;
-
-		BUILD_BUG_ON(sizeof(counter) != 1);
-		if (okmlen - i < HKDF_HASHLEN) {
-			err = crypto_shash_finup(desc, &counter, 1, tmp);
-			if (err)
-				goto out;
-			memcpy(&okm[i], tmp, okmlen - i);
-			memzero_explicit(tmp, sizeof(tmp));
-		} else {
-			err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
-			if (err)
-				goto out;
-		}
-		counter++;
-		prev = &okm[i];
-	}
-	err = 0;
-out:
 	if (unlikely(err))
 		memzero_explicit(okm, okmlen); /* so caller doesn't need to */
-	shash_desc_zero(desc);
 	return err;
 }