[PATCH 4/5] security: DH - use KDF implementation from crypto API

Stephan Müller
2021-01-04T21:55:57Z

The kernel crypto API provides the SP800-108 counter KDF implementation.
Thus, the separate implementation provided as part of the keys subsystem
can be replaced with calls to the KDF offered by the kernel crypto API.

The keys subsystem uses the counter KDF with a hash cipher primitive.
Thus, it only uses the call to crypto_kdf108_ctr_generate.

The change removes the specific code that adds a zero padding that was
intended to be invoked when the DH operation result was smaller than the
modulus. However, this cannot occur any more these days because the
function mpi_write_to_sgl is used in the code path that calculates the
shared secret in dh_compute_value. This MPI service function guarantees
that leading zeros are introduced as needed to ensure the resulting data
is exactly as long as the modulus. This implies that the specific code
to add zero padding is dead code which can be safely removed.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
security/keys/Kconfig | 2 +-
security/keys/dh.c | 118 ++++++------------------------------------
2 files changed, 17 insertions(+), 103 deletions(-)

diff --git a/security/keys/Kconfig b/security/keys/Kconfig
index 83bc23409164..e6604499f0a8 100644
--- a/security/keys/Kconfig
+++ b/security/keys/Kconfig
@@ -106,7 +106,7 @@ config KEY_DH_OPERATIONS
bool "Diffie-Hellman operations on retained keys"
depends on KEYS
select CRYPTO
- select CRYPTO_HASH
+ select CRYPTO_KDF800108_CTR
select CRYPTO_DH
help
This option provides support for calculating Diffie-Hellman
diff --git a/security/keys/dh.c b/security/keys/dh.c
index 1abfa70ed6e1..46fa442b81ec 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -11,6 +11,7 @@
#include <crypto/hash.h>
#include <crypto/kpp.h>
#include <crypto/dh.h>
+#include <crypto/kdf_sp800108.h>
#include <keys/user-type.h>
#include "internal.h"

@@ -79,16 +80,9 @@ static void dh_crypto_done(struct crypto_async_request *req, int err)
complete(&compl->completion);
}

-struct kdf_sdesc {
- struct shash_desc shash;
- char ctx[];
-};
-
-static int kdf_alloc(struct kdf_sdesc **sdesc_ret, char *hashname)
+static int kdf_alloc(struct crypto_shash **hash, char *hashname)
{
struct crypto_shash *tfm;
- struct kdf_sdesc *sdesc;
- int size;
int err;

/* allocate synchronous hash */
@@ -102,14 +96,7 @@ static int kdf_alloc(struct kdf_sdesc **sdesc_ret, char *hashname)
if (crypto_shash_digestsize(tfm) == 0)
goto out_free_tfm;

- err = -ENOMEM;
- size = sizeof(struct shash_desc) + crypto_shash_descsize(tfm);
- sdesc = kmalloc(size, GFP_KERNEL);
- if (!sdesc)
- goto out_free_tfm;
- sdesc->shash.tfm = tfm;
-
- *sdesc_ret = sdesc;
+ *hash = tfm;

return 0;

@@ -118,92 +105,20 @@ static int kdf_alloc(struct kdf_sdesc **sdesc_ret, char *hashname)
return err;
}

-static void kdf_dealloc(struct kdf_sdesc *sdesc)
-{
- if (!sdesc)
- return;
-
- if (sdesc->shash.tfm)
- crypto_free_shash(sdesc->shash.tfm);
-
- kfree_sensitive(sdesc);
-}
-
-/*
- * Implementation of the KDF in counter mode according to SP800-108 section 5.1
- * as well as SP800-56A section 5.8.1 (Single-step KDF).
- *
- * SP800-56A:
- * The src pointer is defined as Z || other info where Z is the shared secret
- * from DH and other info is an arbitrary string (see SP800-56A section
- * 5.8.1.2).
- *
- * 'dlen' must be a multiple of the digest size.
- */
-static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
- u8 *dst, unsigned int dlen, unsigned int zlen)
+static void kdf_dealloc(struct crypto_shash *hash)
{
- struct shash_desc *desc = &sdesc->shash;
- unsigned int h = crypto_shash_digestsize(desc->tfm);
- int err = 0;
- u8 *dst_orig = dst;
- __be32 counter = cpu_to_be32(1);
-
- while (dlen) {
- err = crypto_shash_init(desc);
- if (err)
- goto err;
-
- err = crypto_shash_update(desc, (u8 *)&counter, sizeof(__be32));
- if (err)
- goto err;
-
- if (zlen && h) {
- u8 tmpbuffer[32];
- size_t chunk = min_t(size_t, zlen, sizeof(tmpbuffer));
- memset(tmpbuffer, 0, chunk);
-
- do {
- err = crypto_shash_update(desc, tmpbuffer,
- chunk);
- if (err)
- goto err;
-
- zlen -= chunk;
- chunk = min_t(size_t, zlen, sizeof(tmpbuffer));
- } while (zlen);
- }
-
- if (src && slen) {
- err = crypto_shash_update(desc, src, slen);
- if (err)
- goto err;
- }
-
- err = crypto_shash_final(desc, dst);
- if (err)
- goto err;
-
- dlen -= h;
- dst += h;
- counter = cpu_to_be32(be32_to_cpu(counter) + 1);
- }
-
- return 0;
-
-err:
- memzero_explicit(dst_orig, dlen);
- return err;
+ if (hash)
+ crypto_free_shash(hash);
}

-static int keyctl_dh_compute_kdf(struct kdf_sdesc *sdesc,
+static int keyctl_dh_compute_kdf(struct crypto_shash *hash,
char __user *buffer, size_t buflen,
- uint8_t *kbuf, size_t kbuflen, size_t lzero)
+ uint8_t *kbuf, size_t kbuflen)
{
+ struct kvec kbuf_iov = { .iov_base = kbuf, .iov_len = kbuflen };
uint8_t *outbuf = NULL;
int ret;
- size_t outbuf_len = roundup(buflen,
- crypto_shash_digestsize(sdesc->shash.tfm));
+ size_t outbuf_len = roundup(buflen, crypto_shash_digestsize(hash));

outbuf = kmalloc(outbuf_len, GFP_KERNEL);
if (!outbuf) {
@@ -211,7 +126,7 @@ static int keyctl_dh_compute_kdf(struct kdf_sdesc *sdesc,
goto err;
}

- ret = kdf_ctr(sdesc, kbuf, kbuflen, outbuf, outbuf_len, lzero);
+ ret = crypto_kdf108_ctr_generate(hash, &kbuf_iov, 1, outbuf, outbuf_len);
if (ret)
goto err;

@@ -240,7 +155,7 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
struct kpp_request *req;
uint8_t *secret;
uint8_t *outbuf;
- struct kdf_sdesc *sdesc = NULL;
+ struct crypto_shash *hash = NULL;

if (!params || (!buffer && buflen)) {
ret = -EINVAL;
@@ -273,7 +188,7 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
}

/* allocate KDF from the kernel crypto API */
- ret = kdf_alloc(&sdesc, hashname);
+ ret = kdf_alloc(&hash, hashname);
kfree(hashname);
if (ret)
goto out1;
@@ -383,9 +298,8 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
goto out6;
}

- ret = keyctl_dh_compute_kdf(sdesc, buffer, buflen, outbuf,
- req->dst_len + kdfcopy->otherinfolen,
- outlen - req->dst_len);
+ ret = keyctl_dh_compute_kdf(hash, buffer, buflen, outbuf,
+ req->dst_len + kdfcopy->otherinfolen);
} else if (copy_to_user(buffer, outbuf, req->dst_len) == 0) {
ret = req->dst_len;
} else {
@@ -403,7 +317,7 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
out2:
dh_free_data(&dh_inputs);
out1:
- kdf_dealloc(sdesc);
+ kdf_dealloc(hash);
return ret;
}

--
2.26.2