KEYS: add SP800-56A KDF support for DH

SP800-56A defines the use of DH with key derivation function based on a
counter. The input to the KDF is defined as (DH shared secret || other
information). The value for the "other information" is to be provided by
the caller.

The KDF is implemented using the hash support from the kernel crypto API.
The implementation uses the symmetric hash support as the input to the
hash operation is usually very small. The caller is allowed to specify
the hash name that he wants to use to derive the key material allowing
the use of all supported hashes provided with the kernel crypto API.

As the KDF implements the proper truncation of the DH shared secret to
the requested size, this patch fills the caller buffer up to its size.

The patch is tested with a new test added to the keyutils user space
code which uses a CAVS test vector testing the compliance with
SP800-56A.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 208 insertions, 12 deletions

diff --git a/security/keys/dh.c b/security/keys/dh.c
index 893af4c45038..e603bd912e4c 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -11,6 +11,8 @@
 #include <linux/mpi.h>
 #include <linux/slab.h>
 #include <linux/uaccess.h>
+#include <linux/crypto.h>
+#include <crypto/hash.h>
 #include <keys/user-type.h>
 #include "internal.h"
 
@@ -77,9 +79,146 @@ error:
 	return ret;
 }
 
-long keyctl_dh_compute(struct keyctl_dh_params __user *params,
-		       char __user *buffer, size_t buflen,
-		       void __user *reserved)
+struct kdf_sdesc {
+	struct shash_desc shash;
+	char ctx[];
+};
+
+static int kdf_alloc(struct kdf_sdesc **sdesc_ret, char *hashname)
+{
+	struct crypto_shash *tfm;
+	struct kdf_sdesc *sdesc;
+	int size;
+
+	/* allocate synchronous hash */
+	tfm = crypto_alloc_shash(hashname, 0, 0);
+	if (IS_ERR(tfm)) {
+		pr_info("could not allocate digest TFM handle %s\n", hashname);
+		return PTR_ERR(tfm);
+	}
+
+	size = sizeof(struct shash_desc) + crypto_shash_descsize(tfm);
+	sdesc = kmalloc(size, GFP_KERNEL);
+	if (!sdesc)
+		return -ENOMEM;
+	sdesc->shash.tfm = tfm;
+	sdesc->shash.flags = 0x0;
+
+	*sdesc_ret = sdesc;
+
+	return 0;
+}
+
+static void kdf_dealloc(struct kdf_sdesc *sdesc)
+{
+	if (!sdesc)
+		return;
+
+	if (sdesc->shash.tfm)
+		crypto_free_shash(sdesc->shash.tfm);
+
+	kzfree(sdesc);
+}
+
+/* convert 32 bit integer into its string representation */
+static inline void crypto_kw_cpu_to_be32(u32 val, u8 *buf)
+{
+	__be32 *a = (__be32 *)buf;
+
+	*a = cpu_to_be32(val);
+}
+
+/*
+ * 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).
+ */
+static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
+		   u8 *dst, unsigned int dlen)
+{
+	struct shash_desc *desc = &sdesc->shash;
+	unsigned int h = crypto_shash_digestsize(desc->tfm);
+	int err = 0;
+	u8 *dst_orig = dst;
+	u32 i = 1;
+	u8 iteration[sizeof(u32)];
+
+	while (dlen) {
+		err = crypto_shash_init(desc);
+		if (err)
+			goto err;
+
+		crypto_kw_cpu_to_be32(i, iteration);
+		err = crypto_shash_update(desc, iteration, sizeof(u32));
+		if (err)
+			goto err;
+
+		if (src && slen) {
+			err = crypto_shash_update(desc, src, slen);
+			if (err)
+				goto err;
+		}
+
+		if (dlen < h) {
+			u8 tmpbuffer[h];
+
+			err = crypto_shash_final(desc, tmpbuffer);
+			if (err)
+				goto err;
+			memcpy(dst, tmpbuffer, dlen);
+			memzero_explicit(tmpbuffer, h);
+			return 0;
+		} else {
+			err = crypto_shash_final(desc, dst);
+			if (err)
+				goto err;
+
+			dlen -= h;
+			dst += h;
+			i++;
+		}
+	}
+
+	return 0;
+
+err:
+	memzero_explicit(dst_orig, dlen);
+	return err;
+}
+
+static int keyctl_dh_compute_kdf(struct kdf_sdesc *sdesc,
+				 char __user *buffer, size_t buflen,
+				 uint8_t *kbuf, size_t kbuflen)
+{
+	uint8_t *outbuf = NULL;
+	int ret;
+
+	outbuf = kmalloc(buflen, GFP_KERNEL);
+	if (!outbuf) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	ret = kdf_ctr(sdesc, kbuf, kbuflen, outbuf, buflen);
+	if (ret)
+		goto err;
+
+	ret = buflen;
+	if (copy_to_user(buffer, outbuf, buflen) != 0)
+		ret = -EFAULT;
+
+err:
+	kzfree(outbuf);
+	return ret;
+}
+
+long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
+			 char __user *buffer, size_t buflen,
+			 struct keyctl_kdf_params *kdfcopy)
 {
 	long ret;
 	MPI base, private, prime, result;
@@ -88,6 +227,7 @@ long keyctl_dh_compute(struct keyctl_dh_params __user *params,
 	uint8_t *kbuf;
 	ssize_t keylen;
 	size_t resultlen;
+	struct kdf_sdesc *sdesc = NULL;
 
 	if (!params || (!buffer && buflen)) {
 		ret = -EINVAL;
@@ -98,12 +238,34 @@ long keyctl_dh_compute(struct keyctl_dh_params __user *params,
 		goto out;
 	}
 
-	if (reserved) {
-		ret = -EINVAL;
-		goto out;
+	if (kdfcopy) {
+		char *hashname;
+
+		if (buflen > KEYCTL_KDF_MAX_OUTPUT_LEN ||
+		    kdfcopy->otherinfolen > KEYCTL_KDF_MAX_OI_LEN) {
+			ret = -EMSGSIZE;
+			goto out;
+		}
+
+		/* get KDF name string */
+		hashname = strndup_user(kdfcopy->hashname, CRYPTO_MAX_ALG_NAME);
+		if (IS_ERR(hashname)) {
+			ret = PTR_ERR(hashname);
+			goto out;
+		}
+
+		/* allocate KDF from the kernel crypto API */
+		ret = kdf_alloc(&sdesc, hashname);
+		kfree(hashname);
+		if (ret)
+			goto out;
 	}
 
-	keylen = mpi_from_key(pcopy.prime, buflen, &prime);
+	/*
+	 * If the caller requests postprocessing with a KDF, allow an
+	 * arbitrary output buffer size since the KDF ensures proper truncation.
+	 */
+	keylen = mpi_from_key(pcopy.prime, kdfcopy ? SIZE_MAX : buflen, &prime);
 	if (keylen < 0 || !prime) {
 		/* buflen == 0 may be used to query the required buffer size,
 		 * which is the prime key length.
@@ -133,12 +295,25 @@ long keyctl_dh_compute(struct keyctl_dh_params __user *params,
 		goto error3;
 	}
 
-	kbuf = kmalloc(resultlen, GFP_KERNEL);
+	/* allocate space for DH shared secret and SP800-56A otherinfo */
+	kbuf = kmalloc(kdfcopy ? (resultlen + kdfcopy->otherinfolen) : resultlen,
+		       GFP_KERNEL);
 	if (!kbuf) {
 		ret = -ENOMEM;
 		goto error4;
 	}
 
+	/*
+	 * Concatenate SP800-56A otherinfo past DH shared secret -- the
+	 * input to the KDF is (DH shared secret || otherinfo)
+	 */
+	if (kdfcopy && kdfcopy->otherinfo &&
+	    copy_from_user(kbuf + resultlen, kdfcopy->otherinfo,
+			   kdfcopy->otherinfolen) != 0) {
+		ret = -EFAULT;
+		goto error5;
+	}
+
 	ret = do_dh(result, base, private, prime);
 	if (ret)
 		goto error5;
@@ -147,12 +322,17 @@ long keyctl_dh_compute(struct keyctl_dh_params __user *params,
 	if (ret != 0)
 		goto error5;
 
-	ret = nbytes;
-	if (copy_to_user(buffer, kbuf, nbytes) != 0)
-		ret = -EFAULT;
+	if (kdfcopy) {
+		ret = keyctl_dh_compute_kdf(sdesc, buffer, buflen, kbuf,
+					    resultlen + kdfcopy->otherinfolen);
+	} else {
+		ret = nbytes;
+		if (copy_to_user(buffer, kbuf, nbytes) != 0)
+			ret = -EFAULT;
+	}
 
 error5:
-	kfree(kbuf);
+	kzfree(kbuf);
 error4:
 	mpi_free(result);
 error3:
@@ -162,5 +342,21 @@ error2:
 error1:
 	mpi_free(prime);
 out:
+	kdf_dealloc(sdesc);
 	return ret;
 }
+
+long keyctl_dh_compute(struct keyctl_dh_params __user *params,
+		       char __user *buffer, size_t buflen,
+		       struct keyctl_kdf_params __user *kdf)
+{
+	struct keyctl_kdf_params kdfcopy;
+
+	if (!kdf)
+		return __keyctl_dh_compute(params, buffer, buflen, NULL);
+
+	if (copy_from_user(&kdfcopy, kdf, sizeof(kdfcopy)) != 0)
+		return -EFAULT;
+
+	return __keyctl_dh_compute(params, buffer, buflen, &kdfcopy);
+}