Re: [RFC PATCH] KEYS: add SP800-56A KDF support for DH

Stephan,

On Fri, 15 Jul 2016, Stephan Mueller wrote:

Am Donnerstag, 14. Juli 2016, 17:45:59 schrieb Mat Martineau:

Hi Mat,

quoted

quoted

Signed-off-by: Stephan Mueller <redacted>
---
include/uapi/linux/keyctl.h | 10 +++++
security/keys/Kconfig       |  1 +
security/keys/dh.c          | 98
++++++++++++++++++++++++++++++++++++++++----- security/keys/internal.h
|  5 ++-
security/keys/keyctl.c      |  2 +-
5 files changed, 103 insertions(+), 13 deletions(-)

Be sure to update Documentation/security/keys.txt once the interface is
settled on.

Thanks for the reminder

quoted

quoted

diff --git a/include/uapi/linux/keyctl.h b/include/uapi/linux/keyctl.h
index 86eddd6..cc4ce7c 100644
--- a/include/uapi/linux/keyctl.h
+++ b/include/uapi/linux/keyctl.h

@@ -68,4 +68,14 @@ struct keyctl_dh_params {

	__s32 base;

};

+struct keyctl_kdf_params {
+#define KEYCTL_KDF_MAX_OUTPUTLEN	1024	/* max length of KDF output */
+#define KEYCTL_KDF_MAX_STRING_LEN	64	/* maximum length of strings

*/

quoted

I think these limits should be in the internal headers rather than uapi.

Ok

quoted

quoted

+	char *kdfname;
+	__u32 kdfnamelen;

As noted in the userspace patch, if kdfname is a null-terminated string
then kdfnamelen isn't needed.

Ok

quoted

quoted

+	char *otherinfo;
+	__u32 otherinfolen;
+	__u32 flags;

Looks like flags aren't used anywhere. Do you have a use planned? You
could add some spare capacity like the keyctl_pkey_* structs instead (see
https://git.kernel.org/cgit/linux/kernel/git/dhowells/linux-fs.git/commit/?h
=keys-next&id=dd7e2ea418b4f8851a4eb976b9431fbc03d2ebaf )

I am not sure what to do here: I see the profileration of new syscalls which
just differ from existing syscalls by a new flags field because the initial
implementation simply missed such thing.

I want to avoid something like this happening here.

I am open for any suggestions.

David's approach in the structs I referenced is to add a spare array of 
__u32's:

         __u32 __spare[8];

That way future additions can be added to the space currently used by 
__spare, and that array is made smaller so the overall struct size stays 
the same and the older members are not moved around.

quoted

quoted

+};
+
#endif /*  _LINUX_KEYCTL_H */

diff --git a/security/keys/Kconfig b/security/keys/Kconfig
index f826e87..56491fe 100644
--- a/security/keys/Kconfig
+++ b/security/keys/Kconfig

@@ -90,6 +90,7 @@ config KEY_DH_OPERATIONS

       bool "Diffie-Hellman operations on retained keys"
       depends on KEYS
       select MPILIB

+       select CRYPTO_KDF

       help

	 This option provides support for calculating Diffie-Hellman
	 public keys and shared secrets using values stored as keys

diff --git a/security/keys/dh.c b/security/keys/dh.c
index 531ed2e..4c93969 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c

@@ -77,14 +77,74 @@ error:

	return ret;

}

+static int keyctl_dh_compute_kdf(struct keyctl_kdf_params *kdfcopy,
+				 char __user *buffer, size_t buflen,
+				 uint8_t *kbuf, size_t resultlen)
+{

Minor point: this function name made me think it was a replacement for
keyctl_dh_compute at first (like the userspace counterpart).

Well, initially I had it part of dh_compute, but then extracted it to make the
code nicer and less distracting.

Extracting it is good - my comment was only regarding the name.

quoted

quoted

+	char kdfname[CRYPTO_MAX_ALG_NAME] = { 0 };
+	struct crypto_rng *tfm;
+	uint8_t *outbuf = NULL;
+	int ret;
+
+	BUILD_BUG_ON(CRYPTO_MAX_ALG_NAME != KEYCTL_KDF_MAX_STRING_LEN);

If this is a requirement, why define KEYCTL_KDF_MAX_STRING_LEN? Use
CRYPTO_MAX_ALG_NAME directly.

Ok, I was not sure if I am allowed to add a crypto API header to key header
files.

I don't think you need to include the crypto header in any key headers, 
just here in dh.c. dh.c will be converted to use the DH implementation 
recently added to the crypto subsystem, by the way.

quoted

quoted

+	if (!kdfcopy->kdfnamelen)
+		return -EFAULT;
+	if (copy_from_user(&kdfname, kdfcopy->kdfname,
+			   kdfcopy->kdfnamelen) != 0)

strndup_user works nicely for strings.

yes.

quoted

quoted

+		return -EFAULT;
+

It would be best to validate all of the userspace input before the DH
computation is done.

Uh, that means I cannot have a separate dh_compute_kdf function. But, ok, no
problem.

quoted

quoted

+	/*
+	 * Concatenate otherinfo past DH shared secret -- the
+	 * input to the KDF is (DH shared secret || otherinfo)
+	 */
+	if (kdfcopy->otherinfo &&
+	    copy_from_user(kbuf + resultlen, kdfcopy->otherinfo,
+			   kdfcopy->otherinfolen)
+	    != 0)
+		return -EFAULT;
+
+	tfm = crypto_alloc_rng(kdfname, 0, 0);
+	if (IS_ERR(tfm))
+		return PTR_ERR(tfm);
+
+#if 0
+	/* we do not support HMAC currently */
+	ret = crypto_rng_reset(tfm, xx, xxlen);
+	if (ret) {
+		crypto_free_rng(tfm);
+		goto error5;
+	}
+#endif
+
+	outbuf = kmalloc(buflen, GFP_KERNEL);
+	if (!outbuf) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	ret = crypto_rng_generate(tfm, kbuf, resultlen + kdfcopy-

otherinfolen,

quoted

+				  outbuf, buflen);
+	if (ret)
+		goto err;
+
+	ret = buflen;
+	if (copy_to_user(buffer, outbuf, buflen) != 0)
+		ret = -EFAULT;
+
+err:
+	kzfree(outbuf);
+	crypto_free_rng(tfm);
+	return ret;
+}
long keyctl_dh_compute(struct keyctl_dh_params __user *params,

		       char __user *buffer, size_t buflen,

-		       void __user *reserved)
+		       struct keyctl_kdf_params __user *kdf)
{

	long ret;
	MPI base, private, prime, result;
	unsigned nbytes;
	struct keyctl_dh_params pcopy;

+	struct keyctl_kdf_params kdfcopy;

	uint8_t *kbuf;
	ssize_t keylen;
	size_t resultlen;

@@ -98,12 +158,24 @@ long keyctl_dh_compute(struct keyctl_dh_params __user

*params,>
		goto out;

	}

-	if (reserved) {
-		ret = -EINVAL;
-		goto out;
+	if (kdf) {
+		if (copy_from_user(&kdfcopy, kdf, sizeof(kdfcopy)) != 0) {
+			ret = -EFAULT;
+			goto out;
+		}
+		if (buflen > KEYCTL_KDF_MAX_OUTPUTLEN ||
+		    kdfcopy.otherinfolen > KEYCTL_KDF_MAX_STRING_LEN ||
+		    kdfcopy.kdfnamelen > KEYCTL_KDF_MAX_STRING_LEN) {
+			ret = -EMSGSIZE;
+			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, kdf ? 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.

@@ -133,7 +205,8 @@ long keyctl_dh_compute(struct keyctl_dh_params __user

*params,>
		goto error3;

	}

-	kbuf = kmalloc(resultlen, GFP_KERNEL);
+	kbuf = kmalloc(kdf ? (resultlen + kdfcopy.otherinfolen) : resultlen,
+		       GFP_KERNEL);

	if (!kbuf) {

		ret = -ENOMEM;
		goto error4;

@@ -147,12 +220,17 @@ 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 (kdf) {
+		ret = keyctl_dh_compute_kdf(&kdfcopy, buffer, buflen,
+					    kbuf, resultlen);
+	} else {
+		ret = nbytes;
+		if (copy_to_user(buffer, kbuf, nbytes) != 0)
+			ret = -EFAULT;
+	}

error5:
-	kfree(kbuf);
+	kzfree(kbuf);

Thanks for adjusting this.

I hope it is ok to not have it in a separate patch.

quoted

quoted

error4:
	mpi_free(result);

error3:

diff --git a/security/keys/internal.h b/security/keys/internal.h
index a705a7d..35a8d11 100644
--- a/security/keys/internal.h
+++ b/security/keys/internal.h

@@ -259,12 +259,13 @@ static inline long keyctl_get_persistent(uid_t uid,

key_serial_t destring) #endif

#ifdef CONFIG_KEY_DH_OPERATIONS
+#include <crypto/rng.h>
extern long keyctl_dh_compute(struct keyctl_dh_params __user *, char
__user *, -			      size_t, void __user *);
+			      size_t, struct keyctl_kdf_params __user *);
#else
static inline long keyctl_dh_compute(struct keyctl_dh_params __user
*params,>
				     char __user *buffer, size_t buflen,

-				     void __user *reserved)
+				     struct keyctl_kdf_params __user *kdf)
{

	return -EOPNOTSUPP;

}

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index d580ad0..b106898 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c

@@ -1689,7 +1689,7 @@ SYSCALL_DEFINE5(keyctl, int, option, unsigned long,

arg2, unsigned long, arg3,>
	case KEYCTL_DH_COMPUTE:
		return keyctl_dh_compute((struct keyctl_dh_params __user *) arg2,

					 (char __user *) arg3, (size_t) arg4,

-					 (void __user *) arg5);
+					 (struct keyctl_kdf_params __user *) arg5);

	default:
		return -EOPNOTSUPP;

--
Mat Martineau
Intel OTC