KEYS: Add KEYCTL_DH_COMPUTE command

This adds userspace access to Diffie-Hellman computations through a
new keyctl() syscall command to calculate shared secrets or public
keys using input parameters stored in the keyring.

Input key ids are provided in a struct due to the current 5-arg limit
for the keyctl syscall. Only user keys are supported in order to avoid
exposing the content of logon or encrypted keys.

The output is written to the provided buffer, based on the assumption
that the values are only needed in userspace.

Future support for other types of key derivation would involve a new
command, like KEYCTL_ECDH_COMPUTE.

Once Diffie-Hellman support is included in the crypto API, this code
can be converted to use the crypto API to take advantage of possible
hardware acceleration and reduce redundant code.

Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 160 insertions, 0 deletions

diff --git a/security/keys/dh.c b/security/keys/dh.c
new file mode 100644
index 000000000000..880505a4b9f1
--- /dev/null
+++ b/security/keys/dh.c
@@ -0,0 +1,160 @@
+/* Crypto operations using stored keys
+ *
+ * Copyright (c) 2016, Intel Corporation
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+
+#include <linux/mpi.h>
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+#include <keys/user-type.h>
+#include "internal.h"
+
+/*
+ * Public key or shared secret generation function [RFC2631 sec 2.1.1]
+ *
+ * ya = g^xa mod p;
+ * or
+ * ZZ = yb^xa mod p;
+ *
+ * where xa is the local private key, ya is the local public key, g is
+ * the generator, p is the prime, yb is the remote public key, and ZZ
+ * is the shared secret.
+ *
+ * Both are the same calculation, so g or yb are the "base" and ya or
+ * ZZ are the "result".
+ */
+static int do_dh(MPI result, MPI base, MPI xa, MPI p)
+{
+	return mpi_powm(result, base, xa, p);
+}
+
+static ssize_t mpi_from_key(key_serial_t keyid, size_t maxlen, MPI *mpi)
+{
+	struct key *key;
+	key_ref_t key_ref;
+	long status;
+	ssize_t ret;
+
+	key_ref = lookup_user_key(keyid, 0, KEY_NEED_READ);
+	if (IS_ERR(key_ref)) {
+		ret = -ENOKEY;
+		goto error;
+	}
+
+	key = key_ref_to_ptr(key_ref);
+
+	ret = -EOPNOTSUPP;
+	if (key->type == &key_type_user) {
+		down_read(&key->sem);
+		status = key_validate(key);
+		if (status == 0) {
+			const struct user_key_payload *payload;
+
+			payload = user_key_payload(key);
+
+			if (maxlen == 0) {
+				*mpi = NULL;
+				ret = payload->datalen;
+			} else if (payload->datalen <= maxlen) {
+				*mpi = mpi_read_raw_data(payload->data,
+							 payload->datalen);
+				if (*mpi)
+					ret = payload->datalen;
+			} else {
+				ret = -EINVAL;
+			}
+		}
+		up_read(&key->sem);
+	}
+
+	key_put(key);
+error:
+	return ret;
+}
+
+long keyctl_dh_compute(struct keyctl_dh_params __user *params,
+		       char __user *buffer, size_t buflen)
+{
+	long ret;
+	MPI base, private, prime, result;
+	unsigned nbytes;
+	struct keyctl_dh_params pcopy;
+	uint8_t *kbuf;
+	ssize_t keylen;
+	size_t resultlen;
+
+	if (!params || (!buffer && buflen)) {
+		ret = -EINVAL;
+		goto out;
+	}
+	if (copy_from_user(&pcopy, params, sizeof(pcopy)) != 0) {
+		ret = -EFAULT;
+		goto out;
+	}
+
+	keylen = mpi_from_key(pcopy.prime, buflen, &prime);
+	if (keylen < 0 || !prime) {
+		/* buflen == 0 may be used to query the required buffer size,
+		 * which is the prime key length.
+		 */
+		ret = keylen;
+		goto out;
+	}
+
+	/* The result is never longer than the prime */
+	resultlen = keylen;
+
+	keylen = mpi_from_key(pcopy.base, SIZE_MAX, &base);
+	if (keylen < 0 || !base) {
+		ret = keylen;
+		goto error1;
+	}
+
+	keylen = mpi_from_key(pcopy.private, SIZE_MAX, &private);
+	if (keylen < 0 || !private) {
+		ret = keylen;
+		goto error2;
+	}
+
+	result = mpi_alloc(0);
+	if (!result) {
+		ret = -ENOMEM;
+		goto error3;
+	}
+
+	kbuf = kmalloc(resultlen, GFP_KERNEL);
+	if (!kbuf) {
+		ret = -ENOMEM;
+		goto error4;
+	}
+
+	ret = do_dh(result, base, private, prime);
+	if (ret)
+		goto error5;
+
+	ret = mpi_read_buffer(result, kbuf, resultlen, &nbytes, NULL);
+	if (ret != 0)
+		goto error5;
+
+	ret = nbytes;
+	if (copy_to_user(buffer, kbuf, nbytes) != 0)
+		ret = -EFAULT;
+
+error5:
+	kfree(kbuf);
+error4:
+	mpi_free(result);
+error3:
+	mpi_free(private);
+error2:
+	mpi_free(base);
+error1:
+	mpi_free(prime);
+out:
+	return ret;
+}