crypto: algif_rng - add random number generator support

This patch adds the random number generator support for AF_ALG.

A random number generator's purpose is to generate data without
requiring the caller to provide any data. Therefore, the AF_ALG
interface handler for RNGs only implements a callback handler for
recvmsg.

The following parameters provided with a recvmsg are processed by the
RNG callback handler:

	* sock - to resolve the RNG context data structure accessing the
	  RNG instance private to the socket

	* len - this parameter allows userspace callers to specify how
	  many random bytes the RNG shall produce and return. As the
	  kernel context for the RNG allocates a buffer of 128 bytes to
	  store random numbers before copying them to userspace, the len
	  parameter is checked that it is not larger than 128. If a
	  caller wants more random numbers, a new request for recvmsg
	  shall be made.

The size of 128 bytes is chose because of the following considerations:

	* to increase the memory footprint of the kernel too much (note,
	  that would be 128 bytes per open socket)

	* 128 is divisible by any typical cryptographic block size an
	  RNG may have

	* A request for random numbers typically only shall supply small
	  amount of data like for keys or IVs that should only require
	  one invocation of the recvmsg function.

Note, during instantiation of the RNG, the code checks whether the RNG
implementation requires seeding. If so, the RNG is seeded with output
from get_random_bytes.

A fully working example using all aspects of the RNG interface is
provided at http://www.chronox.de/libkcapi.htmlSigned-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/algif_rng.c

+/*
+ * algif_rng: User-space interface for random number generators
+ *
+ * This file provides the user-space API for random number generators.
+ *
+ * Copyright (C) 2014, Stephan Mueller <smueller@chronox.de>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, and the entire permission notice in its entirety,
+ *    including the disclaimer of warranties.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote
+ *    products derived from this software without specific prior
+ *    written permission.
+ *
+ * ALTERNATIVELY, this product may be distributed under the terms of
+ * the GNU General Public License, in which case the provisions of the GPL2
+ * are required INSTEAD OF the above restrictions.  (This clause is
+ * necessary due to a potential bad interaction between the GPL and
+ * the restrictions contained in a BSD-style copyright.)
+ *
+ * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF
+ * WHICH ARE HEREBY DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
+ * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
+ * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
+ * USE OF THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH
+ * DAMAGE.
+ */
+
+#include <linux/module.h>
+#include <crypto/rng.h>
+#include <linux/random.h>
+#include <crypto/if_alg.h>
+#include <linux/net.h>
+#include <net/sock.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Stephan Mueller <smueller@chronox.de>");
+MODULE_DESCRIPTION("User-space interface for random number generators");
+
+struct rng_ctx {
+#define MAXSIZE 128
+	unsigned int len;
+	struct crypto_rng *drng;
+};
+
+static int rng_recvmsg(struct kiocb *unused, struct socket *sock,
+		       struct msghdr *msg, size_t len, int flags)
+{
+	struct sock *sk = sock->sk;
+	struct alg_sock *ask = alg_sk(sk);
+	struct rng_ctx *ctx = ask->private;
+	int err = -EFAULT;
+	int genlen = 0;
+	u8 result[MAXSIZE];
+
+	if (len == 0)
+		return 0;
+	if (len > MAXSIZE)
+		len = MAXSIZE;
+
+	/*
+	 * although not strictly needed, this is a precaution against coding
+	 * errors
+	 */
+	memset(result, 0, len);
+
+	/*
+	 * The enforcement of a proper seeding of an RNG is done within an
+	 * RNG implementation. Some RNGs (DRBG, krng) do not need specific
+	 * seeding as they automatically seed. The X9.31 DRNG will return
+	 * an error if it was not seeded properly.
+	 */
+	genlen = crypto_rng_get_bytes(ctx->drng, result, len);
+	if (genlen < 0)
+		return genlen;
+
+	err = memcpy_to_msg(msg, result, len);
+	memzero_explicit(result, genlen);
+
+	return err ? err : len;
+}
+
+static struct proto_ops algif_rng_ops = {
+	.family		=	PF_ALG,
+
+	.connect	=	sock_no_connect,
+	.socketpair	=	sock_no_socketpair,
+	.getname	=	sock_no_getname,
+	.ioctl		=	sock_no_ioctl,
+	.listen		=	sock_no_listen,
+	.shutdown	=	sock_no_shutdown,
+	.getsockopt	=	sock_no_getsockopt,
+	.mmap		=	sock_no_mmap,
+	.bind		=	sock_no_bind,
+	.accept		=	sock_no_accept,
+	.setsockopt	=	sock_no_setsockopt,
+	.poll		=	sock_no_poll,
+	.sendmsg	=	sock_no_sendmsg,
+	.sendpage	=	sock_no_sendpage,
+
+	.release	=	af_alg_release,
+	.recvmsg	=	rng_recvmsg,
+};
+
+static void *rng_bind(const char *name, u32 type, u32 mask)
+{
+	return crypto_alloc_rng(name, type, mask);
+}
+
+static void rng_release(void *private)
+{
+	crypto_free_rng(private);
+}
+
+static void rng_sock_destruct(struct sock *sk)
+{
+	struct alg_sock *ask = alg_sk(sk);
+	struct rng_ctx *ctx = ask->private;
+
+	sock_kfree_s(sk, ctx, ctx->len);
+	af_alg_release_parent(sk);
+}
+
+static int rng_accept_parent(void *private, struct sock *sk)
+{
+	struct rng_ctx *ctx;
+	struct alg_sock *ask = alg_sk(sk);
+	unsigned int len = sizeof(*ctx);
+
+	ctx = sock_kmalloc(sk, len, GFP_KERNEL);
+	if (!ctx)
+		return -ENOMEM;
+
+	ctx->len = len;
+
+	/*
+	 * No seeding done at that point -- if multiple accepts are
+	 * done on one RNG instance, each resulting FD points to the same
+	 * state of the RNG.
+	 */
+
+	ctx->drng = private;
+	ask->private = ctx;
+	sk->sk_destruct = rng_sock_destruct;
+
+	return 0;
+}
+
+static int rng_setkey(void *private, const u8 *seed, unsigned int seedlen)
+{
+	/*
+	 * Check whether seedlen is of sufficient size is done in RNG
+	 * implementations.
+	 */
+	return crypto_rng_reset(private, (u8 *)seed, seedlen);
+}
+
+static const struct af_alg_type algif_type_rng = {
+	.bind		=	rng_bind,
+	.release	=	rng_release,
+	.accept		=	rng_accept_parent,
+	.setkey		=	rng_setkey,
+	.ops		=	&algif_rng_ops,
+	.name		=	"rng",
+	.owner		=	THIS_MODULE
+};
+
+static int __init rng_init(void)
+{
+	return af_alg_register_type(&algif_type_rng);
+}
+
+void __exit rng_exit(void)
+{
+	int err = af_alg_unregister_type(&algif_type_rng);
+	BUG_ON(err);
+}
+
+module_init(rng_init);
+module_exit(rng_exit);