[SCTP]: Implement SCTP-AUTH internals

This patch implements the internals operations of the AUTH, such as
key computation and storage.  It also adds necessary variables to
the SCTP data structures.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 745 insertions, 0 deletions

diff --git a/net/sctp/auth.c b/net/sctp/auth.c
new file mode 100644
index 000000000000..2a29409a38d9
--- /dev/null
+++ b/net/sctp/auth.c
@@ -0,0 +1,745 @@
+/* SCTP kernel reference Implementation
+ * (C) Copyright 2007 Hewlett-Packard Development Company, L.P.
+ *
+ * This file is part of the SCTP kernel reference Implementation
+ *
+ * The SCTP reference implementation 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, or (at your option)
+ * any later version.
+ *
+ * The SCTP reference implementation is distributed in the hope that it
+ * will be useful, but WITHOUT ANY WARRANTY; without even the implied
+ *                 ************************
+ * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with GNU CC; see the file COPYING.  If not, write to
+ * the Free Software Foundation, 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Please send any bug reports or fixes you make to the
+ * email address(es):
+ *    lksctp developers <lksctp-developers@lists.sourceforge.net>
+ *
+ * Or submit a bug report through the following website:
+ *    http://www.sf.net/projects/lksctp
+ *
+ * Written or modified by:
+ *   Vlad Yasevich     <vladislav.yasevich@hp.com>
+ *
+ * Any bugs reported given to us we will try to fix... any fixes shared will
+ * be incorporated into the next SCTP release.
+ */
+
+#include <linux/types.h>
+#include <linux/crypto.h>
+#include <linux/scatterlist.h>
+#include <net/sctp/sctp.h>
+#include <net/sctp/auth.h>
+
+static struct sctp_hmac sctp_hmac_list[SCTP_AUTH_NUM_HMACS] = {
+	{
+		/* id 0 is reserved.  as all 0 */
+		.hmac_id = SCTP_AUTH_HMAC_ID_RESERVED_0,
+	},
+	{
+		.hmac_id = SCTP_AUTH_HMAC_ID_SHA1,
+		.hmac_name="hmac(sha1)",
+		.hmac_len = SCTP_SHA1_SIG_SIZE,
+	},
+	{
+		/* id 2 is reserved as well */
+		.hmac_id = SCTP_AUTH_HMAC_ID_RESERVED_2,
+	},
+	{
+		.hmac_id = SCTP_AUTH_HMAC_ID_SHA256,
+		.hmac_name="hmac(sha256)",
+		.hmac_len = SCTP_SHA256_SIG_SIZE,
+	}
+};
+
+
+void sctp_auth_key_put(struct sctp_auth_bytes *key)
+{
+	if (!key)
+		return;
+
+	if (atomic_dec_and_test(&key->refcnt)) {
+		kfree(key);
+		SCTP_DBG_OBJCNT_DEC(keys);
+	}
+}
+
+/* Create a new key structure of a given length */
+static struct sctp_auth_bytes *sctp_auth_create_key(__u32 key_len, gfp_t gfp)
+{
+	struct sctp_auth_bytes *key;
+
+	/* Allocate the shared key */
+	key = kmalloc(sizeof(struct sctp_auth_bytes) + key_len, gfp);
+	if (!key)
+		return NULL;
+
+	key->len = key_len;
+	atomic_set(&key->refcnt, 1);
+	SCTP_DBG_OBJCNT_INC(keys);
+
+	return key;
+}
+
+/* Create a new shared key container with a give key id */
+struct sctp_shared_key *sctp_auth_shkey_create(__u16 key_id, gfp_t gfp)
+{
+	struct sctp_shared_key *new;
+
+	/* Allocate the shared key container */
+	new = kzalloc(sizeof(struct sctp_shared_key), gfp);
+	if (!new)
+		return NULL;
+
+	INIT_LIST_HEAD(&new->key_list);
+	new->key_id = key_id;
+
+	return new;
+}
+
+/* Free the shared key stucture */
+void sctp_auth_shkey_free(struct sctp_shared_key *sh_key)
+{
+	BUG_ON(!list_empty(&sh_key->key_list));
+	sctp_auth_key_put(sh_key->key);
+	sh_key->key = NULL;
+	kfree(sh_key);
+}
+
+/* Destory the entire key list.  This is done during the
+ * associon and endpoint free process.
+ */
+void sctp_auth_destroy_keys(struct list_head *keys)
+{
+	struct sctp_shared_key *ep_key;
+	struct sctp_shared_key *tmp;
+
+	if (list_empty(keys))
+		return;
+
+	key_for_each_safe(ep_key, tmp, keys) {
+		list_del_init(&ep_key->key_list);
+		sctp_auth_shkey_free(ep_key);
+	}
+}
+
+/* Compare two byte vectors as numbers.  Return values
+ * are:
+ * 	  0 - vectors are equal
+ * 	< 0 - vector 1 is smaller then vector2
+ * 	> 0 - vector 1 is greater then vector2
+ *
+ * Algorithm is:
+ * 	This is performed by selecting the numerically smaller key vector...
+ *	If the key vectors are equal as numbers but differ in length ...
+ *	the shorter vector is considered smaller
+ *
+ * Examples (with small values):
+ * 	000123456789 > 123456789 (first number is longer)
+ * 	000123456789 < 234567891 (second number is larger numerically)
+ * 	123456789 > 2345678 	 (first number is both larger & longer)
+ */
+static int sctp_auth_compare_vectors(struct sctp_auth_bytes *vector1,
+			      struct sctp_auth_bytes *vector2)
+{
+	int diff;
+	int i;
+	const __u8 *longer;
+
+	diff = vector1->len - vector2->len;
+	if (diff) {
+		longer = (diff > 0) ? vector1->data : vector2->data;
+
+		/* Check to see if the longer number is
+		 * lead-zero padded.  If it is not, it
+		 * is automatically larger numerically.
+		 */
+		for (i = 0; i < abs(diff); i++ ) {
+			if (longer[i] != 0)
+				return diff;
+		}
+	}
+
+	/* lengths are the same, compare numbers */
+	return memcmp(vector1->data, vector2->data, vector1->len);
+}
+
+/*
+ * Create a key vector as described in SCTP-AUTH, Section 6.1
+ *    The RANDOM parameter, the CHUNKS parameter and the HMAC-ALGO
+ *    parameter sent by each endpoint are concatenated as byte vectors.
+ *    These parameters include the parameter type, parameter length, and
+ *    the parameter value, but padding is omitted; all padding MUST be
+ *    removed from this concatenation before proceeding with further
+ *    computation of keys.  Parameters which were not sent are simply
+ *    omitted from the concatenation process.  The resulting two vectors
+ *    are called the two key vectors.
+ */
+static struct sctp_auth_bytes *sctp_auth_make_key_vector(
+			sctp_random_param_t *random,
+			sctp_chunks_param_t *chunks,
+			sctp_hmac_algo_param_t *hmacs,
+			gfp_t gfp)
+{
+	struct sctp_auth_bytes *new;
+	__u32	len;
+	__u32	offset = 0;
+
+	len = ntohs(random->param_hdr.length) + ntohs(hmacs->param_hdr.length);
+        if (chunks)
+		len += ntohs(chunks->param_hdr.length);
+
+	new = kmalloc(sizeof(struct sctp_auth_bytes) + len, gfp);
+	if (!new)
+		return NULL;
+
+	new->len = len;
+
+	memcpy(new->data, random, ntohs(random->param_hdr.length));
+	offset += ntohs(random->param_hdr.length);
+
+	if (chunks) {
+		memcpy(new->data + offset, chunks,
+			ntohs(chunks->param_hdr.length));
+		offset += ntohs(chunks->param_hdr.length);
+	}
+
+	memcpy(new->data + offset, hmacs, ntohs(hmacs->param_hdr.length));
+
+	return new;
+}
+
+
+/* Make a key vector based on our local parameters */
+struct sctp_auth_bytes *sctp_auth_make_local_vector(
+				    const struct sctp_association *asoc,
+				    gfp_t gfp)
+{
+	return sctp_auth_make_key_vector(
+				    (sctp_random_param_t*)asoc->c.auth_random,
+				    (sctp_chunks_param_t*)asoc->c.auth_chunks,
+				    (sctp_hmac_algo_param_t*)asoc->c.auth_hmacs,
+				    gfp);
+}
+
+/* Make a key vector based on peer's parameters */
+struct sctp_auth_bytes *sctp_auth_make_peer_vector(
+				    const struct sctp_association *asoc,
+				    gfp_t gfp)
+{
+	return sctp_auth_make_key_vector(asoc->peer.peer_random,
+					 asoc->peer.peer_chunks,
+					 asoc->peer.peer_hmacs,
+					 gfp);
+}
+
+
+/* Set the value of the association shared key base on the parameters
+ * given.  The algorithm is:
+ *    From the endpoint pair shared keys and the key vectors the
+ *    association shared keys are computed.  This is performed by selecting
+ *    the numerically smaller key vector and concatenating it to the
+ *    endpoint pair shared key, and then concatenating the numerically
+ *    larger key vector to that.  The result of the concatenation is the
+ *    association shared key.
+ */
+static struct sctp_auth_bytes *sctp_auth_asoc_set_secret(
+			struct sctp_shared_key *ep_key,
+			struct sctp_auth_bytes *first_vector,
+			struct sctp_auth_bytes *last_vector,
+			gfp_t gfp)
+{
+	struct sctp_auth_bytes *secret;
+	__u32 offset = 0;
+	__u32 auth_len;
+
+	auth_len = first_vector->len + last_vector->len;
+	if (ep_key->key)
+		auth_len += ep_key->key->len;
+
+	secret = sctp_auth_create_key(auth_len, gfp);
+	if (!secret)
+		return NULL;
+
+	if (ep_key->key) {
+		memcpy(secret->data, ep_key->key->data, ep_key->key->len);
+		offset += ep_key->key->len;
+	}
+
+	memcpy(secret->data + offset, first_vector->data, first_vector->len);
+	offset += first_vector->len;
+
+	memcpy(secret->data + offset, last_vector->data, last_vector->len);
+
+	return secret;
+}
+
+/* Create an association shared key.  Follow the algorithm
+ * described in SCTP-AUTH, Section 6.1
+ */
+static struct sctp_auth_bytes *sctp_auth_asoc_create_secret(
+				 const struct sctp_association *asoc,
+				 struct sctp_shared_key *ep_key,
+				 gfp_t gfp)
+{
+	struct sctp_auth_bytes *local_key_vector;
+	struct sctp_auth_bytes *peer_key_vector;
+	struct sctp_auth_bytes	*first_vector,
+				*last_vector;
+	struct sctp_auth_bytes	*secret = NULL;
+	int	cmp;
+
+
+	/* Now we need to build the key vectors
+	 * SCTP-AUTH , Section 6.1
+	 *    The RANDOM parameter, the CHUNKS parameter and the HMAC-ALGO
+	 *    parameter sent by each endpoint are concatenated as byte vectors.
+	 *    These parameters include the parameter type, parameter length, and
+	 *    the parameter value, but padding is omitted; all padding MUST be
+	 *    removed from this concatenation before proceeding with further
+	 *    computation of keys.  Parameters which were not sent are simply
+	 *    omitted from the concatenation process.  The resulting two vectors
+	 *    are called the two key vectors.
+	 */
+
+	local_key_vector = sctp_auth_make_local_vector(asoc, gfp);
+	peer_key_vector = sctp_auth_make_peer_vector(asoc, gfp);
+
+	if (!peer_key_vector || !local_key_vector)
+		goto out;
+
+	/* Figure out the order in wich the key_vectors will be
+	 * added to the endpoint shared key.
+	 * SCTP-AUTH, Section 6.1:
+	 *   This is performed by selecting the numerically smaller key
+	 *   vector and concatenating it to the endpoint pair shared
+	 *   key, and then concatenating the numerically larger key
+	 *   vector to that.  If the key vectors are equal as numbers
+	 *   but differ in length, then the concatenation order is the
+	 *   endpoint shared key, followed by the shorter key vector,
+	 *   followed by the longer key vector.  Otherwise, the key
+	 *   vectors are identical, and may be concatenated to the
+	 *   endpoint pair key in any order.
+	 */
+	cmp = sctp_auth_compare_vectors(local_key_vector,
+					peer_key_vector);
+	if (cmp < 0) {
+		first_vector = local_key_vector;
+		last_vector = peer_key_vector;
+	} else {
+		first_vector = peer_key_vector;
+		last_vector = local_key_vector;
+	}
+
+	secret = sctp_auth_asoc_set_secret(ep_key, first_vector, last_vector,
+					    gfp);
+out:
+	kfree(local_key_vector);
+	kfree(peer_key_vector);
+
+	return secret;
+}
+
+/*
+ * Populate the association overlay list with the list
+ * from the endpoint.
+ */
+int sctp_auth_asoc_copy_shkeys(const struct sctp_endpoint *ep,
+				struct sctp_association *asoc,
+				gfp_t gfp)
+{
+	struct sctp_shared_key *sh_key;
+	struct sctp_shared_key *new;
+
+	BUG_ON(!list_empty(&asoc->endpoint_shared_keys));
+
+	key_for_each(sh_key, &ep->endpoint_shared_keys) {
+		new = sctp_auth_shkey_create(sh_key->key_id, gfp);
+		if (!new)
+			goto nomem;
+
+		new->key = sh_key->key;
+		sctp_auth_key_hold(new->key);
+		list_add(&new->key_list, &asoc->endpoint_shared_keys);
+	}
+
+	return 0;
+
+nomem:
+	sctp_auth_destroy_keys(&asoc->endpoint_shared_keys);
+	return -ENOMEM;
+}
+
+
+/* Public interface to creat the association shared key.
+ * See code above for the algorithm.
+ */
+int sctp_auth_asoc_init_active_key(struct sctp_association *asoc, gfp_t gfp)
+{
+	struct sctp_auth_bytes	*secret;
+	struct sctp_shared_key *ep_key;
+
+	/* If we don't support AUTH, or peer is not capable
+	 * we don't need to do anything.
+	 */
+	if (!sctp_auth_enable || !asoc->peer.auth_capable)
+		return 0;
+
+	/* If the key_id is non-zero and we couldn't find an
+	 * endpoint pair shared key, we can't compute the
+	 * secret.
+	 * For key_id 0, endpoint pair shared key is a NULL key.
+	 */
+	ep_key = sctp_auth_get_shkey(asoc, asoc->active_key_id);
+	BUG_ON(!ep_key);
+
+	secret = sctp_auth_asoc_create_secret(asoc, ep_key, gfp);
+	if (!secret)
+		return -ENOMEM;
+
+	sctp_auth_key_put(asoc->asoc_shared_key);
+	asoc->asoc_shared_key = secret;
+
+	return 0;
+}
+
+
+/* Find the endpoint pair shared key based on the key_id */
+struct sctp_shared_key *sctp_auth_get_shkey(
+				const struct sctp_association *asoc,
+				__u16 key_id)
+{
+	struct sctp_shared_key *key = NULL;
+
+	/* First search associations set of endpoint pair shared keys */
+	key_for_each(key, &asoc->endpoint_shared_keys) {
+		if (key->key_id == key_id)
+			break;
+	}
+
+	return key;
+}
+
+/*
+ * Initialize all the possible digest transforms that we can use.  Right now
+ * now, the supported digests are SHA1 and SHA256.  We do this here once
+ * because of the restrictiong that transforms may only be allocated in
+ * user context.  This forces us to pre-allocated all possible transforms
+ * at the endpoint init time.
+ */
+int sctp_auth_init_hmacs(struct sctp_endpoint *ep, gfp_t gfp)
+{
+	struct crypto_hash *tfm = NULL;
+	__u16   id;
+
+	/* if the transforms are already allocted, we are done */
+	if (!sctp_auth_enable) {
+		ep->auth_hmacs = NULL;
+		return 0;
+	}
+
+	if (ep->auth_hmacs)
+		return 0;
+
+	/* Allocated the array of pointers to transorms */
+	ep->auth_hmacs = kzalloc(
+			    sizeof(struct crypto_hash *) * SCTP_AUTH_NUM_HMACS,
+			    gfp);
+	if (!ep->auth_hmacs)
+		return -ENOMEM;
+
+	for (id = 0; id < SCTP_AUTH_NUM_HMACS; id++) {
+
+		/* See is we support the id.  Supported IDs have name and
+		 * length fields set, so that we can allocated and use
+		 * them.  We can safely just check for name, for without the
+		 * name, we can't allocate the TFM.
+		 */
+		if (!sctp_hmac_list[id].hmac_name)
+			continue;
+
+		/* If this TFM has been allocated, we are all set */
+		if (ep->auth_hmacs[id])
+			continue;
+
+		/* Allocate the ID */
+		tfm = crypto_alloc_hash(sctp_hmac_list[id].hmac_name, 0,
+					CRYPTO_ALG_ASYNC);
+		if (IS_ERR(tfm))
+			goto out_err;
+
+		ep->auth_hmacs[id] = tfm;
+	}
+
+	return 0;
+
+out_err:
+	/* Clean up any successfull allocations */
+	sctp_auth_destroy_hmacs(ep->auth_hmacs);
+	return -ENOMEM;
+}
+
+/* Destroy the hmac tfm array */
+void sctp_auth_destroy_hmacs(struct crypto_hash *auth_hmacs[])
+{
+	int i;
+
+	if (!auth_hmacs)
+		return;
+
+	for (i = 0; i < SCTP_AUTH_NUM_HMACS; i++)
+	{
+		if (auth_hmacs[i])
+			crypto_free_hash(auth_hmacs[i]);
+	}
+	kfree(auth_hmacs);
+}
+
+
+struct sctp_hmac *sctp_auth_get_hmac(__u16 hmac_id)
+{
+	return &sctp_hmac_list[hmac_id];
+}
+
+/* Get an hmac description information that we can use to build
+ * the AUTH chunk
+ */
+struct sctp_hmac *sctp_auth_asoc_get_hmac(const struct sctp_association *asoc)
+{
+	struct sctp_hmac_algo_param *hmacs;
+	__u16 n_elt;
+	__u16 id = 0;
+	int i;
+
+	/* If we have a default entry, use it */
+	if (asoc->default_hmac_id)
+		return &sctp_hmac_list[asoc->default_hmac_id];
+
+	/* Since we do not have a default entry, find the first entry
+	 * we support and return that.  Do not cache that id.
+	 */
+	hmacs = asoc->peer.peer_hmacs;
+	if (!hmacs)
+		return NULL;
+
+	n_elt = (ntohs(hmacs->param_hdr.length) - sizeof(sctp_paramhdr_t)) >> 1;
+	for (i = 0; i < n_elt; i++) {
+		id = ntohs(hmacs->hmac_ids[i]);
+
+		/* Check the id is in the supported range */
+		if (id > SCTP_AUTH_HMAC_ID_MAX)
+			continue;
+
+		/* See is we support the id.  Supported IDs have name and
+		 * length fields set, so that we can allocated and use
+		 * them.  We can safely just check for name, for without the
+		 * name, we can't allocate the TFM.
+		 */
+		if (!sctp_hmac_list[id].hmac_name)
+			continue;
+
+		break;
+	}
+
+	if (id == 0)
+		return NULL;
+
+	return &sctp_hmac_list[id];
+}
+
+static int __sctp_auth_find_hmacid(__u16 *hmacs, int n_elts, __u16 hmac_id)
+{
+	int  found = 0;
+	int  i;
+
+	for (i = 0; i < n_elts; i++) {
+		if (hmac_id == hmacs[i]) {
+			found = 1;
+			break;
+		}
+	}
+
+	return found;
+}
+
+/* See if the HMAC_ID is one that we claim as supported */
+int sctp_auth_asoc_verify_hmac_id(const struct sctp_association *asoc,
+				    __u16 hmac_id)
+{
+	struct sctp_hmac_algo_param *hmacs;
+	__u16 n_elt;
+
+	if (!asoc)
+		return 0;
+
+	hmacs = (struct sctp_hmac_algo_param *)asoc->c.auth_hmacs;
+	n_elt = (ntohs(hmacs->param_hdr.length) - sizeof(sctp_paramhdr_t)) >> 1;
+
+	return __sctp_auth_find_hmacid(hmacs->hmac_ids, n_elt, hmac_id);
+}
+
+
+/* Cache the default HMAC id.  This to follow this text from SCTP-AUTH:
+ * Section 6.1:
+ *   The receiver of a HMAC-ALGO parameter SHOULD use the first listed
+ *   algorithm it supports.
+ */
+void sctp_auth_asoc_set_default_hmac(struct sctp_association *asoc,
+				     struct sctp_hmac_algo_param *hmacs)
+{
+	struct sctp_endpoint *ep;
+	__u16   id;
+	int	i;
+	int	n_params;
+
+	/* if the default id is already set, use it */
+	if (asoc->default_hmac_id)
+		return;
+
+	n_params = (ntohs(hmacs->param_hdr.length)
+				- sizeof(sctp_paramhdr_t)) >> 1;
+	ep = asoc->ep;
+	for (i = 0; i < n_params; i++) {
+		id = ntohs(hmacs->hmac_ids[i]);
+
+		/* Check the id is in the supported range */
+		if (id > SCTP_AUTH_HMAC_ID_MAX)
+			continue;
+
+		/* If this TFM has been allocated, use this id */
+		if (ep->auth_hmacs[id]) {
+			asoc->default_hmac_id = id;
+			break;
+		}
+	}
+}
+
+
+/* Check to see if the given chunk is supposed to be authenticated */
+static int __sctp_auth_cid(sctp_cid_t chunk, struct sctp_chunks_param *param)
+{
+	unsigned short len;
+	int found = 0;
+	int i;
+
+	if (!param)
+		return 0;
+
+	len = ntohs(param->param_hdr.length) - sizeof(sctp_paramhdr_t);
+
+	/* SCTP-AUTH, Section 3.2
+	 *    The chunk types for INIT, INIT-ACK, SHUTDOWN-COMPLETE and AUTH
+	 *    chunks MUST NOT be listed in the CHUNKS parameter.  However, if
+	 *    a CHUNKS parameter is received then the types for INIT, INIT-ACK,
+	 *    SHUTDOWN-COMPLETE and AUTH chunks MUST be ignored.
+	 */
+	for (i = 0; !found && i < len; i++) {
+		switch (param->chunks[i]) {
+		    case SCTP_CID_INIT:
+		    case SCTP_CID_INIT_ACK:
+		    case SCTP_CID_SHUTDOWN_COMPLETE:
+		    case SCTP_CID_AUTH:
+			break;
+
+		    default:
+			if (param->chunks[i] == chunk)
+			    found = 1;
+			break;
+		}
+	}
+
+	return found;
+}
+
+/* Check if peer requested that this chunk is authenticated */
+int sctp_auth_send_cid(sctp_cid_t chunk, const struct sctp_association *asoc)
+{
+	if (!sctp_auth_enable || !asoc || !asoc->peer.auth_capable)
+		return 0;
+
+	return __sctp_auth_cid(chunk, asoc->peer.peer_chunks);
+}
+
+/* Check if we requested that peer authenticate this chunk. */
+int sctp_auth_recv_cid(sctp_cid_t chunk, const struct sctp_association *asoc)
+{
+	if (!sctp_auth_enable || !asoc)
+		return 0;
+
+	return __sctp_auth_cid(chunk,
+			      (struct sctp_chunks_param *)asoc->c.auth_chunks);
+}
+
+/* SCTP-AUTH: Section 6.2:
+ *    The sender MUST calculate the MAC as described in RFC2104 [2] using
+ *    the hash function H as described by the MAC Identifier and the shared
+ *    association key K based on the endpoint pair shared key described by
+ *    the shared key identifier.  The 'data' used for the computation of
+ *    the AUTH-chunk is given by the AUTH chunk with its HMAC field set to
+ *    zero (as shown in Figure 6) followed by all chunks that are placed
+ *    after the AUTH chunk in the SCTP packet.
+ */
+void sctp_auth_calculate_hmac(const struct sctp_association *asoc,
+			      struct sk_buff *skb,
+			      struct sctp_auth_chunk *auth,
+			      gfp_t gfp)
+{
+	struct scatterlist sg;
+	struct hash_desc desc;
+	struct sctp_auth_bytes *asoc_key;
+	__u16 key_id, hmac_id;
+	__u8 *digest;
+	unsigned char *end;
+	int free_key = 0;
+
+	/* Extract the info we need:
+	 * - hmac id
+	 * - key id
+	 */
+	key_id = ntohs(auth->auth_hdr.shkey_id);
+	hmac_id = ntohs(auth->auth_hdr.hmac_id);
+
+	if (key_id == asoc->active_key_id)
+		asoc_key = asoc->asoc_shared_key;
+	else {
+		struct sctp_shared_key *ep_key;
+
+		ep_key = sctp_auth_get_shkey(asoc, key_id);
+		if (!ep_key)
+			return;
+
+		asoc_key = sctp_auth_asoc_create_secret(asoc, ep_key, gfp);
+		if (!asoc_key)
+			return;
+
+		free_key = 1;
+	}
+
+	/* set up scatter list */
+	end = skb_tail_pointer(skb);
+	sg.page = virt_to_page(auth);
+	sg.offset = (unsigned long)(auth) % PAGE_SIZE;
+	sg.length = end - (unsigned char *)auth;
+
+	desc.tfm = asoc->ep->auth_hmacs[hmac_id];
+	desc.flags = 0;
+
+	digest = auth->auth_hdr.hmac;
+	if (crypto_hash_setkey(desc.tfm, &asoc_key->data[0], asoc_key->len))
+		goto free;
+
+	crypto_hash_digest(&desc, &sg, sg.length, digest);
+
+free:
+	if (free_key)
+		sctp_auth_key_put(asoc_key);
+}