sctp: Convert cookie authentication to use HMAC-SHA256

Convert SCTP cookies to use HMAC-SHA256, instead of the previous choice
of the legacy algorithms HMAC-MD5 and HMAC-SHA1. Simplify and optimize
the code by using the HMAC-SHA256 library instead of crypto_shash, and
by preparing the HMAC key when it is generated instead of per-operation.

This doesn't break compatibility, since the cookie format is an
implementation detail, not part of the SCTP protocol itself.

Note that the cookie size doesn't change either. The HMAC field was
already 32 bytes, even though previously at most 20 bytes were actually
compared. 32 bytes exactly fits an untruncated HMAC-SHA256 value. So,
although we could safely truncate the MAC to something slightly shorter,
for now just keep the cookie size the same.

I also considered SipHash, but that would generate only 8-byte MACs. An
8-byte MAC *might* suffice here. However, there's quite a lot of
information in the SCTP cookies: more than in TCP SYN cookies. So
absent an analysis that occasional forgeries of all that information is
okay in SCTP, I errored on the side of caution.

Remove HMAC-MD5 and HMAC-SHA1 as options, since the new HMAC-SHA256
option is just better. It's faster as well as more secure. For
example, benchmarking on x86_64, cookie authentication is now nearly 3x
as fast as the previous default choice and implementation of HMAC-MD5.

Also just make the kernel always support cookie authentication if SCTP
is supported at all, rather than making it optional in the build. (It
was sort of optional before, but it didn't really work properly. E.g.,
a kernel with CONFIG_SCTP_COOKIE_HMAC_MD5=n still supported HMAC-MD5
cookie authentication if CONFIG_CRYPTO_HMAC and CONFIG_CRYPTO_MD5
happened to be enabled in the kconfig for other reasons.)

Acked-by: Xin Long <[e-mail]>
Signed-off-by: Eric Biggers <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/ip-sysctl.rst

···
 	a listening sctp socket to a connecting client in the INIT-ACK chunk.
 	Valid values are:
 
-	* md5
-	* sha1
+	* sha256
 	* none
 
-	Ability to assign md5 or sha1 as the selected alg is predicated on the
-	configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
-	CONFIG_CRYPTO_SHA1).
+	md5 and sha1 are also accepted for backwards compatibility, but cause
+	sha256 to be selected.
 
-	Default: Dependent on configuration.  MD5 if available, else SHA1 if
-	available, else none.
+	Default: sha256
 
 rcvbuf_policy - INTEGER
 	Determines if the receive buffer is attributed to the socket or to

include/net/netns/sctp.h

···
 	/* Whether Cookie Preservative is enabled(1) or not(0) */
 	int cookie_preserve_enable;
 
-	/* The namespace default hmac alg */
-	char *sctp_hmac_alg;
+	/* Whether cookie authentication is enabled(1) or not(0) */
+	int cookie_auth_enable;
 
 	/* Valid.Cookie.Life	    - 60  seconds  */
 	unsigned int valid_cookie_life;

include/net/sctp/constants.h

···
 					 */
 #define SCTP_DEFAULT_MINSEGMENT 512	/* MTU size ... if no mtu disc */
 
-#define SCTP_SECRET_SIZE 32		/* Number of octets in a 256 bits. */
-
-#define SCTP_SIGNATURE_SIZE 20	        /* size of a SLA-1 signature */
+#define SCTP_COOKIE_KEY_SIZE 32	/* size of cookie HMAC key */
+#define SCTP_COOKIE_MAC_SIZE 32	/* size of HMAC field in cookies */
 
 #define SCTP_COOKIE_MULTIPLE 32 /* Pad out our cookie to make our hash
 				 * functions simpler to write.

include/net/sctp/structs.h

···
 #ifndef __sctp_structs_h__
 #define __sctp_structs_h__
 
+#include <crypto/sha2.h>
 #include <linux/ktime.h>
 #include <linux/generic-radix-tree.h>
 #include <linux/rhashtable-types.h>
···
 struct sctp_bind_addr;
 struct sctp_ulpq;
 struct sctp_ep_common;
-struct crypto_shash;
 struct sctp_stream;
 
 
···
 	/* PF_ family specific functions.  */
 	struct sctp_pf *pf;
 
-	/* Access to HMAC transform. */
-	struct crypto_shash *hmac;
-	char *sctp_hmac_alg;
-
 	/* What is our base endpointer? */
 	struct sctp_endpoint *ep;
 
···
 		frag_interleave:1,
 		recvrcvinfo:1,
 		recvnxtinfo:1,
-		data_ready_signalled:1;
+		data_ready_signalled:1,
+		cookie_auth_enable:1;
 
 	atomic_t pd_mode;
 
···
 
 /* The format of our cookie that we send to our peer. */
 struct sctp_signed_cookie {
-	__u8 signature[SCTP_SECRET_SIZE];
+	__u8 mac[SCTP_COOKIE_MAC_SIZE];
 	__u32 __pad;		/* force sctp_cookie alignment to 64 bits */
 	struct sctp_cookie c;
 } __packed;
···
 	/* This is really a list of struct sctp_association entries. */
 	struct list_head asocs;
 
-	/* Secret Key: A secret key used by this endpoint to compute
-	 *	      the MAC.	This SHOULD be a cryptographic quality
-	 *	      random number with a sufficient length.
-	 *	      Discussion in [RFC1750] can be helpful in
-	 *	      selection of the key.
-	 */
-	__u8 secret_key[SCTP_SECRET_SIZE];
+	/* Cookie authentication key used by this endpoint */
+	struct hmac_sha256_key cookie_auth_key;
 
- 	/* digest:  This is a digest of the sctp cookie.  This field is
- 	 * 	    only used on the receive path when we try to validate
- 	 * 	    that the cookie has not been tampered with.  We put
- 	 * 	    this here so we pre-allocate this once and can re-use
- 	 * 	    on every receive.
- 	 */
- 	__u8 *digest;
- 
 	/* sendbuf acct. policy.	*/
 	__u32 sndbuf_policy;
 

net/sctp/Kconfig

···
 	  'cat /proc/net/sctp/sctp_dbg_objcnt'
 
 	  If unsure, say N
+
 choice
-	prompt "Default SCTP cookie HMAC encoding"
-	default SCTP_DEFAULT_COOKIE_HMAC_MD5
+	prompt "Default SCTP cookie authentication method"
+	default SCTP_DEFAULT_COOKIE_HMAC_SHA256
 	help
-	  This option sets the default sctp cookie hmac algorithm
-	  when in doubt select 'md5'
+	  This option sets the default SCTP cookie authentication method, for
+	  when a method hasn't been explicitly selected via the
+	  net.sctp.cookie_hmac_alg sysctl.
 
-config SCTP_DEFAULT_COOKIE_HMAC_MD5
-	bool "Enable optional MD5 hmac cookie generation"
-	help
-	  Enable optional MD5 hmac based SCTP cookie generation
-	select SCTP_COOKIE_HMAC_MD5
+	  If unsure, choose the default (HMAC-SHA256).
 
-config SCTP_DEFAULT_COOKIE_HMAC_SHA1
-	bool "Enable optional SHA1 hmac cookie generation"
-	help
-	  Enable optional SHA1 hmac based SCTP cookie generation
-	select SCTP_COOKIE_HMAC_SHA1
+config SCTP_DEFAULT_COOKIE_HMAC_SHA256
+	bool "HMAC-SHA256"
 
 config SCTP_DEFAULT_COOKIE_HMAC_NONE
-	bool "Use no hmac alg in SCTP cookie generation"
-	help
-	  Use no hmac algorithm in SCTP cookie generation
+	bool "None"
 
 endchoice
-
-config SCTP_COOKIE_HMAC_MD5
-	bool "Enable optional MD5 hmac cookie generation"
-	help
-	  Enable optional MD5 hmac based SCTP cookie generation
-	select CRYPTO
-	select CRYPTO_HMAC
-	select CRYPTO_MD5
-
-config SCTP_COOKIE_HMAC_SHA1
-	bool "Enable optional SHA1 hmac cookie generation"
-	help
-	  Enable optional SHA1 hmac based SCTP cookie generation
-	select CRYPTO
-	select CRYPTO_HMAC
-	select CRYPTO_SHA1
 
 config INET_SCTP_DIAG
 	depends on INET_DIAG

net/sctp/endpointola.c

···
 /* Forward declarations for internal helpers. */
 static void sctp_endpoint_bh_rcv(struct work_struct *work);
 
+static void gen_cookie_auth_key(struct hmac_sha256_key *key)
+{
+	u8 raw_key[SCTP_COOKIE_KEY_SIZE];
+
+	get_random_bytes(raw_key, sizeof(raw_key));
+	hmac_sha256_preparekey(key, raw_key, sizeof(raw_key));
+	memzero_explicit(raw_key, sizeof(raw_key));
+}
+
 /*
  * Initialize the base fields of the endpoint structure.
  */
···
 {
 	struct net *net = sock_net(sk);
 	struct sctp_shared_key *null_key;
-
-	ep->digest = kzalloc(SCTP_SIGNATURE_SIZE, gfp);
-	if (!ep->digest)
-		return NULL;
 
 	ep->asconf_enable = net->sctp.addip_enable;
 	ep->auth_enable = net->sctp.auth_enable;
···
 	/* Get the receive buffer policy for this endpoint */
 	ep->rcvbuf_policy = net->sctp.rcvbuf_policy;
 
-	/* Initialize the secret key used with cookie. */
-	get_random_bytes(ep->secret_key, sizeof(ep->secret_key));
+	/* Generate the cookie authentication key. */
+	gen_cookie_auth_key(&ep->cookie_auth_key);
 
 	/* SCTP-AUTH extensions*/
 	INIT_LIST_HEAD(&ep->endpoint_shared_keys);
···
 nomem_shkey:
 	sctp_auth_free(ep);
 nomem:
-	kfree(ep->digest);
 	return NULL;
 
 }
···
 		return;
 	}
 
-	/* Free the digest buffer */
-	kfree(ep->digest);
-
 	/* SCTP-AUTH: Free up AUTH releated data such as shared keys
 	 * chunks and hmacs arrays that were allocated
 	 */
···
 	sctp_inq_free(&ep->base.inqueue);
 	sctp_bind_addr_free(&ep->base.bind_addr);
 
-	memset(ep->secret_key, 0, sizeof(ep->secret_key));
+	memzero_explicit(&ep->cookie_auth_key, sizeof(ep->cookie_auth_key));
 
 	sk = ep->base.sk;
 	/* Remove and free the port */

net/sctp/protocol.c

···
 	/* Whether Cookie Preservative is enabled(1) or not(0) */
 	net->sctp.cookie_preserve_enable 	= 1;
 
-	/* Default sctp sockets to use md5 as their hmac alg */
-#if defined (CONFIG_SCTP_DEFAULT_COOKIE_HMAC_MD5)
-	net->sctp.sctp_hmac_alg			= "md5";
-#elif defined (CONFIG_SCTP_DEFAULT_COOKIE_HMAC_SHA1)
-	net->sctp.sctp_hmac_alg			= "sha1";
-#else
-	net->sctp.sctp_hmac_alg			= NULL;
-#endif
+	/* Whether cookie authentication is enabled(1) or not(0) */
+	net->sctp.cookie_auth_enable =
+		!IS_ENABLED(CONFIG_SCTP_DEFAULT_COOKIE_HMAC_NONE);
 
 	/* Max.Burst		    - 4 */
 	net->sctp.max_burst			= SCTP_DEFAULT_MAX_BURST;

net/sctp/sm_make_chunk.c

···
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
-#include <crypto/hash.h>
 #include <crypto/utils.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
···
 	 * out on the network.
 	 */
 	retval = kzalloc(*cookie_len, GFP_ATOMIC);
-	if (!retval)
-		goto nodata;
+	if (!retval) {
+		*cookie_len = 0;
+		return NULL;
+	}
 
 	cookie = (struct sctp_signed_cookie *) retval->body;
 
···
 	memcpy((__u8 *)(cookie + 1) +
 	       ntohs(init_chunk->chunk_hdr->length), raw_addrs, addrs_len);
 
-	if (sctp_sk(ep->base.sk)->hmac) {
-		struct crypto_shash *tfm = sctp_sk(ep->base.sk)->hmac;
-		int err;
-
-		/* Sign the message.  */
-		err = crypto_shash_setkey(tfm, ep->secret_key,
-					  sizeof(ep->secret_key)) ?:
-		      crypto_shash_tfm_digest(tfm, (u8 *)&cookie->c, bodysize,
-					      cookie->signature);
-		if (err)
-			goto free_cookie;
+	/* Sign the cookie, if cookie authentication is enabled. */
+	if (sctp_sk(ep->base.sk)->cookie_auth_enable) {
+		static_assert(sizeof(cookie->mac) == SHA256_DIGEST_SIZE);
+		hmac_sha256(&ep->cookie_auth_key, (const u8 *)&cookie->c,
+			    bodysize, cookie->mac);
 	}
 
 	return retval;
-
-free_cookie:
-	kfree(retval);
-nodata:
-	*cookie_len = 0;
-	return NULL;
 }
 
 /* Unpack the cookie from COOKIE ECHO chunk, recreating the association.  */
···
 	struct sctp_signed_cookie *cookie;
 	struct sk_buff *skb = chunk->skb;
 	struct sctp_cookie *bear_cookie;
-	__u8 *digest = ep->digest;
 	enum sctp_scope scope;
 	unsigned int len;
 	ktime_t kt;
···
 	cookie = chunk->subh.cookie_hdr;
 	bear_cookie = &cookie->c;
 
-	if (!sctp_sk(ep->base.sk)->hmac)
-		goto no_hmac;
+	/* Verify the cookie's MAC, if cookie authentication is enabled. */
+	if (sctp_sk(ep->base.sk)->cookie_auth_enable) {
+		u8 mac[SHA256_DIGEST_SIZE];
 
-	/* Check the signature.  */
-	{
-		struct crypto_shash *tfm = sctp_sk(ep->base.sk)->hmac;
-		int err;
-
-		err = crypto_shash_setkey(tfm, ep->secret_key,
-					  sizeof(ep->secret_key)) ?:
-		      crypto_shash_tfm_digest(tfm, (u8 *)bear_cookie, bodysize,
-					      digest);
-		if (err) {
-			*error = -SCTP_IERROR_NOMEM;
+		hmac_sha256(&ep->cookie_auth_key, (const u8 *)bear_cookie,
+			    bodysize, mac);
+		static_assert(sizeof(cookie->mac) == sizeof(mac));
+		if (crypto_memneq(mac, cookie->mac, sizeof(mac))) {
+			*error = -SCTP_IERROR_BAD_SIG;
 			goto fail;
 		}
 	}
 
-	if (crypto_memneq(digest, cookie->signature, SCTP_SIGNATURE_SIZE)) {
-		*error = -SCTP_IERROR_BAD_SIG;
-		goto fail;
-	}
-
-no_hmac:
 	/* IG Section 2.35.2:
 	 *  3) Compare the port numbers and the verification tag contained
 	 *     within the COOKIE ECHO chunk to the actual port numbers and the

net/sctp/socket.c

···
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
-#include <crypto/hash.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <linux/wait.h>
···
 	sp->default_rcv_context = 0;
 	sp->max_burst = net->sctp.max_burst;
 
-	sp->sctp_hmac_alg = net->sctp.sctp_hmac_alg;
+	sp->cookie_auth_enable = net->sctp.cookie_auth_enable;
 
 	/* Initialize default setup parameters. These parameters
 	 * can be modified with the SCTP_INITMSG socket option or
···
 	if (!sp->ep)
 		return -ENOMEM;
 
-	sp->hmac = NULL;
-
 	sk->sk_destruct = sctp_destruct_sock;
 
 	SCTP_DBG_OBJCNT_INC(sock);
···
 	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
 }
 
-/* Triggered when there are no references on the socket anymore */
-static void sctp_destruct_common(struct sock *sk)
-{
-	struct sctp_sock *sp = sctp_sk(sk);
-
-	/* Free up the HMAC transform. */
-	crypto_free_shash(sp->hmac);
-}
-
 static void sctp_destruct_sock(struct sock *sk)
 {
-	sctp_destruct_common(sk);
 	inet_sock_destruct(sk);
 }
 
···
 {
 	struct sctp_sock *sp = sctp_sk(sk);
 	struct sctp_endpoint *ep = sp->ep;
-	struct crypto_shash *tfm = NULL;
-	char alg[32];
 	int err;
-
-	/* Allocate HMAC for generating cookie. */
-	if (!sp->hmac && sp->sctp_hmac_alg) {
-		sprintf(alg, "hmac(%s)", sp->sctp_hmac_alg);
-		tfm = crypto_alloc_shash(alg, 0, 0);
-		if (IS_ERR(tfm)) {
-			net_info_ratelimited("failed to load transform for %s: %ld\n",
-					     sp->sctp_hmac_alg, PTR_ERR(tfm));
-			return -ENOSYS;
-		}
-		sctp_sk(sk)->hmac = tfm;
-	}
 
 	/*
 	 * If a bind() or sctp_bindx() is not called prior to a listen()
···
 	 * copy.
 	 */
 	newsp->ep = newep;
-	newsp->hmac = NULL;
 
 	/* Hook this new socket in to the bind_hash list. */
 	head = &sctp_port_hashtable[sctp_phashfn(sock_net(oldsk),
···
 
 static void sctp_v6_destruct_sock(struct sock *sk)
 {
-	sctp_destruct_common(sk);
 	inet6_sock_destruct(sk);
 }
 

net/sctp/sysctl.c

···
 	},
 	{
 		.procname	= "cookie_hmac_alg",
-		.data		= &init_net.sctp.sctp_hmac_alg,
+		.data		= &init_net.sctp.cookie_auth_enable,
 		.maxlen		= 8,
 		.mode		= 0644,
 		.proc_handler	= proc_sctp_do_hmac_alg,
···
 				 void *buffer, size_t *lenp, loff_t *ppos)
 {
 	struct net *net = container_of(ctl->data, struct net,
-				       sctp.sctp_hmac_alg);
+				       sctp.cookie_auth_enable);
 	struct ctl_table tbl;
-	bool changed = false;
-	char *none = "none";
 	char tmp[8] = {0};
 	int ret;
 
···
 
 	if (write) {
 		tbl.data = tmp;
-		tbl.maxlen = sizeof(tmp);
-	} else {
-		tbl.data = net->sctp.sctp_hmac_alg ? : none;
-		tbl.maxlen = strlen(tbl.data);
+		tbl.maxlen = sizeof(tmp) - 1;
+		ret = proc_dostring(&tbl, 1, buffer, lenp, ppos);
+		if (ret)
+			return ret;
+		if (!strcmp(tmp, "sha256") ||
+		    /* for backwards compatibility */
+		    !strcmp(tmp, "md5") || !strcmp(tmp, "sha1")) {
+			net->sctp.cookie_auth_enable = 1;
+			return 0;
+		}
+		if (!strcmp(tmp, "none")) {
+			net->sctp.cookie_auth_enable = 0;
+			return 0;
+		}
+		return -EINVAL;
 	}
-
-	ret = proc_dostring(&tbl, write, buffer, lenp, ppos);
-	if (write && ret == 0) {
-#ifdef CONFIG_CRYPTO_MD5
-		if (!strncmp(tmp, "md5", 3)) {
-			net->sctp.sctp_hmac_alg = "md5";
-			changed = true;
-		}
-#endif
-#ifdef CONFIG_CRYPTO_SHA1
-		if (!strncmp(tmp, "sha1", 4)) {
-			net->sctp.sctp_hmac_alg = "sha1";
-			changed = true;
-		}
-#endif
-		if (!strncmp(tmp, "none", 4)) {
-			net->sctp.sctp_hmac_alg = NULL;
-			changed = true;
-		}
-		if (!changed)
-			ret = -EINVAL;
-	}
-
-	return ret;
+	if (net->sctp.cookie_auth_enable)
+		tbl.data = (char *)"sha256";
+	else
+		tbl.data = (char *)"none";
+	tbl.maxlen = strlen(tbl.data);
+	return proc_dostring(&tbl, 0, buffer, lenp, ppos);
 }
 
 static int proc_sctp_do_rto_min(const struct ctl_table *ctl, int write,