sctp: Make hmac algorithm selection for cookie generation dynamic

Currently sctp allows for the optional use of md5 of sha1 hmac algorithms to
generate cookie values when establishing new connections via two build time
config options. Theres no real reason to make this a static selection. We can
add a sysctl that allows for the dynamic selection of these algorithms at run
time, with the default value determined by the corresponding crypto library
availability.
This comes in handy when, for example running a system in FIPS mode, where use
of md5 is disallowed, but SHA1 is permitted.

Note: This new sysctl has no corresponding socket option to select the cookie
hmac algorithm. I chose not to implement that intentionally, as RFC 6458
contains no option for this value, and I opted not to pollute the socket option
namespace.

Change notes:
v2)
* Updated subject to have the proper sctp prefix as per Dave M.
* Replaced deafult selection options with new options that allow
developers to explicitly select available hmac algs at build time
as per suggestion by Vlad Y.

Signed-off-by: Neil Horman <[e-mail]>
CC: Vlad Yasevich <[e-mail]>
CC: "David S. Miller" <[e-mail]>
CC: [e-mail]
Acked-by: Vlad Yasevich <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/ip-sysctl.txt

···
 
 	Default: 1
 
+cookie_hmac_alg - STRING
+	Select the hmac algorithm used when generating the cookie value sent by
+	a listening sctp socket to a connecting client in the INIT-ACK chunk.
+	Valid values are:
+	* md5
+	* sha1
+	* none
+	Ability to assign md5 or sha1 as the selected alg is predicated on the
+	configuarion of those algorithms at build time (CONFIG_CRYPTO_MD5 and
+	CONFIG_CRYPTO_SHA1).
+
+	Default: Dependent on configuration.  MD5 if available, else SHA1 if
+	available, else none.
+
 rcvbuf_policy - INTEGER
 	Determines if the receive buffer is attributed to the socket or to
 	association.   SCTP supports the capability to create multiple

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;
+
 	/* Valid.Cookie.Life	    - 60  seconds  */
 	unsigned int valid_cookie_life;
 

include/net/sctp/constants.h

···
 				 * functions simpler to write.
 				 */
 
-#if defined (CONFIG_SCTP_HMAC_MD5)
-#define SCTP_COOKIE_HMAC_ALG "hmac(md5)"
-#elif defined (CONFIG_SCTP_HMAC_SHA1)
-#define SCTP_COOKIE_HMAC_ALG "hmac(sha1)"
-#else
-#define SCTP_COOKIE_HMAC_ALG NULL
-#endif
-
 /* These return values describe the success or failure of a number of
  * routines which form the lower interface to SCTP_outqueue.
  */

include/net/sctp/structs.h

···
 
 	/* Access to HMAC transform. */
 	struct crypto_hash *hmac;
+	char *sctp_hmac_alg;
 
 	/* What is our base endpointer? */
 	struct sctp_endpoint *ep;

net/sctp/Kconfig

···
 	select CRYPTO
 	select CRYPTO_HMAC
 	select CRYPTO_SHA1
-	select CRYPTO_MD5 if SCTP_HMAC_MD5
 	select LIBCRC32C
 	---help---
 	  Stream Control Transmission Protocol
···
 
 	  If unsure, say N
 
-choice
-	prompt "SCTP: Cookie HMAC Algorithm"
-	default SCTP_HMAC_MD5
+config SCTP_COOKIE_HMAC_MD5
+	bool "Enable optional MD5 hmac cookie generation"
 	help
-	  HMAC algorithm to be used during association initialization.  It
-	  is strongly recommended to use HMAC-SHA1 or HMAC-MD5.  See 
-	  configuration for Cryptographic API and enable those algorithms
-          to make usable by SCTP. 
+	  Enable optional MD5 hmac based SCTP cookie generation
+	default y
+	select CRYPTO_HMAC if SCTP_COOKIE_HMAC_MD5
+	select CRYPTO_MD5 if SCTP_COOKIE_HMAC_MD5
 
-config SCTP_HMAC_NONE
-	bool "None"
-	help 
-	  Choosing this disables the use of an HMAC during association 
-	  establishment.  It is advised to use either HMAC-MD5 or HMAC-SHA1.
-
-config SCTP_HMAC_SHA1
-	bool "HMAC-SHA1"
-	help 
-	  Enable the use of HMAC-SHA1 during association establishment.  It 
-	  is advised to use either HMAC-MD5 or HMAC-SHA1.
-
-config SCTP_HMAC_MD5
-	bool "HMAC-MD5"
+config SCTP_COOKIE_HMAC_SHA1
+	bool "Enable optional SHA1 hmac cookie generation"
 	help
-	  Enable the use of HMAC-MD5 during association establishment.  It is 
-	  advised to use either HMAC-MD5 or HMAC-SHA1.
+	  Enable optional SHA1 hmac based SCTP cookie generation
+	default y
+	select CRYPTO_HMAC if SCTP_COOKIE_HMAC_SHA1
+	select CRYPTO_SHA1 if SCTP_COOKIE_HMAC_SHA1
 
-endchoice
 
 endif # IP_SCTP

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_CRYPTO_MD5)
+	net->sctp.sctp_hmac_alg			= "md5";
+#elif defined (CONFIG_CRYPTO_SHA1)
+	net->sctp.sctp_hmac_alg			= "sha1";
+#else
+	net->sctp.sctp_hmac_alg			= NULL;
+#endif
+
 	/* Max.Burst		    - 4 */
 	net->sctp.max_burst			= SCTP_DEFAULT_MAX_BURST;
 

net/sctp/socket.c

···
 static int sctp_autobind(struct sock *sk);
 static void sctp_sock_migrate(struct sock *, struct sock *,
 			      struct sctp_association *, sctp_socket_type_t);
-static char *sctp_hmac_alg = SCTP_COOKIE_HMAC_ALG;
 
 extern struct kmem_cache *sctp_bucket_cachep;
 extern long sysctl_sctp_mem[3];
···
 	sp->default_rcv_context = 0;
 	sp->max_burst = net->sctp.max_burst;
 
+	sp->sctp_hmac_alg = net->sctp.sctp_hmac_alg;
+
 	/* Initialize default setup parameters. These parameters
 	 * can be modified with the SCTP_INITMSG socket option or
 	 * overridden by the SCTP_INIT CMSG.
···
 	struct sctp_sock *sp = sctp_sk(sk);
 	struct sctp_endpoint *ep = sp->ep;
 	struct crypto_hash *tfm = NULL;
+	char alg[32];
 
 	/* Allocate HMAC for generating cookie. */
-	if (!sctp_sk(sk)->hmac && sctp_hmac_alg) {
-		tfm = crypto_alloc_hash(sctp_hmac_alg, 0, CRYPTO_ALG_ASYNC);
+	if (!sp->hmac && sp->sctp_hmac_alg) {
+		sprintf(alg, "hmac(%s)", sp->sctp_hmac_alg);
+		tfm = crypto_alloc_hash(alg, 0, CRYPTO_ALG_ASYNC);
 		if (IS_ERR(tfm)) {
 			net_info_ratelimited("failed to load transform for %s: %ld\n",
-					     sctp_hmac_alg, PTR_ERR(tfm));
+					     sp->sctp_hmac_alg, PTR_ERR(tfm));
 			return -ENOSYS;
 		}
 		sctp_sk(sk)->hmac = tfm;

net/sctp/sysctl.c

···
 extern int sysctl_sctp_rmem[3];
 extern int sysctl_sctp_wmem[3];
 
+static int proc_sctp_do_hmac_alg(ctl_table *ctl,
+				int write,
+				void __user *buffer, size_t *lenp,
+
+				loff_t *ppos);
 static ctl_table sctp_table[] = {
 	{
 		.procname	= "sctp_mem",
···
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
+	},
+	{
+		.procname	= "cookie_hmac_alg",
+		.maxlen		= 8,
+		.mode		= 0644,
+		.proc_handler	= proc_sctp_do_hmac_alg,
 	},
 	{
 		.procname	= "valid_cookie_life",
···
 
 	{ /* sentinel */ }
 };
+
+static int proc_sctp_do_hmac_alg(ctl_table *ctl,
+				int write,
+				void __user *buffer, size_t *lenp,
+				loff_t *ppos)
+{
+	struct net *net = current->nsproxy->net_ns;
+	char tmp[8];
+	ctl_table tbl;
+	int ret;
+	int changed = 0;
+	char *none = "none";
+
+	memset(&tbl, 0, sizeof(struct ctl_table));
+
+	if (write) {
+		tbl.data = tmp;
+		tbl.maxlen = 8;
+	} else {
+		tbl.data = net->sctp.sctp_hmac_alg ? : none;
+		tbl.maxlen = strlen(tbl.data);
+	}
+		ret = proc_dostring(&tbl, write, buffer, lenp, ppos);
+
+	if (write) {
+#ifdef CONFIG_CRYPTO_MD5
+		if (!strncmp(tmp, "md5", 3)) {
+			net->sctp.sctp_hmac_alg = "md5";
+			changed = 1;
+		}
+#endif
+#ifdef CONFIG_CRYPTO_SHA1
+		if (!strncmp(tmp, "sha1", 4)) {
+			net->sctp.sctp_hmac_alg = "sha1";
+			changed = 1;
+		}
+#endif
+		if (!strncmp(tmp, "none", 4)) {
+			net->sctp.sctp_hmac_alg = NULL;
+			changed = 1;
+		}
+
+		if (!changed)
+			ret = -EINVAL;
+	}
+
+	return ret;
+}
 
 int sctp_sysctl_net_register(struct net *net)
 {