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 <nhorman@tuxdriver.com>
CC: Vlad Yasevich <vyasevich@gmail.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: netdev@vger.kernel.org
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 4 deletions

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 59d16ea927f0..fa81bdee00a5 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -110,7 +110,6 @@ static int sctp_do_bind(struct sock *, union sctp_addr *, int);
 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];
@@ -3890,6 +3889,8 @@ SCTP_STATIC int sctp_init_sock(struct sock *sk)
 	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.
@@ -5981,13 +5982,15 @@ SCTP_STATIC int sctp_listen_start(struct sock *sk, int backlog)
 	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;