socket: Add osmo_sock_init flag to enable SCTP ASCONF features

The SCTP_ASCONF_SUPPORTED sockopt, which enables ASCONF features
for the SCTP associations managed by the SCTP socket, gets its default
value from to sysctl "net.sctp.addip_enable", which, at least up to
current kernel 6.4.8 is set to "0" (disabled) by default.
As a result, ASCONF related features such as
setsockopt(SCTP_SET_PEER_PRIMARY_ADDR) fail with -EPERM
since ASNCONF extensions where not negotiated during the SCTP init
handshake.
This commit adds support for setting SCTP_ASCONF_SUPPORTED=1 during
socket creation, since that needs to be applied before the first
bind()/connect() (before first assoc is created).

Furthermore, for ASCONF features to work properly, the assoc also needs
to announce/use the AUTH extension, as per RFC5061 section 4.2.7.
Otherwise, the peer receiving an SCTP INIT with
ExtensionFeatures=ASCONF,ASCONF_ACK but without AUTH, will reject the
assoc with an ABORT since it's not complying with spec (this behavior
can be tweaked through sysctl "net.sctp.addip_noauth_enable").

The AUTH extension is enabled through sockopt SCTP_AUTH_SUPPORTED, and
its default value is that of sysctl "net.sctp.auth_enable", which is
also disabled (0) by default.

Kernel support for those is relatively new:
SCTP_FUTURE_ASSOC added in: 80df2704a375bb4b3c9c5cce9c00052361b16d61
Follows: v5.0-rc4
Precedes: v5.1-rc1

SCTP_ASCONF_SUPPORTED added in: df2c71ffdfae58961981d7cbcccea93688fc4e96
Follows: v5.3-rc5
Precedes: v5.4-rc1

SCTP_AUTH_SUPPORTED added in: 56dd525abd56f7acd7b44a52935726e3ada4916c
Follows: v5.3-rc5
Precedes: v5.4-rc1

Related: OS#6076
Change-Id: Iac07031927b66a9d32d2bb2faab817e4c922a359

2 files changed, 55 insertions, 0 deletions

diff --git a/include/osmocom/core/socket.h b/include/osmocom/core/socket.h
index 6bac87d5..c68bb6f0 100644
--- a/include/osmocom/core/socket.h
+++ b/include/osmocom/core/socket.h
@@ -94,6 +94,8 @@ char *osmo_sockaddr_to_str_c(void *ctx, const struct osmo_sockaddr *sockaddr);
 #define OSMO_SOCK_F_NO_MCAST_ALL  (1 << 4)
 /*! use SO_REUSEADDR on UDP ports (required for multicast) */
 #define OSMO_SOCK_F_UDP_REUSEADDR (1 << 5)
+/*! use SCTP_AUTH_SUPPORTED + SCTP_ASCONF_SUPPORTED on SCTP socket (required for ASCONF / Peer Primary Address feature) */
+#define OSMO_SOCK_F_SCTP_ASCONF_SUPPORTED (1 << 6)
 
 /*! use OSMO_SOCK_F_DSCP(x) to set IP DSCP 'x' for packets transmitted on the socket */
 #define OSMO_SOCK_F_DSCP(x)	(((x)&0x3f) << 24)
diff --git a/src/core/socket.c b/src/core/socket.c
index 5a31c8f4..02d05e2a 100644
--- a/src/core/socket.c
+++ b/src/core/socket.c
@@ -696,6 +696,36 @@ static int addrinfo_to_sockaddr(uint16_t family, const struct addrinfo **result,
 	return 0;
 }
 
+static int setsockopt_sctp_auth_supported(int fd)
+{
+#ifdef SCTP_AUTH_SUPPORTED
+	struct sctp_assoc_value assoc_val = {
+		.assoc_id = SCTP_FUTURE_ASSOC,
+		.assoc_value = 1,
+	};
+	return setsockopt(fd, IPPROTO_SCTP, SCTP_AUTH_SUPPORTED, &assoc_val, sizeof(assoc_val));
+#else
+#pragma message "setsockopt(SCTP_AUTH_SUPPORTED) not supported! some SCTP features may not be available!"
+	LOGP(DLGLOBAL, LOGL_NOTICE, "Built without support for setsockopt(SCTP_AUTH_SUPPORTED), skipping\n");
+	return 0;
+#endif
+}
+
+static int setsockopt_sctp_asconf_supported(int fd)
+{
+#ifdef SCTP_ASCONF_SUPPORTED
+	struct sctp_assoc_value assoc_val = {
+		.assoc_id = SCTP_FUTURE_ASSOC,
+		.assoc_value = 1,
+	};
+	return setsockopt(fd, IPPROTO_SCTP, SCTP_ASCONF_SUPPORTED, &assoc_val, sizeof(assoc_val));
+#else
+#pragma message "setsockopt(SCTP_ASCONF_SUPPORTED) not supported! some SCTP features may not be available!"
+	LOGP(DLGLOBAL, LOGL_NOTICE, "Built without support for setsockopt(SCTP_ASCONF_SUPPORTED), skipping\n");
+	return 0;
+#endif
+}
+
 /*! Initialize a socket (including bind and/or connect) with multiple local or remote addresses.
  *  \param[in] family Address Family like AF_INET, AF_INET6, AF_UNSPEC
  *  \param[in] type Socket type like SOCK_DGRAM, SOCK_STREAM
@@ -811,6 +841,29 @@ int osmo_sock_init2_multiaddr(uint16_t family, uint16_t type, uint8_t proto,
 			goto ret_close;
 		}
 
+		if (flags & OSMO_SOCK_F_SCTP_ASCONF_SUPPORTED) {
+			/* RFC 5061 4.2.7: ASCONF also requires AUTH feature. */
+			rc = setsockopt_sctp_auth_supported(sfd);
+			if (rc < 0) {
+				int err = errno;
+				multiaddr_snprintf(strbuf, sizeof(strbuf), local_hosts, local_hosts_cnt);
+				LOGP(DLGLOBAL, LOGL_ERROR,
+				     "cannot setsockopt(SCTP_AUTH_SUPPORTED) socket: %s:%u: %s\n",
+				     strbuf, local_port, strerror(err));
+				goto ret_close;
+			}
+
+			rc = setsockopt_sctp_asconf_supported(sfd);
+			if (rc < 0) {
+				int err = errno;
+				multiaddr_snprintf(strbuf, sizeof(strbuf), local_hosts, local_hosts_cnt);
+				LOGP(DLGLOBAL, LOGL_ERROR,
+				     "cannot setsockopt(SCTP_ASCONF_SUPPORTED) socket: %s:%u: %s\n",
+				     strbuf, local_port, strerror(err));
+				goto ret_close;
+			}
+		}
+
 		/* Build array of addresses taking first entry for each host.
 		   TODO: Ideally we should use backtracking storing last used
 		   indexes and trying next combination if connect() fails .*/