Add RADIUS blast attack protection with Message-Authenticator

Recently FreeRadius started to complain accel-ppp doesn't pass
BlastRADIUS check. This commit fixes that.

This commit implements protection against RADIUS blast attacks
by adding support for the Message-Authenticator attribute in
Access-Request packets. This security enhancement helps
prevent unauthorized access attempts and replay attacks
on RADIUS authentication.

- Added new configuration option `blast-protection=1`
  in [radius] to enable Message-Authenticator inclusion
- Implemented HMAC-MD5 calculation for
  Message-Authenticator attribute (RFC 2869)
- Modified packet building to include 18-byte Message-Authenticator
  attribute when enabled
- Updated packet structure to support signing with shared secret

Enable blast protection by adding to the `[radius]` section:
```
blast-protection=1
```

When enabled, all Access-Request packets will include a
Message-Authenticator attribute with HMAC-MD5 signature,
providing cryptographic integrity verification and protection
against packet modification attacks.

Signed-off-by: Denys Fedoryshchenko <denys.f@collabora.com>

8 files changed, 81 insertions, 1 deletions

diff --git a/accel-pppd/accel-ppp.conf b/accel-pppd/accel-ppp.conf
index 99d35c79..3f277a16 100644
--- a/accel-pppd/accel-ppp.conf
+++ b/accel-pppd/accel-ppp.conf
@@ -227,6 +227,7 @@ verbose=1
 #strip-realm=0
 #attr-tunnel-type=My-Tunnel-Type
 #nas-port-id-in-req=1
+blast-protection=1
 
 [client-ip-range]
 10.0.0.0/8
diff --git a/accel-pppd/accel-ppp.conf.5 b/accel-pppd/accel-ppp.conf.5
index 72376c35..8860d520 100644
--- a/accel-pppd/accel-ppp.conf.5
+++ b/accel-pppd/accel-ppp.conf.5
@@ -1008,6 +1008,11 @@ Specifies should accel-ppp send NAS-Port-Id on Access-Request and Accounting-Req
 .br
 Configuration of log and log_file modules.
 .TP
+.BI "blast-protection=" 0|1
+If this option is given and
+.B 1
+is specified then radius module will include Message-Authenticator attribute in Access-Request packets.
+.TP
 .BI "log-file=" file
 Path to file to write general log.
 .TP
diff --git a/accel-pppd/radius/dm_coa.c b/accel-pppd/radius/dm_coa.c
index e88cc347..8eb45a60 100644
--- a/accel-pppd/radius/dm_coa.c
+++ b/accel-pppd/radius/dm_coa.c
@@ -106,6 +106,7 @@ static int dm_coa_send_nak(int fd, struct rad_packet_t *req, struct sockaddr_in
 	if (err_code)
 		rad_packet_add_int(reply, NULL, "Error-Cause", err_code);
 
+	// TODO: We need to add Message-Authenticator attribute here
 	if (rad_packet_build(reply, RA)) {
 		rad_packet_free(reply);
 		return -1;
diff --git a/accel-pppd/radius/packet.c b/accel-pppd/radius/packet.c
index 3c2ef99d..0ef9a0e3 100644
--- a/accel-pppd/radius/packet.c
+++ b/accel-pppd/radius/packet.c
@@ -8,14 +8,20 @@
 #include <sys/mman.h>
 #include <linux/mman.h>
 #include <arpa/inet.h>
+#include <openssl/hmac.h>
+#include <openssl/evp.h>
 
 #include "log.h"
 #include "mempool.h"
-
 #include "radius_p.h"
+#include "attr_defs.h"
 
 #include "memdebug.h"
 
+#define HMAC_MD5_LEN 16
+/* Radius header + attribute: type + length */
+#define PACKET_SIGNED_OFFSET (20 + 2)
+
 static mempool_t packet_pool;
 static mempool_t attr_pool;
 static mempool_t buf_pool;
@@ -47,6 +53,34 @@ void print_buf(uint8_t *buf,int size)
 	printf("\n");
 }
 
+
+int hmac_md5(const uint8_t *key,  size_t key_len,
+             const uint8_t *data, size_t data_len,
+             uint8_t out[HMAC_MD5_LEN])
+{
+    unsigned int len = 0;
+    HMAC_CTX *ctx = HMAC_CTX_new();
+    if (!ctx)
+        return -1;
+
+    if (HMAC_Init_ex(ctx, key, (int)key_len, EVP_md5(), NULL) != 1)
+        goto err;
+
+    if (HMAC_Update(ctx, data, data_len) != 1)
+        goto err;
+
+    if (HMAC_Final(ctx, out, &len) != 1 || len != HMAC_MD5_LEN)
+        goto err;
+
+    HMAC_CTX_free(ctx);
+    return 0;
+
+err:
+    HMAC_CTX_free(ctx);
+    return -1;
+}
+
+
 int rad_packet_build(struct rad_packet_t *pack, uint8_t *RA)
 {
 	struct rad_attr_t *attr;
@@ -302,6 +336,9 @@ void rad_packet_free(struct rad_packet_t *pack)
 		mempool_free(pack->buf);
 		//munmap(pack->buf, REQ_LENGTH_MAX);
 
+	if (pack->secret)
+	  _free(pack->secret);
+
 	while(!list_empty(&pack->attrs)) {
 		attr = list_entry(pack->attrs.next, typeof(*attr), entry);
 		list_del(&attr->entry);
@@ -811,6 +848,18 @@ int rad_packet_send(struct rad_packet_t *pack, int fd, struct sockaddr_in *addr)
 
 	clock_gettime(CLOCK_MONOTONIC, &pack->tv);
 
+	if (pack->secret && pack->message_authenticator) {
+		uint8_t hmac[HMAC_MD5_LEN];
+		uint8_t *ptr = pack->buf;
+		uint8_t *hmac_ptr = ptr + PACKET_SIGNED_OFFSET;
+		if (hmac_md5((const uint8_t *)pack->secret, strlen(pack->secret), pack->buf, pack->len, hmac) < 0) {
+			log_emerg("radius:packet: failed to calculate HMAC\n");
+			return -1;
+		}
+		memcpy(hmac_ptr, hmac, HMAC_MD5_LEN);
+	}
+
+
 	while (1) {
 		if (addr)
 			n = sendto(fd, pack->buf, pack->len, 0, (struct sockaddr *)addr, sizeof(*addr));
diff --git a/accel-pppd/radius/radius.c b/accel-pppd/radius/radius.c
index 2406ba04..9fbbc467 100644
--- a/accel-pppd/radius/radius.c
+++ b/accel-pppd/radius/radius.c
@@ -58,6 +58,7 @@ static int conf_strip_realm;
 const char *conf_attr_tunnel_type;
 
 int conf_acct_delay_start;
+int conf_blast_protection;
 
 static LIST_HEAD(sessions);
 static pthread_rwlock_t sessions_lock = PTHREAD_RWLOCK_INITIALIZER;
@@ -1086,6 +1087,13 @@ static int load_config(void)
 	else
 		conf_acct_delay_start = 0;
 
+	opt = conf_get_opt("radius", "blast-protection");
+	if (opt && atoi(opt) > 0) {
+		conf_blast_protection = 1;
+	} else {
+		conf_blast_protection = 0;
+	}
+
 	return 0;
 }
 
diff --git a/accel-pppd/radius/radius.h b/accel-pppd/radius/radius.h
index 1925c973..c19b25c2 100644
--- a/accel-pppd/radius/radius.h
+++ b/accel-pppd/radius/radius.h
@@ -103,6 +103,8 @@ struct rad_attr_t
 
 struct rad_packet_t
 {
+	int message_authenticator; // 1 if message authenticator is required
+	uint8_t *secret; // shared secret for this packet for Message-Authenticator signature
 	int code;
 	uint8_t id;
 	int len;
diff --git a/accel-pppd/radius/radius_p.h b/accel-pppd/radius/radius_p.h
index 87613624..06588ead 100644
--- a/accel-pppd/radius/radius_p.h
+++ b/accel-pppd/radius/radius_p.h
@@ -198,6 +198,7 @@ extern int conf_acct_interim_jitter;
 extern int conf_accounting;
 extern const char *conf_attr_tunnel_type;
 extern int conf_acct_delay_start;
+extern int conf_blast_protection;
 
 int rad_check_nas_pack(struct rad_packet_t *pack);
 struct radius_pd_t *rad_find_session(const char *sessionid, const char *username, const char *port_id, int port, in_addr_t ipaddr, const char *csid);
diff --git a/accel-pppd/radius/req.c b/accel-pppd/radius/req.c
index be74f047..863174a2 100644
--- a/accel-pppd/radius/req.c
+++ b/accel-pppd/radius/req.c
@@ -15,6 +15,8 @@
 
 #include "memdebug.h"
 
+#define HMAC_MD5_LEN 16
+
 static int make_socket(struct rad_req_t *req);
 static mempool_t req_pool;
 
@@ -73,6 +75,17 @@ static struct rad_req_t *__rad_req_alloc(struct radius_pd_t *rpd, int code, cons
 	if (!req->pack)
 		goto out_err;
 
+	if (code == CODE_ACCESS_REQUEST && conf_blast_protection) {
+		uint8_t buf[HMAC_MD5_LEN] = {0};
+		req->pack->message_authenticator = 1;
+		req->pack->secret = strdup(req->serv->secret);
+		if (rad_packet_add_octets(req->pack, NULL, "Message-Authenticator", buf, HMAC_MD5_LEN)) {
+                        free(req->pack->secret);
+                        req->pack->secret == NULL;
+			goto out_err;
+                }
+	}
+
 	if (code == CODE_ACCOUNTING_REQUEST && rpd->acct_username)
 		username = rpd->acct_username;