Allow connections based on authorized_keys file.

When run with option `-a /path/to/authorized_keys`, tmate-slave will only
authenticate public keys present in the file `/path/to/authorized_keys`.

The expected format is the same as specified in sshd(8) manpage, section
"AUTHORIZED_KEYS FILE FORMAT".

See: tmate-io/tmate-slave#37

Closes #38

tmate-slave.c

@@ -33,14 +33,15 @@ extern int server_create_socket(void);
 extern int client_connect(struct event_base *base, const char *path, int start_server);
 
 struct tmate_settings _tmate_settings = {
-	.keys_dir        = TMATE_SSH_DEFAULT_KEYS_DIR,
-	.ssh_port        = TMATE_SSH_DEFAULT_PORT,
-	.proxy_hostname  = NULL,
-	.bind_addr	 = NULL,
-	.proxy_port      = TMATE_DEFAULT_PROXY_PORT,
-	.tmate_host      = NULL,
-	.log_level       = LOG_NOTICE,
-	.use_syslog      = false,
+	.keys_dir        	= TMATE_SSH_DEFAULT_KEYS_DIR,
+	.authorized_keys_path 	= NULL,
+	.ssh_port        	= TMATE_SSH_DEFAULT_PORT,
+	.proxy_hostname  	= NULL,
+	.bind_addr	 	= NULL,
+	.proxy_port      	= TMATE_DEFAULT_PROXY_PORT,
+	.tmate_host      	= NULL,
+	.log_level      	= LOG_NOTICE,
+	.use_syslog      	= false,
 };
 
 struct tmate_settings *tmate_settings = &_tmate_settings;
@@ -102,7 +103,7 @@ void request_server_termination(void)
 
 static void usage(void)
 {
-	fprintf(stderr, "usage: tmate-slave [-b ip] [-h hostname] [-k keys_dir] [-p port] [-x proxy_hostname] [-q proxy_port] [-s] [-v]\n");
+	fprintf(stderr, "usage: tmate-slave [-b ip] [-h hostname] [-k keys_dir] [-a authorized_keys_path] [-p port] [-x proxy_hostname] [-q proxy_port] [-s] [-v]\n");
 }
 
 static char* get_full_hostname(void)
@@ -155,7 +156,7 @@ int main(int argc, char **argv, char **envp)
 {
 	int opt;
 
-	while ((opt = getopt(argc, argv, "b:h:k:p:x:q:sv")) != -1) {
+	while ((opt = getopt(argc, argv, "b:h:k:a:p:x:q:sv")) != -1) {
 		switch (opt) {
 		case 'b':
 			tmate_settings->bind_addr = xstrdup(optarg);
@@ -166,6 +167,9 @@ int main(int argc, char **argv, char **envp)
 		case 'k':
 			tmate_settings->keys_dir = xstrdup(optarg);
 			break;
+		case 'a':
+			tmate_settings->authorized_keys_path = xstrdup(optarg);
+			break;
 		case 'p':
 			tmate_settings->ssh_port = atoi(optarg);
 			break;

tmate-ssh-server.c

@@ -4,6 +4,7 @@
 #include <sys/socket.h>
 #include <netinet/tcp.h>
 #include <sys/wait.h>
+#include <ctype.h>
 #include <stdio.h>
 #include <event.h>
 #include <arpa/inet.h>
@@ -106,6 +107,61 @@ static ssh_channel channel_open_request_cb(ssh_session session, void *userdata)
 	return client->channel;
 }
 
+static int check_authorized_keys(struct ssh_key_struct *client_pubkey) {
+	#define MAX_PUBKEY_SIZE 0x4000
+
+	const char *authorized_keys_path = tmate_settings->authorized_keys_path;
+	const char *token_delim = " ";
+
+	FILE *file;
+	char key_buf[MAX_PUBKEY_SIZE], *key_type, *key_content;
+	enum ssh_keytypes_e type;
+	ssh_key pkey;
+
+	if (authorized_keys_path == NULL)
+		return SSH_AUTH_SUCCESS;
+
+	file = fopen(authorized_keys_path, "rb");
+	if (file == NULL) {
+		tmate_fatal("Could not open authorized_keys file: \"%s\"", authorized_keys_path);
+		return SSH_AUTH_DENIED;
+	}
+
+	while (fgets(key_buf, MAX_PUBKEY_SIZE, file)) {
+		if (key_buf[0] == '#' || key_buf[0] == '\0')
+			continue;
+
+		key_type = strtok(key_buf, token_delim);
+		if (key_type == NULL)
+			continue;
+
+		type = ssh_key_type_from_name(key_type);
+		if (type == SSH_KEYTYPE_UNKNOWN)
+			continue;
+
+		key_content = strtok(NULL, token_delim);
+		if (key_content == NULL)
+			continue;
+
+		pkey = ssh_key_new();
+		if (ssh_pki_import_pubkey_base64(key_content, type, &pkey) != SSH_OK) {
+			ssh_key_free(pkey);
+			continue;
+		}
+
+		if (!ssh_key_cmp(pkey, client_pubkey, SSH_KEY_CMP_PUBLIC)) {
+			ssh_key_free(pkey);
+			fclose(file);
+			return SSH_AUTH_SUCCESS;
+		}
+
+		ssh_key_free(pkey);
+	}
+
+	fclose(file);
+	return SSH_AUTH_DENIED;
+}
+
 static int auth_pubkey_cb(__unused ssh_session session,
 			  const char *user,
 			  struct ssh_key_struct *pubkey,
@@ -118,7 +174,8 @@ static int auth_pubkey_cb(__unused ssh_session session,
 		client->username = xstrdup(user);
 		if (ssh_pki_export_pubkey_base64(pubkey, &client->pubkey) != SSH_OK)
 			tmate_fatal("error getting public key");
-		return SSH_AUTH_SUCCESS;
+
+		return check_authorized_keys(pubkey);
 	case SSH_PUBLICKEY_STATE_NONE:
 		return SSH_AUTH_SUCCESS;
 	default:

tmate.h

@@ -205,6 +205,7 @@ extern void tmate_ssh_server_main(struct tmate_session *session,
 
 struct tmate_settings {
 	const char *keys_dir;
+	const char *authorized_keys_path;
 	int ssh_port;
 	const char *proxy_hostname;
 	int proxy_port;