upstream: Allow forwarding a different agent socket to the path

specified by $SSH_AUTH_SOCK, by extending the existing ForwardAgent option to
accepting an explicit path or the name of an environment variable in addition
to yes/no.

Patch by Eric Chiang, manpage by me; ok markus@

OpenBSD-Commit-ID: 98f2ed80bf34ea54d8b2ddd19ac14ebbf40e9265

authfd.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: authfd.c,v 1.120 2019/11/13 04:47:52 deraadt Exp $ */
+/* $OpenBSD: authfd.c,v 1.121 2019/12/21 02:19:13 djm Exp $ */
 /*
  * Author: Tatu Ylonen <[email protected]>
  * Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
@@ -82,21 +82,16 @@ decode_reply(u_char type)
 		return SSH_ERR_INVALID_FORMAT;
 }
 
-/* Returns the number of the authentication fd, or -1 if there is none. */
+/*
+ * Opens an authentication socket at the provided path and stores the file
+ * descriptor in fdp. Returns 0 on success and an error on failure.
+ */
 int
-ssh_get_authentication_socket(int *fdp)
+ssh_get_authentication_socket_path(const char *authsocket, int *fdp)
 {
-	const char *authsocket;
 	int sock, oerrno;
 	struct sockaddr_un sunaddr;
 
-	if (fdp != NULL)
-		*fdp = -1;
-
-	authsocket = getenv(SSH_AUTHSOCKET_ENV_NAME);
-	if (authsocket == NULL || *authsocket == '\0')
-		return SSH_ERR_AGENT_NOT_PRESENT;
-
 	memset(&sunaddr, 0, sizeof(sunaddr));
 	sunaddr.sun_family = AF_UNIX;
 	strlcpy(sunaddr.sun_path, authsocket, sizeof(sunaddr.sun_path));
@@ -119,6 +114,25 @@ ssh_get_authentication_socket(int *fdp)
 	return 0;
 }
 
+/*
+ * Opens the default authentication socket and stores the file descriptor in
+ * fdp. Returns 0 on success and an error on failure.
+ */
+int
+ssh_get_authentication_socket(int *fdp)
+{
+	const char *authsocket;
+
+	if (fdp != NULL)
+		*fdp = -1;
+
+	authsocket = getenv(SSH_AUTHSOCKET_ENV_NAME);
+	if (authsocket == NULL || *authsocket == '\0')
+		return SSH_ERR_AGENT_NOT_PRESENT;
+
+	return ssh_get_authentication_socket_path(authsocket, fdp);
+}
+
 /* Communicate with agent: send request and read reply */
 static int
 ssh_request_reply(int sock, struct sshbuf *request, struct sshbuf *reply)

authfd.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: authfd.h,v 1.47 2019/10/31 21:19:15 djm Exp $ */
+/* $OpenBSD: authfd.h,v 1.48 2019/12/21 02:19:13 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <[email protected]>
@@ -24,6 +24,7 @@ struct ssh_identitylist {
 };
 
 int	ssh_get_authentication_socket(int *fdp);
+int	ssh_get_authentication_socket_path(const char *authsocket, int *fdp);
 void	ssh_close_authentication_socket(int sock);
 
 int	ssh_lock_agent(int sock, int lock, const char *password);

clientloop.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: clientloop.c,v 1.329 2019/11/25 00:51:37 djm Exp $ */
+/* $OpenBSD: clientloop.c,v 1.330 2019/12/21 02:19:13 djm Exp $ */
 /*
  * Author: Tatu Ylonen <[email protected]>
  * Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
@@ -134,6 +134,12 @@ extern int muxserver_sock; /* XXX use mux_client_cleanup() instead */
  */
 extern char *host;
 
+/*
+ * If this field is not NULL, the ForwardAgent socket is this path and different
+ * instead of SSH_AUTH_SOCK.
+ */
+extern char *forward_agent_sock_path;
+
 /*
  * Flag to indicate that we have received a window change signal which has
  * not yet been processed.  This will cause a message indicating the new
@@ -1618,7 +1624,12 @@ client_request_agent(struct ssh *ssh, const char *request_type, int rchan)
 		    "malicious server.");
 		return NULL;
 	}
-	if ((r = ssh_get_authentication_socket(&sock)) != 0) {
+	if (forward_agent_sock_path == NULL) {
+		r = ssh_get_authentication_socket(&sock);
+	} else {
+		r = ssh_get_authentication_socket_path(forward_agent_sock_path, &sock);
+	}
+	if (r != 0) {
 		if (r != SSH_ERR_AGENT_NOT_PRESENT)
 			debug("%s: ssh_get_authentication_socket: %s",
 			    __func__, ssh_err(r));

readconf.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.c,v 1.318 2019/12/20 02:42:42 dtucker Exp $ */
+/* $OpenBSD: readconf.c,v 1.319 2019/12/21 02:19:13 djm Exp $ */
 /*
  * Author: Tatu Ylonen <[email protected]>
  * Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
@@ -919,6 +919,34 @@ process_config_line_depth(Options *options, struct passwd *pw, const char *host,
 
 	case oForwardAgent:
 		intptr = &options->forward_agent;
+
+		arg = strdelim(&s);
+		if (!arg || *arg == '\0')
+			fatal("%s line %d: missing argument.",
+			    filename, linenum);
+
+		value = -1;
+		multistate_ptr = multistate_flag;
+		for (i = 0; multistate_ptr[i].key != NULL; i++) {
+			if (strcasecmp(arg, multistate_ptr[i].key) == 0) {
+				value = multistate_ptr[i].value;
+				break;
+			}
+		}
+		if (value != -1) {
+			if (*activep && *intptr == -1)
+				*intptr = value;
+			break;
+		}
+		/* ForwardAgent wasn't 'yes' or 'no', assume a path */
+		if (*activep && *intptr == -1)
+			*intptr = 1;
+
+		charptr = &options->forward_agent_sock_path;
+		goto parse_agent_path;
+
+	case oForwardX11:
+		intptr = &options->forward_x11;
  parse_flag:
 		multistate_ptr = multistate_flag;
  parse_multistate:
@@ -940,10 +968,6 @@ process_config_line_depth(Options *options, struct passwd *pw, const char *host,
 			*intptr = value;
 		break;
 
-	case oForwardX11:
-		intptr = &options->forward_x11;
-		goto parse_flag;
-
 	case oForwardX11Trusted:
 		intptr = &options->forward_x11_trusted;
 		goto parse_flag;
@@ -1736,6 +1760,7 @@ process_config_line_depth(Options *options, struct passwd *pw, const char *host,
 		if (!arg || *arg == '\0')
 			fatal("%.200s line %d: Missing argument.",
 			    filename, linenum);
+  parse_agent_path:
 		/* Extra validation if the string represents an env var. */
 		if (arg[0] == '$' && !valid_env_name(arg + 1)) {
 			fatal("%.200s line %d: Invalid environment name %s.",
@@ -1853,6 +1878,7 @@ initialize_options(Options * options)
 {
 	memset(options, 'X', sizeof(*options));
 	options->forward_agent = -1;
+	options->forward_agent_sock_path = NULL;
 	options->forward_x11 = -1;
 	options->forward_x11_trusted = -1;
 	options->forward_x11_timeout = -1;
@@ -2636,7 +2662,6 @@ dump_client_config(Options *o, const char *host)
 	dump_cfg_fmtint(oClearAllForwardings, o->clear_forwardings);
 	dump_cfg_fmtint(oExitOnForwardFailure, o->exit_on_forward_failure);
 	dump_cfg_fmtint(oFingerprintHash, o->fingerprint_hash);
-	dump_cfg_fmtint(oForwardAgent, o->forward_agent);
 	dump_cfg_fmtint(oForwardX11, o->forward_x11);
 	dump_cfg_fmtint(oForwardX11Trusted, o->forward_x11_trusted);
 	dump_cfg_fmtint(oGatewayPorts, o->fwd_opts.gateway_ports);
@@ -2712,6 +2737,12 @@ dump_client_config(Options *o, const char *host)
 
 	/* Special cases */
 
+	/* oForwardAgent */
+	if (o->forward_agent_sock_path == NULL)
+		dump_cfg_fmtint(oForwardAgent, o->forward_agent);
+	else
+		dump_cfg_string(oForwardAgent, o->forward_agent_sock_path);
+
 	/* oConnectTimeout */
 	if (o->connection_timeout == -1)
 		printf("connecttimeout none\n");

readconf.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.h,v 1.130 2019/10/31 21:18:28 djm Exp $ */
+/* $OpenBSD: readconf.h,v 1.131 2019/12/21 02:19:13 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <[email protected]>
@@ -29,6 +29,7 @@ struct allowed_cname {
 
 typedef struct {
 	int     forward_agent;	/* Forward authentication agent. */
+	char   *forward_agent_sock_path; /* Optional path of the agent. */
 	int     forward_x11;	/* Forward X11 display. */
 	int     forward_x11_timeout;	/* Expiration for Cookies */
 	int     forward_x11_trusted;	/* Trust Forward X11 display. */

ssh.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh.c,v 1.509 2019/11/18 16:10:05 naddy Exp $ */
+/* $OpenBSD: ssh.c,v 1.510 2019/12/21 02:19:13 djm Exp $ */
 /*
  * Author: Tatu Ylonen <[email protected]>
  * Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
@@ -168,6 +168,12 @@ char *config = NULL;
  */
 char *host;
 
+/*
+ * A config can specify a path to forward, overriding SSH_AUTH_SOCK. If this is
+ * not NULL, forward the socket at this path instead.
+ */
+char *forward_agent_sock_path = NULL;
+
 /* Various strings used to to percent_expand() arguments */
 static char thishost[NI_MAXHOST], shorthost[NI_MAXHOST], portstr[NI_MAXSERV];
 static char uidstr[32], *host_arg, *conn_hash_hex;
@@ -1498,6 +1504,32 @@ main(int ac, char **av)
 		}
 	}
 
+	if (options.forward_agent && (options.forward_agent_sock_path != NULL)) {
+		p = tilde_expand_filename(options.forward_agent_sock_path, getuid());
+		cp = percent_expand(p,
+		    "d", pw->pw_dir,
+		    "h", host,
+		    "i", uidstr,
+		    "l", thishost,
+		    "r", options.user,
+		    "u", pw->pw_name,
+		    (char *)NULL);
+		free(p);
+
+		if (cp[0] == '$') {
+			if (!valid_env_name(cp + 1)) {
+				fatal("Invalid ForwardAgent environment variable name %s", cp);
+			}
+			if ((p = getenv(cp + 1)) != NULL)
+				forward_agent_sock_path = p;
+			else
+				options.forward_agent = 0;
+			free(cp);
+		} else {
+			forward_agent_sock_path = cp;
+		}
+	}
+
 	/* Expand ~ in known host file names. */
 	tilde_expand_paths(options.system_hostfiles,
 	    options.num_system_hostfiles);

ssh_config.5

@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: ssh_config.5,v 1.311 2019/12/19 15:09:30 naddy Exp $
-.Dd $Mdocdate: December 19 2019 $
+.\" $OpenBSD: ssh_config.5,v 1.312 2019/12/21 02:19:13 djm Exp $
+.Dd $Mdocdate: December 21 2019 $
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
@@ -669,11 +669,14 @@ and
 .It Cm ForwardAgent
 Specifies whether the connection to the authentication agent (if any)
 will be forwarded to the remote machine.
-The argument must be
-.Cm yes
-or
+The argument may be
+.Cm yes ,
 .Cm no
-(the default).
+(the default),
+an explicit path to an agent socket or the name of an environment variable
+(beginning with
+.Sq $ )
+in which to find the path.
 .Pp
 Agent forwarding should be enabled with caution.
 Users with the ability to bypass file permissions on the remote host