Add an ACL list for users connecting to the tmux socket. Users may be

forbidden from attaching, forced to attach read-only, or allowed to
attach read-write. A new command, server-access, configures the list.
tmux gets the user using getpeereid(3) of the client socket. Users must
still configure file system permissions manually. From Dallas Lyons and
others.

Makefile

@@ -55,6 +55,7 @@ SRCS=	alerts.c \
 	cmd-select-pane.c \
 	cmd-select-window.c \
 	cmd-send-keys.c \
+	cmd-server-access.c \
 	cmd-set-buffer.c \
 	cmd-set-environment.c \
 	cmd-set-option.c \
@@ -104,6 +105,7 @@ SRCS=	alerts.c \
 	screen-redraw.c \
 	screen-write.c \
 	screen.c \
+	server-acl.c \
 	server-client.c \
 	server-fn.c \
 	server.c \

client.c

@@ -360,6 +360,7 @@ client_main(struct event_base *base, int argc, char **argv, uint64_t flags,
 	/* Send identify messages. */
 	client_send_identify(ttynam, termname, caps, ncaps, cwd, feat);
 	tty_term_free_list(caps, ncaps);
+	proc_flush_peer(client_peer);
 
 	/* Send first command. */
 	if (msg == MSG_COMMAND) {

cmd-attach-session.c

@@ -43,7 +43,7 @@ const struct cmd_entry cmd_attach_session_entry = {
 
 	/* -t is special */
 
-	.flags = CMD_STARTSERVER,
+	.flags = CMD_STARTSERVER|CMD_READONLY,
 	.exec = cmd_attach_session_exec
 };
 
@@ -69,6 +69,7 @@ cmd_attach_session(struct cmdq_item *item, const char *tflag, int dflag,
 
 	if (c == NULL)
 		return (CMD_RETURN_NORMAL);
+
 	if (server_client_check_nested(c)) {
 		cmdq_error(item, "sessions should be nested with care, "
 		    "unset $TMUX to force");

cmd-server-access.c

@@ -0,0 +1,147 @@
+/* $OpenBSD$ */
+
+/*
+ * Copyright (c) 2021 Dallas Lyons <dallasdlyons@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
+ * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#include <pwd.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "tmux.h"
+
+/*
+ * Controls access to session.
+ */
+
+static enum cmd_retval cmd_server_access_exec(struct cmd *, struct cmdq_item *);
+
+const struct cmd_entry cmd_server_access_entry = {
+	.name = "server-access",
+	.alias = NULL,
+
+	.args = { "adlrw", 0, 1, NULL },
+	.usage = "[-adlrw]" CMD_TARGET_PANE_USAGE " [user]",
+
+	.flags = CMD_CLIENT_CANFAIL,
+	.exec = cmd_server_access_exec
+};
+
+static enum cmd_retval
+cmd_server_access_deny(struct cmdq_item *item, struct passwd *pw)
+{
+	struct client		*loop;
+	struct server_acl_user	*user;
+	uid_t			 uid;
+
+	if ((user = server_acl_user_find(pw->pw_uid)) == NULL) {
+		cmdq_error(item, "user %s not found", pw->pw_name);
+		return (CMD_RETURN_ERROR);
+	}
+	TAILQ_FOREACH(loop, &clients, entry) {
+		uid = proc_get_peer_uid(loop->peer);
+		if (uid == server_acl_get_uid(user)) {
+			loop->exit_message = xstrdup("access not allowed");
+			loop->flags |= CLIENT_EXIT;
+		}
+	}
+	server_acl_user_deny(pw->pw_uid);
+
+	return (CMD_RETURN_NORMAL);
+}
+
+static enum cmd_retval
+cmd_server_access_exec(struct cmd *self, struct cmdq_item *item)
+{
+
+	struct args	*args = cmd_get_args(self);
+	struct client	*c = cmdq_get_target_client(item);
+	char		*name;
+	struct passwd	*pw = NULL;
+
+	if (args_has(args, 'l')) {
+		server_acl_display(item);
+		return (CMD_RETURN_NORMAL);
+	}
+	if (args_count(args) == 0) {
+		cmdq_error(item, "missing user arguement");
+		return (CMD_RETURN_ERROR);
+	}
+
+	name = format_single(item, args_string(args, 0), c, NULL, NULL, NULL);
+	if (*name != '\0')
+		pw = getpwnam(name);
+	if (pw == NULL) {
+		cmdq_error(item, "unknown user: %s", name);
+		return (CMD_RETURN_ERROR);
+	}
+	free(name);
+
+	if (pw->pw_uid == 0 || pw->pw_uid == getuid()) {
+		cmdq_error(item, "%s owns the server, can't change access",
+		    pw->pw_name);
+		return (CMD_RETURN_ERROR);
+	}
+
+	if (args_has(args, 'a') && args_has(args, 'd')) {
+		cmdq_error(item, "-a and -d cannot be used together");
+		return (CMD_RETURN_ERROR);
+	}
+	if (args_has(args, 'w') && args_has(args, 'r')) {
+		cmdq_error(item, "-r and -w cannot be used together");
+		return (CMD_RETURN_ERROR);
+	}
+
+	if (args_has(args, 'd'))
+		return (cmd_server_access_deny(item, pw));
+	if (args_has(args, 'a')) {
+		if (server_acl_user_find(pw->pw_uid) != NULL) {
+			cmdq_error(item, "user %s is already added",
+			    pw->pw_name);
+			return (CMD_RETURN_ERROR);
+		}
+		server_acl_user_allow(pw->pw_uid);
+		/* Do not return - allow -r or -w with -a. */
+	} else if (args_has(args, 'r') || args_has(args, 'w')) {
+		/* -r or -w implies -a if user does not exist. */
+		if (server_acl_user_find(pw->pw_uid) == NULL)
+			server_acl_user_allow(pw->pw_uid);
+	}
+
+	if (args_has(args, 'w')) {
+		if (server_acl_user_find(pw->pw_uid) == NULL) {
+			cmdq_error(item, "user %s not found", pw->pw_name);
+			return (CMD_RETURN_ERROR);
+		}
+		server_acl_user_allow_write(pw->pw_uid);
+		return (CMD_RETURN_NORMAL);
+	}
+
+	if (args_has(args, 'r')) {
+		if (server_acl_user_find(pw->pw_uid) == NULL) {
+			cmdq_error(item, "user %s not found", pw->pw_name);
+			return (CMD_RETURN_ERROR);
+		}
+		server_acl_user_deny_write(pw->pw_uid);
+		return (CMD_RETURN_NORMAL);
+	}
+
+	return (CMD_RETURN_NORMAL);
+}

cmd.c

@@ -96,6 +96,7 @@ extern const struct cmd_entry cmd_select_pane_entry;
 extern const struct cmd_entry cmd_select_window_entry;
 extern const struct cmd_entry cmd_send_keys_entry;
 extern const struct cmd_entry cmd_send_prefix_entry;
+extern const struct cmd_entry cmd_server_access_entry;
 extern const struct cmd_entry cmd_set_buffer_entry;
 extern const struct cmd_entry cmd_set_environment_entry;
 extern const struct cmd_entry cmd_set_hook_entry;
@@ -188,6 +189,7 @@ const struct cmd_entry *cmd_table[] = {
 	&cmd_select_window_entry,
 	&cmd_send_keys_entry,
 	&cmd_send_prefix_entry,
+	&cmd_server_access_entry,
 	&cmd_set_buffer_entry,
 	&cmd_set_environment_entry,
 	&cmd_set_hook_entry,

proc.c

@@ -337,6 +337,12 @@ proc_kill_peer(struct tmuxpeer *peer)
 	peer->flags |= PEER_BAD;
 }
 
+void
+proc_flush_peer(struct tmuxpeer *peer)
+{
+	imsg_flush(&peer->ibuf);
+}
+
 void
 proc_toggle_log(struct tmuxproc *tp)
 {

server.c

@@ -239,6 +239,8 @@ server_start(struct tmuxproc *client, int flags, struct event_base *base,
 	evtimer_set(&server_ev_tidy, server_tidy_event, NULL);
 	evtimer_add(&server_ev_tidy, &tv);
 
+	server_acl_init();
+
 	server_add_accept(0);
 	proc_loop(server_proc, server_loop);
 
@@ -355,9 +357,10 @@ server_update_socket(void)
 static void
 server_accept(int fd, short events, __unused void *data)
 {
-	struct sockaddr_storage	sa;
-	socklen_t		slen = sizeof sa;
-	int			newfd;
+	struct sockaddr_storage	 sa;
+	socklen_t		 slen = sizeof sa;
+	int			 newfd;
+	struct client		*c;
 
 	server_add_accept(0);
 	if (!(events & EV_READ))
@@ -374,11 +377,16 @@ server_accept(int fd, short events, __unused void *data)
 		}
 		fatal("accept failed");
 	}
+
 	if (server_exit) {
 		close(newfd);
 		return;
 	}
-	server_client_create(newfd);
+	c = server_client_create(newfd);
+	if (!server_acl_join(c)) {
+		c->exit_message = xstrdup("access not allowed");
+		c->flags |= CLIENT_EXIT;
+	}
 }
 
 /*

tmux.1

@@ -1485,6 +1485,44 @@ option.
 .D1 Pq alias: Ic rename
 Rename the session to
 .Ar new-name .
+.It Xo Ic server-access
+.Op Fl adlrw
+.Op Ar user
+.Xc
+Change the access or read/write permission of
+.Ar user .
+The user running the
+.Nm
+server (its owner) and the root user cannot be changed and are always
+permitted access.
+.Pp
+.Fl a
+and
+.Fl d
+are used to give or revoke access for the specified user.
+If the user is already attached, the
+.Fl d
+flag causes their clients to be detached.
+.Pp
+.Fl r
+and
+.Fl w
+change the permissions for
+.Ar user :
+.Fl r
+makes their clients read-only and
+.Fl w
+writable.
+.Fl l
+lists current access permissions.
+.Pp
+By default, the access list is empty and
+.Nm
+creates sockets with file system permissions preventing access by any user
+other than the owner (and root).
+These permissions must be changed manually.
+Great care should be taken not to allow access to untrusted users even
+read-only.
 .Tg showmsgs
 .It Xo Ic show-messages
 .Op Fl JT
@@ -5069,7 +5107,7 @@ The following variables are available, where appropriate:
 .It Li "client_name" Ta "" Ta "Name of client"
 .It Li "client_pid" Ta "" Ta "PID of client process"
 .It Li "client_prefix" Ta "" Ta "1 if prefix key has been pressed"
-.It Li "client_readonly" Ta "" Ta "1 if client is readonly"
+.It Li "client_readonly" Ta "" Ta "1 if client is read-only"
 .It Li "client_session" Ta "" Ta "Name of the client's session"
 .It Li "client_termfeatures" Ta "" Ta "Terminal features of client, if any"
 .It Li "client_termname" Ta "" Ta "Terminal name of client"