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.

Makefile.am

@@ -123,6 +123,7 @@ dist_tmux_SOURCES = \
 	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 \
@@ -172,6 +173,7 @@ dist_tmux_SOURCES = \
 	screen-redraw.c \
 	screen-write.c \
 	screen.c \
+	server-acl.c \
 	server-client.c \
 	server-fn.c \
 	server.c \

client.c

@@ -361,6 +361,7 @@ 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-queue.c

@@ -19,9 +19,11 @@
 #include <sys/types.h>
 
 #include <ctype.h>
+#include <pwd.h>
 #include <stdlib.h>
 #include <string.h>
 #include <time.h>
+#include <unistd.h>
 
 #include "tmux.h"
 
@@ -558,17 +560,31 @@ cmdq_add_message(struct cmdq_item *item)
 {
 	struct client		*c = item->client;
 	struct cmdq_state	*state = item->state;
-	const char		*name, *key;
+	const char		*key;
 	char			*tmp;
+	uid_t                    uid;
+	struct passwd		*pw;
+	char                    *user = NULL;
 
 	tmp = cmd_print(item->cmd);
 	if (c != NULL) {
-		name = c->name;
+		uid = proc_get_peer_uid(c->peer);
+		if (uid != getuid()) {
+			if ((pw = getpwuid(uid)) != NULL)
+				xasprintf(&user, "[%s]", pw->pw_name);
+			else
+				user = xstrdup("[unknown]");
+		} else
+			user = xstrdup("");
 		if (c->session != NULL && state->event.key != KEYC_NONE) {
 			key = key_string_lookup_key(state->event.key, 0);
-			server_add_message("%s key %s: %s", name, key, tmp);
+			server_add_message("%s%s key %s: %s", c->name, user,
-		} else
+			    key, tmp);
-			server_add_message("%s command: %s", name, tmp);
+		} else {
+			server_add_message("%s%s command: %s", c->name, user,
+			    tmp);
+		}
+		free(user);
 	} else
 		server_add_message("command: %s", tmp);
 	free(tmp);

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

@@ -95,6 +95,7 @@ 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;
@@ -187,6 +188,7 @@ 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

@@ -349,6 +349,12 @@ 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-acl.c

@@ -0,0 +1,182 @@
+/* $OpenBSD$ */
+
+/*
+ * Copyright (c) 2021 Holland Schutte, Jayson Morberg
+ * 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/types.h>
+#include <sys/stat.h>
+#include <sys/socket.h>
+
+#include <ctype.h>
+#include <pwd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "tmux.h"
+
+struct server_acl_user {
+	uid_t				uid;
+
+	int				flags;
+#define SERVER_ACL_READONLY 0x1
+
+	RB_ENTRY(server_acl_user)	entry;
+};
+
+static int
+server_acl_cmp(struct server_acl_user *user1, struct server_acl_user *user2)
+{
+	if (user1->uid < user2->uid)
+		return -1;
+	return user1->uid > user2->uid;
+}
+
+RB_HEAD(server_acl_entries, server_acl_user) server_acl_entries;
+RB_GENERATE_STATIC(server_acl_entries, server_acl_user, entry, server_acl_cmp);
+
+/* Initialize server_acl tree. */
+void
+server_acl_init(void)
+{
+	RB_INIT(&server_acl_entries);
+
+	if (getuid() != 0)
+		server_acl_user_allow(0);
+	server_acl_user_allow(getuid());
+}
+
+/* Find user entry. */
+struct server_acl_user*
+server_acl_user_find(uid_t uid)
+{
+	struct server_acl_user	find = { .uid = uid };
+
+	return RB_FIND(server_acl_entries, &server_acl_entries, &find);
+}
+
+/* Display the tree. */
+void
+server_acl_display(struct cmdq_item *item)
+{
+	struct server_acl_user	*loop;
+	struct passwd		*pw;
+	const char		*name;
+
+	RB_FOREACH(loop, server_acl_entries, &server_acl_entries) {
+		if (loop->uid == 0)
+			continue;
+		if ((pw = getpwuid(loop->uid)) != NULL)
+			name = pw->pw_name;
+		else
+			name = "unknown";
+		if (loop->flags == SERVER_ACL_READONLY)
+			cmdq_print(item, "%s (R)", name);
+		else
+			cmdq_print(item, "%s (W)", name);
+	}
+}
+
+/* Allow a user. */
+void
+server_acl_user_allow(uid_t uid)
+{
+	struct server_acl_user	*user;
+
+	user = server_acl_user_find(uid);
+	if (user == NULL) {
+		user = xcalloc(1, sizeof *user);
+		user->uid = uid;
+		RB_INSERT(server_acl_entries, &server_acl_entries, user);
+	}
+}
+
+/* Deny a user (remove from the tree). */
+void
+server_acl_user_deny(uid_t uid)
+{
+	struct server_acl_user	*user;
+
+	user = server_acl_user_find(uid);
+	if (user != NULL) {
+		RB_REMOVE(server_acl_entries, &server_acl_entries, user);
+		free(user);
+	}
+}
+
+/* Allow this user write access. */
+void
+server_acl_user_allow_write(uid_t uid)
+{
+	struct server_acl_user	*user;
+	struct client		*c;
+
+	user = server_acl_user_find(uid);
+	if (user == NULL)
+		return;
+	user->flags &= ~SERVER_ACL_READONLY;
+
+	TAILQ_FOREACH(c, &clients, entry) {
+		uid = proc_get_peer_uid(c->peer);
+		if (uid != (uid_t)-1 && uid == user->uid)
+			c->flags &= ~CLIENT_READONLY;
+	}
+}
+
+/* Deny this user write access. */
+void
+server_acl_user_deny_write(uid_t uid)
+{
+	struct server_acl_user	*user;
+	struct client		*c;
+
+	user = server_acl_user_find(uid);
+	if (user == NULL)
+		return;
+	user->flags |= SERVER_ACL_READONLY;
+
+	TAILQ_FOREACH(c, &clients, entry) {
+		uid = proc_get_peer_uid(c->peer);
+		if (uid == user->uid && uid == user->uid)
+			c->flags |= CLIENT_READONLY;
+	}
+}
+
+/*
+ * Check if the client's UID exists in the ACL list and if so, set as read only
+ * if needed. Return false if the user does not exist.
+ */
+int
+server_acl_join(struct client *c)
+{
+	struct server_acl_user	*user;
+	uid_t			 uid = proc_get_peer_uid(c->peer);
+
+	user = server_acl_user_find(uid);
+	if (user == NULL)
+		return (0);
+	if (user->flags & SERVER_ACL_READONLY)
+		c->flags |= CLIENT_READONLY;
+	return (1);
+}
+
+/* Get UID for user entry. */
+uid_t
+server_acl_get_uid(struct server_acl_user *user)
+{
+	return (user->uid);
+}

server-client.c

@@ -2772,6 +2772,14 @@ server_client_dispatch(struct imsg *imsg, void *arg)
 	}
 }
 
+/* Callback when command is not allowed. */
+static enum cmd_retval
+server_client_read_only(struct cmdq_item *item, __unused void *data)
+{
+	cmdq_error(item, "client is read-only");
+	return (CMD_RETURN_ERROR);
+}
+
 /* Callback when command is done. */
 static enum cmd_retval
 server_client_command_done(struct cmdq_item *item, __unused void *data)
@@ -2796,6 +2804,7 @@ server_client_dispatch_command(struct client *c, struct imsg *imsg)
 	char			**argv, *cause;
 	struct cmd_parse_result	 *pr;
 	struct args_value	 *values;
+	struct cmdq_item	 *new_item;
 
 	if (c->flags & CLIENT_EXIT)
 		return;
@@ -2834,7 +2843,12 @@ server_client_dispatch_command(struct client *c, struct imsg *imsg)
 	free(values);
 	cmd_free_argv(argc, argv);
 
-	cmdq_append(c, cmdq_get_command(pr->cmdlist, NULL));
+	if ((c->flags & CLIENT_READONLY) &&
+	    !cmd_list_all_have(pr->cmdlist, CMD_READONLY))
+		new_item = cmdq_get_callback(server_client_read_only, NULL);
+	else
+		new_item = cmdq_get_command(pr->cmdlist, NULL);
+	cmdq_append(c, new_item);
 	cmdq_append(c, cmdq_get_callback(server_client_command_done, NULL));
 
 	cmd_list_free(pr->cmdlist);
@@ -3072,9 +3086,11 @@ server_client_set_flags(struct client *c, const char *flags)
 			continue;
 
 		log_debug("client %s set flag %s", c->name, next);
-		if (not)
+		if (not) {
+			if (c->flags & CLIENT_READONLY)
+				flag &= ~CLIENT_READONLY;
 			c->flags &= ~flag;
-		else
+		} else
 			c->flags |= flag;
 		if (flag == CLIENT_CONTROL_NOOUTPUT)
 			control_reset_offsets(c);

server.c

@@ -245,6 +245,8 @@ 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);
 
@@ -361,9 +363,10 @@ server_update_socket(void)
 static void
 server_accept(int fd, short events, __unused void *data)
 {
-	struct sockaddr_storage	sa;
+	struct sockaddr_storage	 sa;
-	socklen_t		slen = sizeof sa;
+	socklen_t		 slen = sizeof sa;
-	int			newfd;
+	int			 newfd;
+	struct client		*c;
 
 	server_add_accept(0);
 	if (!(events & EV_READ))
@@ -380,11 +383,16 @@ 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

@@ -1488,6 +1488,44 @@ 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
@@ -5072,7 +5110,7 @@ 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"

tmux.h

@@ -2025,6 +2025,7 @@ struct tmuxpeer *proc_add_peer(struct tmuxproc *, int,
 	    void (*)(struct imsg *, void *), void *);
 void	proc_remove_peer(struct tmuxpeer *);
 void	proc_kill_peer(struct tmuxpeer *);
+void	proc_flush_peer(struct tmuxpeer *);
 void	proc_toggle_log(struct tmuxproc *);
 pid_t	proc_fork_and_daemon(int *);
 uid_t	proc_get_peer_uid(struct tmuxpeer *);
@@ -3269,4 +3270,15 @@ struct window_pane *spawn_pane(struct spawn_context *, char **);
 /* regsub.c */
 char		*regsub(const char *, const char *, const char *, int);
 
+/* server-acl.c */
+void			 server_acl_init(void);
+struct server_acl_user	*server_acl_user_find(uid_t);
+void 			 server_acl_display(struct cmdq_item *);
+void			 server_acl_user_allow(uid_t);
+void			 server_acl_user_deny(uid_t);
+void			 server_acl_user_allow_write(uid_t);
+void			 server_acl_user_deny_write(uid_t);
+int			 server_acl_join(struct client *);
+uid_t			 server_acl_get_uid(struct server_acl_user *);
+
 #endif /* TMUX_H */