sesman: Prevent the use of 'alternate shell'

By setting the new config value 'AllowAlternateShell' to 'no' it is now
possible to prevent the use of an alternate shell, which can be set by
the connecting user.
The default remains unchanged and any shell is allowed if the config
value is not specified. It can also be set explicitly to 'yes' to achieve
the same outcome.

Fixes: #850

docs/man/sesman.ini.5.in

@@ -292,6 +292,10 @@ To keep compatibility, the following aliases are also available.
 If set to \fB1\fR, \fBtrue\fR or \fByes\fR, require group membership even
 if the group specified in \fBTerminalServerUsers\fR doesn't exist.
 
+.TP
+\fBAllowAlternateShell\fR=\fI[true|false]\fR
+If set to \fB0\fR, \fBfalse\fR or \fBno\fR, prevent usage of alternate shells by users.
+
 .SH "X11 SERVER"
 Following parameters can be used in the \fB[Xvnc]\fR and
 \fB[Xorg]\fR sections.

sesman/libsesman/sesman_config.c

@@ -69,6 +69,7 @@
 #define SESMAN_CFG_SEC_ALWAYSGROUPCHECK            "AlwaysGroupCheck"
 #define SESMAN_CFG_SEC_RESTRICT_OUTBOUND_CLIPBOARD "RestrictOutboundClipboard"
 #define SESMAN_CFG_SEC_RESTRICT_INBOUND_CLIPBOARD  "RestrictInboundClipboard"
+#define SESMAN_CFG_SEC_ALLOW_ALTERNATE_SHELL       "AllowAlternateShell"
 
 #define SESMAN_CFG_SESSIONS          "Sessions"
 #define SESMAN_CFG_SESS_MAX          "MaxSessions"
@@ -306,6 +307,7 @@ config_read_security(int file, struct config_security *sc,
     sc->ts_admins_enable = 0;
     sc->restrict_outbound_clipboard = 0;
     sc->restrict_inbound_clipboard = 0;
+    sc->allow_alternate_shell = 1;
 
     file_read_section(file, SESMAN_CFG_SECURITY, param_n, param_v);
 
@@ -373,6 +375,11 @@ config_read_security(int file, struct config_security *sc,
                     unrecognised);
             }
         }
+        if (0 == g_strcasecmp(buf, SESMAN_CFG_SEC_ALLOW_ALTERNATE_SHELL))
+        {
+            sc->allow_alternate_shell =
+                g_text2bool((char *)list_get_item(param_v, i));
+        }
 
     }
 
@@ -641,6 +648,7 @@ config_dump(struct config_sesman *config)
     g_writeln("    AllowRootLogin:            %d", sc->allow_root);
     g_writeln("    MaxLoginRetry:             %d", sc->login_retry);
     g_writeln("    AlwaysGroupCheck:          %d", sc->ts_always_group_check);
+    g_writeln("    AllowAlternateShell:       %d", sc->allow_alternate_shell);
     sesman_clip_restrict_mask_to_string(sc->restrict_outbound_clipboard,
                                         restrict_s, sizeof(restrict_s));
     g_writeln("    RestrictOutboundClipboard: %s", restrict_s);

sesman/libsesman/sesman_config.h

@@ -89,6 +89,15 @@ struct config_security
      * @brief if the clipboard should be enforced restricted. If true only allow server -> client, not vice versa.
      */
     int restrict_inbound_clipboard;
+
+    /**
+     * @var allow_alternate_shell
+     * @brief allow an user-supplied alternate shell.
+     * @details 'YES' for all programs allowed, 'NO' for disabling of alternate
+     *          shells.
+     *          If not specified, 'YES' is assumed.
+     */
+    int allow_alternate_shell;
 };
 
 /**

sesman/sesman.ini.in

@@ -37,6 +37,8 @@ RestrictOutboundClipboard=none
 ;   false: an alias of none
 ;   yes: an alias of all
 RestrictInboundClipboard=none
+; Set to 'no' to prevent users from logging in with alternate shells
+#AllowAlternateShell=true
 
 [Sessions]
 ;; X11DisplayOffset - x11 display number offset

sesman/session.c

@@ -275,26 +275,46 @@ start_window_manager(struct auth_info *auth_info,
     auth_set_env(auth_info);
     if (s->directory[0] != '\0')
     {
-        g_set_current_dir(s->directory);
+        if (g_cfg->sec.allow_alternate_shell)
+        {
+            g_set_current_dir(s->directory);
+        }
+        else
+        {
+            LOG(LOG_LEVEL_WARNING,
+                "Directory change to %s requested, but not "
+                "allowed by AllowAlternateShell config value.",
+                s->directory);
+        }
     }
 
     if (s->shell[0] != '\0')
     {
-        if (g_strchr(s->shell, ' ') != 0 || g_strchr(s->shell, '\t') != 0)
+        if (g_cfg->sec.allow_alternate_shell)
         {
-            LOG(LOG_LEVEL_INFO,
-                "Using user requested window manager on "
-                "display %u with embedded arguments using a shell: %s",
-                s->display, s->shell);
-            const char *argv[] = {"sh", "-c", s->shell, NULL};
-            g_execvp("/bin/sh", (char **)argv);
+            if (g_strchr(s->shell, ' ') != 0 || g_strchr(s->shell, '\t') != 0)
+            {
+                LOG(LOG_LEVEL_INFO,
+                    "Using user requested window manager on "
+                    "display %u with embedded arguments using a shell: %s",
+                    s->display, s->shell);
+                const char *argv[] = {"sh", "-c", s->shell, NULL};
+                g_execvp("/bin/sh", (char **)argv);
+            }
+            else
+            {
+                LOG(LOG_LEVEL_INFO,
+                    "Using user requested window manager on "
+                    "display %d: %s", s->display, s->shell);
+                g_execlp3(s->shell, s->shell, 0);
+            }
         }
         else
         {
-            LOG(LOG_LEVEL_INFO,
-                "Using user requested window manager on "
-                "display %d: %s", s->display, s->shell);
-            g_execlp3(s->shell, s->shell, 0);
+            LOG(LOG_LEVEL_WARNING,
+                "Shell %s requested by user, but not allowed by "
+                "AllowAlternateShell config value.",
+                s->shell);
         }
     }
     else