Meta Guard Fallback External Connections Crash Recovery Data Sharing Clipboard Access GPU Access Display Control Screen Recording Rate Limiting Protected Terminal Delete Protect

Security and Safety Features

There are a number of mitigations in place to prevent abuse, loss of work and loss of access due to device or software failure. Some require cooperation from Arcan, some are implemented as part of Durden.

Meta Guard

The meta guard is the first feature you were exposed to, when active, it waits a certain number (~30) of keypresses and expects one of them to be a valid meta- menu path. If not, it assumes your keyboard is broken or has switched layout and provides an option to rebind the keys necessary to regain control.

This feature is reactivated if the keyboard device is lost or replaced.

Delete Protect

Keyboard repeat actions are disabled on meta bound paths for the reason that a possible I/O stall in the wrong moment on a loaded system could trigger dangerous cascades like a destroy-window action being triggered accidentally.

To further protect against unwanted destruction of windows with important contents, you can enable delete protect through the target/window/delete protect path. This blocks the window-destroy action until the delete protection is removed.

Fallback Application

A fallback application is simply an alternate set of scripts that can be activated if a scripting error occurs in the ones that are currently running.

This is part of arcan and needs to be enabled explicitly (see the -b argument). It can be set to re-run the same application (:self) or switch to another.

The engine will try and migrate external to the new script context. This means that unsaved window settings will be lost, along with negotiated extra resources and protocols, but a client will retain its primary connection and be adopted into a setting where it should be possible to save / recovery any important work.

The same feature is used for resetting/reloading (global/system/reset) and switching (global/system/reset/switch appl).

External Connections

The default settings expose an external connection point with the name ‘durden’. You can rename this connection point through the global/settings/system/connection path menu path.

Doing so can help you to avoid conflicts if you want to run multiple instances, or set it to the special ‘:disabled’ to remove support for external connections entirely. If so, only whitelisted targets/configs and builtin- frameservers can be hooked up to the arcan instance.

Crash Recovery

This is part of arcan, but is enabled explicitly in durden IF external connections are allowed. Should arcan crash, connected/compliant clients will attempt to reconnect (with a backoff in delays) to the recovery connection point provided by durden on external client connection. Clients that have been assigned additional privileges interactively by the user will have those privileges revoked before recovery.

Data Sharing

If your threat model does not include hostile local clients, you may want to open up further features for external control. Default settings do not permit any external control over sensitive data-paths, but some can be opened up - if desired.

Clipboard Access

As mentioned in clipboard, external clipboard managers are disabled by default but can be activated via the global/settings/system/clipboard bridge path, allowing clients that identify with their primary segment id as CLIPBOARD and CLIPBOARD_PASTE to inject and/or monitor global clipboard events.

Display Control

As mentioned in the color section in display management, clients are not granted access to the color ramp subprotocol that allows access to monitor hardware information and modification of accelerated display LUTs.

This can be toggled on a client-by-client basis via the target/video/advanced/color sync option, with the added risk that a malfunctioning client can make the screen contents unreadable.

It can also be globally allowed via the global/settings/system/gamma bridge path, with the added risk of clients fighting eachother for control over the LUTs. This will be mitigated later by allowing LUT state to follow client window selection.

GPU Access

The GPU and its seemingly infinite amount of attack surface is one of the absolute scariest and biggest risks for privilege escalation there is. Due to the value and utility for many clients, it is also a risk that is hard to mitigate and it comes at a cost. Although the engine tries, as best the drivers permit, to reduce the risk for DoS via priority inversion/resource starving, there is also the possibility to dynamically enable and disable the passing of accelerated buffer handles, forcing the clients to draw into shared memory and copy its contents down from the GPU. See “toggle handle passing” in target/video/advanced.

Furthermore, proper bridging of X clients pose additional risk due to an old and mostly unknown buffer passing mechanism that is likely as insecure as it gets - but it essentially provides the Xorg server with the same set of privileges as the main display server itself. Even though it is a high risk of broken clients, particularly games, this mechanism is disabled by default. To unlock it, go to global/config/system/GPU delegation.

Screen Recording

Clients that requests an output segment, either as primary registration segid or as secondary subsegments will have their request rejected for the time being. To compenstate, the ‘encode’ frameserver role can still be used for user-initiated screen sharing and recording and you can still interactively force-push an output segment into a client. See record, stream, share for more information.

Rate Limiting

To protect against fork-bombing style denial of service (clients stalling the server by requesting lots of connections or subsegments), there is configurable rate-limiting in place, though it is not enabled by default.

The relevant menu paths are found in global/settings/system/rate limiting:

  • Rate Limit : Force n ticks between each accepted external connection
  • Grace Period : Disable Rate Limit for the n first ticks (faster recovery)
  • External Windows Limit : Set an upper limit to the number of concurrent external connections
  • Window Subsegment Limit : Limit the amount of windows a client is allowed to spawn

It is also possible to force-disable handle passing (GPU resource starvation) through the target/video/advanced/toggle handle passing.

Arcan already favors platform- delivered events over external clients when multiplexing on a half-saturated event queue, and each individual client has a short event-queue to prevent starvation and priority-inversion.

IPC Controls

The control, status and output IPC pipes are only protected by their default unix permissions. There is no authentication step and preventing access to the appl_temp_path is the responsibility of the user when setting up arcan namespacing. To disable these features, simply set their respective names to :disabled, or enable whitelisting (global/settings/) and only permit certain menu paths from being activated via external IPC.

Protected Terminal

The drop-down terminal tool is designed to be used for a command-line shell that needs extra protection. It has a direct input path, separate management of font typeface, size and color scheme. It is not accessible from any of the normal menu paths, no clipboard interaction and is drawn with a separate border in a non-client controllable part of the UI with clear indication when it is active or not.

Future Changes

  • Display-Control LUT safeguards
  • Privilege Level Border indicators (color)
  • Event-queue load-balacing factors split into internal/whitelist/external