Qubes-Community-Content/docs/common-tasks/opening-urls-in-vms.md

How to open URLs/files in other VMs

Qubes' philosophy is to assume you are already compromised and to partition your work / data in a way that even if all your VMs are compromised an attacker would not be able to extract any information. This document describes how to implement such compartmentalization when opening URLs and files from "secure" offline or firewalled VMs. Configuration samples throughout this document show how to setup a flexible and powerful workflow, mitigating the long starting time and resource usage of dispVMs that unfortunately often results in users not taking advantage of them.

Naming convention:

• srcVM is the VM where the files/URLs are
• dstVM is the VM we want to open them in ; dstVM can be any VM type - a DispVM, a regular AppVM, a Whonix workstation dvm, ...

Configuring dom0 RPC permissions

Opening files and URLs in other VMs rely on the qubes.OpenInVM and qubes.OpenURL RPC services, which are called by srcVM's qvm-open-in-vm and qvm-open-in-dvm shell scripts.

Qubes RPC policies allow to fine tune how those RPC services can be used between VMs.

The (powerful) ask policy

A very powerful and convenient RPC policy rule is ask: in that case a dialog with the list of destination VMs pops up each time the RPC service is called, allowing the user to select a destination VM depending on his work's context (eg. the target URL's level of trust, protocol, file MIME type, ...).

It is impossible to overstate how flexible this is and how much security it can add to one's workflow: while opening things in dispVMs is the most secure approach the problem is starting a dispVM for each URL/file takes far too much time and resources, leading people to open files/URLs in persistent VMs instead.

The ask policy's VM selection dialog allows one to start any type of VM or dispVM (see section "Considerations on dispVMs" below), or send the URL/file to an already running (disp)VM. The first time an URL/file is opened the (disp)VM will start if it wasn't running. The next time another URL/file is sent, there's no need start a new (disp)VM, one can instead select the already running (disp)VM. It is also possible to choose 'cancel' in the dialog and nothing will launch.

This setup makes it possible to control if and on which network (eg. "clearnet", TOR, VPN) an URL is requested - always. It also effectively mitigates the long starting times of dispVMs.

Note: when using the ask policy, the destination VM specified in srcVM by qvm-open-in-vm is ignored if no allow match exists for that given srcVM/dstVM combo.

The allow policy

If an allow policy is configured with a destination other than $dispvm it is obviously up to srcVM to provide the name of the destination VM. The RPC policies should then be configured accordingly.

Caveat: even with offline srcVMs, allow policies allow applications in srcVM to leak data through URLs. You might notice that an URL has been open in the destination VM but it would be too late.

Sample policy

In the following example, opening URLs in specific VMs is explicitely forbidden to prevent mistakenly selecting such VM, opening URLs in regular dispVMs is always allowed (see notes below), and the default policy is to have the selection dialog pop up for everything else with the "dispBrowser" VM preselected.

/etc/qubes-rpc/policy/qubes.OpenURL:

@anyvm vault deny
@anyvm private deny
@anyvm banking deny
@anyvm @dispvm allow
@anyvm @anyvm ask,default_target=dispBrowser

/etc/qubes-rpc/policy/qubes.OpenInVM:

@anyvm @anyvm ask

Notes about the @dispvm syntax:

• it is possible to further restrict the target dispVM by specifying the template it's based on with the @dispvm:templatename syntax. See the official doc for further details.
• caveat: @dispvm means "DisposableVMs based on the default DisposableVM template of the calling VM", not "any DisposableVMs". If you were to run qvm-open-in-vm @dispvm:web https://www.qubes-os.org with the policy sample above and web wasn't the default dvm template for the calling VM, @anyvm @dispvm allow wouldn't be matched and you'd be shown the selection dialog window because of the last ask line.

Considerations on dispVMs

Re-using dispVMs

In the section above we've seen how using the 'ask' RPC policy allowed us to start a (disp)VM once and use it for opening subsequent URLs (or files) to avoid having to wait insane amounts of time for dispVMs to start. However this comes at the price of a loss in compartmentalization. It is thus up to the user to carefully pick destination VMs and to manage the lifecycle of dispVMs, killing it/them when necessary when a clean state is required.

Managing changes

When opening and modifying a document in a dispVM the content is sent back to srcVM when the dispVM's process (eg. LibreOffice) closes. The dispVM's private volume is then wiped and any change that was made to the VM are discarded - eg. automatically updated add-ons, blacklists, tweaked browser preferences, ... ; The following ideas show how to cope with those "deliberate" changes:

• inter-VM copy/paste is probably the easiest way to synchronize small amounts of data in text form from the dispVM to srcVM (or to another dedicated VM like the oft-used 'vault' VM). Eg.:
  • passwords: copy/paste from/to KeepassX (or one of its forks).
  • bookmarks: copy/paste from/to
    • a plain text file
    • or an html bookmark file (most browsers can export/import such file)
    • or a dedicated bookmark manager like buku (command line manager, available in Fedora 28 repo - dnf install buku).
• other content/changes will have to be copied, usually to the dispVM templateVM. Care must be taken not to replicate compromised files: working with a freshly started dispVM and performing only the required update actions before synchronizing files with the templateVM is a good idea.

Using "named" dispVMs

If for some reason a user needs to have use a dispVM with a given name - which is for instance handy when using allow RPC policies - he/she can do like so (replace fedora-28-dvm with the dvm template you want to use):

qvm-create -C DispVM -t fedora-28-dvm -l red dstVM

This VM works like a regular VM, with the difference that its private disk is wiped after it's powered off. However it doesn't "auto power off" like random dispVMs so it's up to the user to power off (and optionaly restart) the VM when he/she deems necessary.

Sample real-world workflow

Here's an example of a real-world, thoroughly used setup/workflow:

Disposable VMs are based on the following templates:

• dvm-offline (many apps, libreoffice, VLC etc. -- no network)
• dvm-online (minimal with firefox only)
• dvm-anon (whonix workstation)

AppVMs are highly specialized: vault (offline), documents (offline), media (offline), email (firewalled). Those is where information lives. But files do not get opened nor worked on there ... only on instances of dvm-offline.

Configuring srcVM

The subsections below list various approaches on opening URLs/files from srcVM in destination VMs. A hassle-free but very powerful setup is to use the application-independent approach documented in the next subsection with the ask RPC policy.

Application-independent setup

It is possible to (re)define a default handler for programs/URLs so that this handler is automatically called by all the applications in srcVM - provided that the applications adhere to the freedesktop standard which is most always the case nowadays.

Defining a new handler simply requires creating a .desktop file and registering it. The following example shows how to open http/https URLs (along with some other common "web" Mime types) with qvm-open-in-vm:

• create $HOME/.local/share/applications/browser_vm.desktop with the following text:

  [Desktop Entry]
  Encoding=UTF-8
  Name=BrowserVM
  Exec=qvm-open-in-vm dstVM %u
  Terminal=false
  X-MultipleArgs=false
  Type=Application
  Categories=Network;WebBrowser;
  MimeType=x-scheme-handler/unknown;x-scheme-handler/about;text/html;text/xml;application/xhtml+xml;application/xml;application/vnd.mozilla.xul+xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;
  

• register the .desktop file you've just created with xdg-settings set default-web-browser browser_vm.desktop.

The same can be done with any Mime type (see man xdg-mime and xdg-settings); you could either reuse the .desktop created above and add Mime types to the MimeType= line, or create and register another .desktop file.

Notes:

• for some reasons some applications may not honor the new xdg application/handler (eg. if you had previously configured default applications), in which case you'd have to manually select the xdg application (see section below).
• qvm-open-in-vm dstVM can be replaced by a user written wrapper with custom logic for selecting a specific dstVM depending on the URL/file type, site level of trust, ... ; The RPC policies should be configured accordingly.
• very security conscious users should consider basing AppVMs on minimal templates; that way, unless the default handler is set, nothing else is usually there to open those files (little risk, plus the VMs are firewalled or offline).

Application-specific setup

Most applications provide a way to select a given program to use for opening specific URL/file (MIME) types. We can use that feature to select the /usr/bin/qvm-open-in-{vm,dvm} scripts instead of the default programs.

The subsections below show how to configure popular applications.

Firefox, Chrome/Chromium

Those browsers have an option to define programs associated to a file (MIME) type. It is pretty straightforward to configure and is outside the scope of this document.

An alternative is to use Raffaele Florio's qubes-url-redirector add-on, which provides a lot of flexibility when opening links without the hassle of having to write custom shell wrappers to qvm-open-in-vm. For instance links can be opened with a context menu and the add-on's default behavior can be configured, even with whitelist regexes.

Notes:

• the qubes-url-redirector add-on will likely be included officialy in Qubes (see this issue).
• the add-on can actually be used with applications other than firefox/chrome/chromium, the only requirement is that URLs open in a browser in srcVM. It works like so:
  • the application in srcVM opens an URL in the default browser in srcVM (eg. firefox)
  • firefox starts on srcVM, the add-on processes the URL and according to its configuration "sends" the URL to the destination VM with qubes.OpenURL
  • the URL opens in the destination VM's browser

Thunderbird

Opening attachements: "actions" must be defined, see section "Download Actions" settings" in this document.

Opening URLs: changing the way http and https URLs are opened requires tweaking configuration options; see this and this document for more information. Those changes can be made in Thunderbird's built-in config editor, or by adding the following lines to $HOME/.thunderbird/user.js:

user_pref("network.protocol-handler.warn-external.http", true);
user_pref("network.protocol-handler.warn-external.https", true);
user_pref("network.protocol-handler.expose-all", true);

Thunderbird will then ask which program to use the next time a link is opened. If dstVM is a standard (random) dispVM, choose /usr/bin/qvm-open-in-dvm. Otherwise you'll have to create a wrapper to qvm-open-in-vm since arguments cannot be passed to programs selected in Thunderbird's dialog gui. For instance, put the following text in $HOME/bin/thunderbird-open-url, make it executable, and select that program when asked which program to use:

#!/bin/sh
qvm-open-in-vm dstVM "$@"

Vi

Opening URLs: put the following in $HOME/.vimrc:

let g:netrw_browsex_viewer = 'qvm-open-in-vm dstVM'

Typing gx when the cursor is over an URL will then open it in dstVM (or will trigger a dialog if ask policy is configured, ignoring the dstVM argument).

Inter-VM copy/paste and file copy

This approach is obvious and is the simplest one:

• URLs: copy/paste the link in dstVM.
• Files: copy the file to dstVM (provided that qubes.Filecopy RPC service's policy allows it - it does by default), and open it from there.

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Contributors: @neowutran, @SvenSemmler, @Aekez, @taradiddles

Credits: @raffaeleflorio, Micah Lee