qubes-doc/VMSudo.md

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Password-less root access in VM

Background (/etc/sudoers.d/qubes in VM):

{% highlight trac-wiki %} user ALL=(ALL) NOPASSWD: ALL

WTF?! Have you lost your mind?!

In Qubes VMs there is no point in isolating the root account from

the user account. This is because all the user data are already

accessible from the user account, so there is no direct benefit for

the attacker if she could escalate to root (there is even no benefit

in trying to install some persistent rootkits, as the VM's root

filesystem modifications are lost upon each start of a VM).

One might argue that some hypothetical attacks against the

hypervisor or the few daemons/backends in Dom0 (so VM escape

attacks) most likely would require root access in the VM to trigger

the attack.

That's true, but mere existence of such a bug in the hypervisor or

Dom0 that could be exploited by a malicious VM, no matter whether

requiring user, root, or even kernel access in the VM, would be

FATAL. In such situation (if there was such a bug in Xen) there

really is no comforting that: "oh, but the mitigating factor was

that the attacker needed root in VM!" We're not M$, and we're not

gonna BS our users that there are mitigating factors in that case,

and for sure, root/user isolation is not a mitigating factor.

Because, really, if somebody could find and exploit a bug in the Xen

hypervisor -- so far there have been only one (!) publicly disclosed

exploitable bug in the Xen hypervisor from a VM, found in 2008,

incidentally by one of the Qubes developers (RW) -- then it would be

highly unlikely if that person couldn't also found a user-to-root

escalation in VM (which as we know from history of UNIX/Linux

happens all the time).

At the same time allowing for easy user-to-root escalation in a VM

is simply convenient for users, especially for update installation.

Currently this still doesn't work as expected, because some idotic

piece of software called PolKit uses own set of policies. We're

planning to address this in Beta 2. (Why PolKit is an idiocy? Do a

simple experiment: start 'xinput test' in one xterm, running as

user, then open some app that uses PolKit and asks for root

password, e.g. gpk-update-viewer -- observe how all the keystrokes

with root password you enter into the "secure" PolKit dialog box can

be seen by the xinput program...)

joanna.

{% endhighlight %}

Below is a complete list of configuration made according to the above statement, with (not necessary complete) list of mechanisms depending on each of them:

1. sudo (/etc/sudoers.d/qubes):

   {% highlight trac-wiki %} user ALL=(ALL) NOPASSWD: ALL (...) {% endhighlight %}

   • easy user->root access (main option for the user)
   • qvm-usb (not really working, as of R2)

2. PolicyKit (/etc/polkit-1/rules.d/00-qubes-allow-all.rules):

   {% highlight trac-wiki %} //allow any action, detailed reasoning in sudoers.d/qubes polkit.addRule(function(action,subject) { return polkit.Result.YES; }); {% endhighlight %}

   and /etc/polkit-1/localauthority/50-local.d/qubes-allow-all.pkla:

   {% highlight trac-wiki %} [Qubes allow all] Identity=* Action=* ResultAny=yes ResultInactive=yes ResultActive=yes {% endhighlight %}

   • NetworkManager configuration from normal user (nm-applet)
   • updates installation (gpk-update-viewer)
   • user can use pkexec just like sudo Note: above is needed mostly because Qubes user GUI session isn't treated by PolicyKit/logind as "local" session because of the way in which X server and session is started. Perhaps we will address this issue in the future, but this is really low priority. Patches welcomed anyway.

3. Empty root password

   • used for access to 'root' account from text console (xl console) - the only way to access the VM when GUI isn't working
   • can be used for easy 'su -' from user to root

Replacing password-less root access with Dom0 user prompt

While ITL still supports the statement above, some Qubes users may want to enable user/root isolation in VMs anyway. We do not support it in any of our packages, but of course nothing can stop the user from making some modifications his or her own system. A list of steps to do so is provided here without guarantee of completeness (read: do not rely on this for extra security):

1. Adding Dom0 "VMAuth" service:

   {% highlight trac-wiki %} [root@dom0 /]# echo -n "/usr/bin/echo 1" >/etc/qubes-rpc/qubes.VMAuth [root@dom0 /]# echo -n "$anyvm dom0 ask" >/etc/qubes-rpc/policy/qubes.VMAuth {% endhighlight %}

   (Note: any VMs you would like still to have password-less root access (e.g. TemplateVMs) can be specified in the second file with "<vmname> dom0 allow")

2. Configuring TemplateVM to prompt Dom0 for any authorization request:

   • In /etc/pam.d/system-auth, replace all lines beginning with "auth" with one line:

     {% highlight trac-wiki %} auth [success=done default=die] pam_exec.so seteuid /usr/lib/qubes/qrexec-client-vm dom0 qubes.VMAuth /usr/bin/grep -q ^1$ {% endhighlight %}

   • Require authentication for sudo. Replace the first line of /etc/sudoers.d/qubes with:

     {% highlight trac-wiki %} user ALL=(ALL) ALL {% endhighlight %}

   • Disable PolKit's default-allow behavior:

     {% highlight trac-wiki %} [root@fedora-20-x64]# rm /etc/polkit-1/rules.d/00-qubes-allow-all.rules [root@fedora-20-x64]# rm /etc/polkit-1/localauthority/50-local.d/qubes-allow-all.pkla {% endhighlight %}

Dom0 password-less root access

There is also password-less user->root access in dom0. As stated in comment in sudo configuration there (different one than VMs one), there is really no point in user/root isolation, because all the user data (and VM management interface) is already accessible from dom0 user level, so there is nothing more to get from dom0 root account.