Currently, non-existing VMs match $anyvm in qrexec-policy. I suggest
that this is potentially dangerous when combined with innocent
mistakes elsewhere, and should not be the default.

From https://github.com/QubesOS/qubes-gui-daemon/pull/10:

> I did not detect this sooner because if qubes-clipboard.bin.source had
> a trailing newline, then evaluate_clipboard_policy() would not fail,
> and my dom0 clipboard-copyout script produced such a trailing newline.

This to me sounds like something that may cause trouble in the future,
both in false-denies and more importantly potentially false-allows.

If source vm names are being mangled somehow, `--assume-yes-for-ask`
allows a specific policy to fall through in a potentially surprising
way.

Consider a policy like:
    $anyvm  protected-thing  deny
    $anyvm  $anyvm  ask
or
    bad-vm  $anyvm  deny
    $anyvm  $anyvm  ask

Consider this example:
[user@dom0 qubes]$ /usr/lib/qubes/qrexec-policy --assume-yes-for-ask \
> --just-evaluate dummy_id some_source_vm_name_that_got_mangled_somehow \
> sys-firewall qubes.OpenInVM 0 && echo pwned || echo safe
pwned

This invocation of qrexec-policy was taken from
gui-daemon/gui-daemon/xside.c:
https://github.com/QubesOS/qubes-gui-daemon/blob/95417c573d9b24269d50b3733164c3c9e390851c/gui-daemon/xside.c#L753-L754

So, I propose we make qrexec-policy actually verify that an evaluated
VM actually exists in order to match $anyvm. This would mean invalid
sources would fall all the way through to the implicit deny, even in
case of `--assume-yes-for-ask`.

Thoughts?

On Sat, Nov 26, 2016 at 7:05 PM, Jean-Philippe Ouellet <j...@vt.edu> wrote:
> Consider a policy like:
>     $anyvm  protected-thing  deny
>     $anyvm  $anyvm  ask
> or
>     bad-vm  $anyvm  deny
>     $anyvm  $anyvm  ask

This pattern is even recommended in the official docs [1].

[1]: https://www.qubes-os.org/doc/copy-paste/#clipboard-automatic-policy-enforcement

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Generally source VM name always come from trusted place - out of VM
control. If you wrote it manually to qubes-clipboard.bin.source, well,
that's also "trusted" place.

Anyway, this looks like a good idea to verify it and deny the call if
there is no such VM.

- --
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
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On Sat, Nov 26, 2016 at 7:33 PM, Marek Marczykowski-Górecki
<marm...@invisiblethingslab.com> wrote:
> Generally source VM name always come from trusted place - out of VM
> control. If you wrote it manually to qubes-clipboard.bin.source, well,
> that's also "trusted" place.

Yes, but trusted in no way implies bug free.

I may not be the only one to make this particular mistake in the
future, therefore a general mitigation sounds like a good idea to me.

> Anyway, this looks like a good idea to verify it and deny the call if
> there is no such VM.

Noted. Tracking here:
- https://github.com/QubesOS/qubes-issues/issues/2460
- https://github.com/QubesOS/qubes-issues/issues/2461

Feel free to assign those issues to me if you'd like. I will get to
them when I can.