[Xen-devel] [Fwd: Xen 3.4 feature request (security)]

["Stephen Spector" <stephen.spector@xxxxxxxxxx>]

From: Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>
Organization: Invisible Things Lab
 

Hello,

On behalf of the Invisible Things Lab team I propose addition of the
following security features/changes into Xen 3.4 roadmap:

1) No self-modifying code in the hypervisor.

Currently there is at least one place where the hypervisor builds a
trampoline
that is later executed (emulate_privileged_op() which builds a "stub" on
the
stack for I/O emulation). Such approach has not only a problem of being
inelegant and potentially dangerous, the bigger problem is that it makes
runtime
verification of the hypervisor code integrity more difficult.

2) Strict application of the NX/XD bit in the hypervisor.

Every page that doesn't contain code should be marked as non-executable.
Currently this is not the case, as e.g. the Xen's heap is executable.
There
should be no pages that contain both the code and data. Strict NX
application
would not only make the exploitation of bugs in the hypervisor harder
(as e.g.
the FLASK overflow that we exploited during Black Hat last month [1]),
but would
also allow to create much simpler and more effective runtime integrity
scanners
for the hypervisor (see our Black Hat slides about the HyperGuard
project done
together with Phoenix [2]).

3) Make the Xen system more auditable. Currently the Xen hypervisor does
not
expose several important information about the system. For example the
"xm list"
command (and the underlying hypercall) do not present an information
whether
each domain is privileged or not. As a consequence it is possible for an
attacker, that got access to the hypervisor, to set the privileged bit
for one
of the DomUs, allowing it to e.g. map memory at will from other domains
to his
or her domain (See [3] for details of a practical implementation of such
backdoor).

With current Xen implementation such Xen compromise would be
undetectable. We
believe such scenario could be used e.g. in the hosting scenarios, where
various
domains are rented to different customers. During one of our Black Hat
presentation we have presented a rootkit that does just that. Note that
such
rootkit doesn't require modifications of any code in the hypervisor, nor
adding
any new code, just setting the is_privileged flag for some domain. Thus
if we
imagined a runtime hypervisor integrity scanner, like the mentioned
above
HyperGuard, it would still not detect the system compromise.

To make this auditing complete, the hypervisor should also expose a set
of MFNs
that a given domain effectively is allowed to use (so, the set
containing any
MFN that is present in any page table that the domain can set as its
current
page table). This is to address a more complex variation of the
compromise
described above where, instead of privileging a domain, the attacker
simply maps
some MFN to the domain, e.g. by manually modifying the page tables
inside the
hypervisor.

We would be happy to answer/discuss any questions/doubts you might have
regarding the above propositions.

Regards,
Joanna Rutkowska
Invisible Things Lab, CEO
http://invisiblethingslab.com/

[1]
http://theinvisiblethings.blogspot.com/2008/08/our-xen-0wning-trilogy-hi
ghlights.html

[2] http://invisiblethingslab.com/bh08/part2.pdf

[3] http://invisiblethingslab.com/bh08/part1.pdf
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