---===[ Qubes Security Bulletin #27 ]===---

                          November 22, 2016


      Xen 64-bit bit test instruction emulation broken (XSA 195)


Quick Summary
==============

The Xen Security Team has discovered a serious security bug (XSA 195)
in the hypervisor instruction emulation code which might potentially
lead to privilege escalation [1]:

| When Xen needs to emulate [BT, or BT{C,R,S}] instruction (...) the
| memory address and register operand are recalculated internally to
| Xen. In this process, the high bits of an intermediate expression
| were discarded, leading to both the memory location and the register
| operand being wrong.
|  
| The wrong memory location would have only a guest local effect
| (either access to an unintended location, or a fault delivered to
| the guest), whereas the wrong register value could lead to either a
| host crash or an unintended host memory access.

There have been also a few other Xen-related bugs that are being
disclosed today by the Xen team [2], most likely with none or only
minor impacts on Qubes OS (e.g. information leaks from the hypervisor
memory, which might ease exploitation of other potential bugs, but
unlikely to reveal any information from other VMs).

Description of the bug
=======================

The x86_emulate() function, which is internally used by the hypervisor
to emulate guest instructions in some situations, contains an
arithmetic bug resulting from implicit type conversions between 64-bit
and 32-bit integers:

    byte_offset = op_bytes + (((-src.val-1) >> 3) & ~(op_bytes-1));
    ea.mem.off += (src.val >> 3) & ~(op_bytes - 1);

The '1' immediate above should have been suffixed by 'L', letting the
compiler know it should not be clamped to 32-bit, which in turn causes
the above expressions to incorrectly allow for huge offsets when
emulating the BT* instructions.

A malicious VM can trigger a condition in which: 1) the execution of
the BT* instruction will be intercepted and emulated by the
hypervisor, and 2) the calculated offset will fall far beyond the page
mapped by the hypervisor for the purpose of emulating the instruction
(the hypervisor needs to map a page from the guest to emulate the BT
instruction when the destination operand is a memory address). This
will likely lead to the host crash, but a possibility of exploiting
this bug for privilege escalation cannot be excluded.

The Xen Security team has pointed out in a discussion with us that a
reliable exploitation will likely be complicated by the uncertainty of
the Xen stack absolute address, as the emulated opcodes will use
stack-relative addressing, which, among other things, would also
depend on which VCPU the attacker's VM happens to be currently
scheduled on.

Impact
=======

In theory this is a critical bug allowing full privilege escalation
from a gust to the hypervisor, but for the reasons described above
reliable exploitation might not be trivial. Nevertheless, one should
never underestimate creativity of exploit authors, and thus should
assume the bug is exploitable and patch immediately, just to stay on
the safe side.

More discussion
===============

This is the second serious bug in Xen's x86_emulate() that has
affected Qubes OS. The first such bug has been described in QSB18
(March 2015).

It's tempting to think that instruction emulation (on x86) might be
required only for PV guests and could be ditched for HVM guests,
specifically those using SLAT (HAP in Xen parlance). Sadly this
doesn't seem to be feasible. The main culprit, as pointed out by Xen
developers, is the MMIO virtualization. And while it might be possible
to run some HVM guests without the need for instruction emulator, this
doesn't seem feasible for the VMs with assigned PCI devices. This is
rather unfortunate, especially for Qubes OS, as the VMs with PCI
devices assigned, such as the USB or Network VMs, are often considered
untrusted.

Apparently the x86 architecture cannot escape its clumsiness despite
all the efforts by Intel to add new technologies on top of it.

Another solution might be to attempt to isolate the x86 instruction
emulator, similarly to how the I/O emulator is already isolated in a
dedicated stub domain on Qubes OS (thanks to Xen stub domains). The
Xen Security Team pointed out, however, that this has already been
attempted in the past, but the effort was ultimately gave up on, due
to apparently an increase of the interface complexity required to make
that work. It's unclear what the performance impact of such a change
might be.

On the other hand though some other, arguably more experimental,
hypervisors, such as Nova [3], do isolate instruction-emulating code,
together with most of the VMM functionality, away from the
(micro)hypervisor. Again, it's unclear to us what the performance (or
other) impact might be of that solution, but it surely looks
interesting from the security point of view (at least considering x86
platform).

On the positive side though, the bug has been found by the Xen
Security Team during the internal effort for improving critical code
security, and the team promises even more efforts in the near future
to further improve the hypervisor code quality.

We shall stress one more time that the instruction emulator
(implemented by the x86_emulate() function on Xen) discussed in this
bulletin should not be confused with the I/O device emulator
(typically implemented by the qemu process on many virtualization
systems). The latter is already well isolated on Qubes OS, thanks to
Xen's stub domain features.

Patching
=========

The specific packages that resolve the problem discussed in this
bulletin (and also brining a few other reliability patches to Xen)
have been uploaded to the security-testing repository:

For Qubes 3.1 and 3.2:
* xen packages, version 4.6.3-24

The packages are to be installed in Dom0 via qubes-dom0-update command
or via the Qubes graphical manager.

A system restart will be required afterwards.

If you use Anti Evil Maid, you will need to reseal your secret
passphrase to new PCR values, as PCR18 will change because of a new
xen.gz binary.

These packages will be moved to the current repository over the coming
days once they receive more testing from the community.

Credits
========

This bugs has been found by Xen Security Team, and reported via Xen
Security mailing list.

Additionally, we would like to thank members of the Xen Security Team
for discussing with us the details of the bugs as well as sharing
their thoughts on possible future workarounds.

We also would like to thank Norman Feske of Genode Labs for explaining
to us how does the Nova hypervisor handle x86 instruction emulation.


References
===========

[1] http://xenbits.xen.org/xsa/advisory-195.html
[2] http://xenbits.xen.org/xsa/
[3] http://hypervisor.org/

--
The Qubes Security Team
https://www.qubes-os.org/security/