[Xen-devel] Xen Security Advisory 208 (CVE-2017-2615) - oob access in cirrus bitblt copy

[Xen.org security team <security@xxxxxxx>]

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            Xen Security Advisory CVE-2017-2615 / XSA-208

                   oob access in cirrus bitblt copy

ISSUE DESCRIPTION
=================

When doing bitblt copy backwards, qemu should negate the blit width.
This avoids an oob access before the start of video memory.

IMPACT
======

A malicious guest administrator can cause an out of bounds memory
access, possibly leading to information disclosure or privilege
escalation.

VULNERABLE SYSTEMS
==================

Versions of qemu shipped with all Xen versions are vulnerable.

Xen systems running on x86 with HVM guests, with the qemu process
running in dom0 are vulnerable.

Only guests provided with the "cirrus" emulated video card can exploit
the vulnerability.  The non-default "stdvga" emulated video card is
not vulnerable.  (With xl the emulated video card is controlled by the
"stdvga=" and "vga=" domain configuration options.)

ARM systems are not vulnerable.  Systems using only PV guests are not
vulnerable.

For VMs whose qemu process is running in a stub domain, a successful
attacker will only gain the privileges of that stubdom, which should
be only over the guest itself.

Both upstream-based versions of qemu (device_model_version="qemu-xen")
and `traditional' qemu (device_model_version="qemu-xen-traditional")
are vulnerable.

MITIGATION
==========

Running only PV guests will avoid the issue.

Running HVM guests with the device model in a stubdomain will mitigate
the issue.

Changing the video card emulation to stdvga (stdvga=1, vga="stdvga",
in the xl domain configuration) will avoid the vulnerability.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa208-qemuu.patch    qemu-xen, mainline qemu
xsa208-qemut.patch    qemu-xen-traditional

$ sha256sum xsa208*
4369cce9b72daf2418a1b9dd7be6529c312b447b814c44d634bab462e80a15f5  
xsa208-qemut.patch
1e516e3df1091415b6ba34aaf54fa67eac91e22daceaad569b11baa2316c78ba  
xsa208-qemuu.patch
$


NOTE REGARDING LACK OF EMBARGO
==============================

This issue has already been publicly disclosed.
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Attachment: xsa208-qemut.patch
Description: Binary data

Attachment: xsa208-qemuu.patch
Description: Binary data

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