[Xen-devel] Xen Security Advisory 204 - x86: Mishandling of SYSCALL singlestep during emulation

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

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                    Xen Security Advisory XSA-204

        x86: Mishandling of SYSCALL singlestep during emulation

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

The typical behaviour of singlestepping exceptions is determined at the
start of the instruction, with a #DB trap being raised at the end of the
instruction.

SYSCALL (and SYSRET, although we don't implement it) behave differently
because the typical behaviour allows userspace to escalate its
privilege.  (This difference in behaviour seems to be undocumented.)

Xen wrongly raised the exception based on the flags at the start of
the instruction.

IMPACT
======

Guest userspace which can invoke the instruction emulator can use this
flaw to escalate its privilege to that of the guest kernel.

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

All Xen versions are affected.

The vulnerability is only exposed to 64-bit x86 HVM guests.

On Xen 4.6 and earlier the vulnerability is exposed to all guest user
processes, including unprivileged processes, in such guests.

On Xen 4.7 and later, the vulnerability is exposed only to guest user
processes granted a degree of privilege (such as direct hardware access)
by the guest administrator; or, to all user processes when the VM has
been explicitly configured with a non-default cpu vendor string (in
xm/xl, this would be done with a `cpuid=' domain config option).

A 64-bit guest kernel which uses an IST for #DB handling will most likely
mitigate the issue, but will have a single unexpected #DB exception
frame to deal with.  This in practice means that Linux is not
vulnerable.

The vulnerability is not exposed to 32-bit HVM guests.  This is because
the emulation bug also matches real hardware behaviour, and a 32-bit
guest kernel using SYSCALL will already have to be using a Task Gate for
handling #DB to avoid being susceptible to an escalation of privilege.

The vulnerability is not exposed to PV guests.

ARM systems are not vulnerable.

MITIGATION
==========

There is no known mitigation.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa204.patch           xen-unstable
xsa204-4.8.patch       Xen 4.8.x
xsa204-4.7.patch       Xen 4.7.x, Xen 4.6.x
xsa204-4.5.patch       Xen 4.5.x, Xen 4.4.x

$ sha256sum xsa204*
251c33905f86d386cc07240041108ec0664e5e9dddb2b88685d9b4b8ca7fdc24  xsa204.patch
e523b65ba122c8e22d32004d2035facaf06295094fdc8b67c151b6f44799ef0b  
xsa204-4.5.patch
d0359f26e9be783672896200e14d85a3111c29d7da580313b593fca04688fef2  
xsa204-4.7.patch
fa2a69682868104b6263655abbfc6b326f76deebdac3273b4b65da6673f5d977  
xsa204-4.8.patch
$

NOTE REGARDING EMBARGO
======================

This issue was discussed publicly on qemu-devel before its impact was
realised.
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Attachment: xsa204.patch
Description: Binary data

Attachment: xsa204-4.5.patch
Description: Binary data

Attachment: xsa204-4.7.patch
Description: Binary data

Attachment: xsa204-4.8.patch
Description: Binary data

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