There is a workaround in Xen to deal with the fact that AMD CPUs don't
load the x86 registers FIP (and possibly FCS), FDP (and possibly FDS),
and FOP from memory (via XRSTOR or FXRSTOR) when there is no pending
However, this workaround does not cover all possible input cases.
This is because writes to the hardware FSW.ES bit, which the current
workaround is based on, are ignored; instead, the CPU calculates
FSW.ES from the pending exception and exception mask bits. Xen
therefore needs to do the same.
Note that part of said workaround was the subject of XSA-52.
A malicious domain may be able to leverage this to obtain sensitive
information such as cryptographic keys from another domain.
Xen versions 4.0 and onwards are vulnerable. Any kind of guest can
exploit the vulnerability.
The vulnerability is exposed only on AMD x86 systems. Intel and ARM
systems do not expose this vulnerability.
Both PV and HVM guests are affected.
The vulnerability can be avoided if the guest kernel is controlled by
the host rather than guest administrator, provided that further steps
are taken to prevent the guest administrator from loading code into
the kernel (e.g. by disabling loadable modules etc) or from using
other mechanisms which allow them to run code at kernel privilege.
On Xen versions 4.3 and earlier, turning off XSAVE support via the
"no-xsave" hypervisor command line option will avoid the vulnerability.
On Xen versions 4.4 and onwards there is no other known mitigation.
Created attachment 1136653 [details]
This patch covers versions:
xen-unstable, Xen 4.6.x, Xen 4.5.x, Xen 4.4.x
Name: the Xen project
UPDATES IN VERSION 2
Impact is less severe than previously thought: sensitive information
is very unlikely to reside in the leaked registers.
NOTE REGARDING CVE
CVE-2016-3158 is for the code change which is required for all
versions (but which is sufficient only on Xen 4.3.x, and insufficient
on later versions). Ie for the second hunk in xsa172.patch (the only
hunk in xsa172-4.3.patch), which patches the function xrstor.
CVE-2016-3159 is for the code change which is applicable for later
versions only, but which must always be combined with the code change
for CVE-2016-3158. Ie for the first hunk in xsa172.patch, which
patches the function fpu_fxrstor.
Created xen tracking bugs for this issue:
Affects: fedora-all [bug 1321944]
xen-4.5.3-1.fc22 has been pushed to the Fedora 22 stable repository. If problems still persist, please make note of it in this bug report.
xen-4.5.3-1.fc23 has been pushed to the Fedora 23 stable repository. If problems still persist, please make note of it in this bug report.
This issue does not affect the Xen hypervisor packages as shipped with Red Hat Enterprise Linux 5.