When instantiating an emulated VGA device for an x86 HVM guest qemu
will by default enable a backend to expose that device, either SDL or
VNC depending on the version of qemu and the build time configuration.
The libxl toolstack library does not explicitly disable these default
backends when they are not enabled, leading to an unexpected backend
If either SDL or VNC is explicitly enabled in the guest configuration
then only the expected backends will be enabled.
This affects qemu-xen and qemu-xen-traditional differently.
If qemu-xen was compiled with SDL support then this would result in an
SDL window being opened if $DISPLAY is valid, or a failure to start
the guest if not.
If qemu-xen was compiled without SDL support then qemu would instead
start a VNC server listening on ::1 (IPv6 localhost) or 127.0.0.1
(IPv4 localhost) with IPv6 preferred if available. A VNC password will
not be configured even if one is present in the guest configuration.
qemu-xen-traditional will never start a vnc backend unless explicitly
configured. However by default it will start an SDL backend if it was
built with SDL support and $DISPLAY is valid.
For qemu-xen compiled without SDL support (unexpected VNC server):
Any local user on the domain 0 hosting the VM will be able to access
the guest's emulated VGA console.
For any qemu compiled with SDL support (unexpected SDL backend):
Users who are able to control the DISPLAY environment variable of the
toolstack process which creates the VM will be able to direct the SDL
output to an X server of their choosing and from there gain access to
the guest's emulated console.
This is a practical attack only on systems where arrangements have
been made for lower-privileged users to execute Xen toolstack code via
means which do not sufficiently launder the process environment. This
would include some restricted sudo command configurations.
In both cases unexpected access to the guest console may then,
depending on the guest configuration, grant further privilege or
opportunities for attack.
Both cases also open up the qemu process to attacks via the VNC or X
The qemu monitor is not exposed via this means unless it is explicitly
enabled in the guest configuration.
ARM systems are not vulnerable.
PV domains are not vulnerable.
Systems where either SDL or VNC is explicitly enabled in the guest
configuration (eg `sdl=1' or `vnc=1' in the guest config file) are not
Systems using qemu-xen-traditional, or systems using qemu-xen where
SDL support is built into qemu-xen, are not vulnerable; unless the Xen
toolstack code runs in a process environment partially controlled by
x86 systems running HVM domains, configured to disable both SDL and
VNC access to the emulated VGA device, may be vulnerable.
Versions of Xen from 4.2 onwards are known to be affected. Older
versions have not been inspected.
Running qemu in a stub domain will avoid this issue.
Setting nographic to true on the domain (i.e. nographic=1 in an xl
configuration file) will completely disable the emulated VGA device
and therefore avoid this issue. (NB that publicly visible deployment
of this mitigation during the embargo is forbidden.)
In order to disable the backends while retaining the emulated VGA then
prepending "-vnc none -display none" to the qemu-xen command-line or
"-vnc none" to the qemu-xen-traditional command-line, using e.g. a
wrapper script will avoid the issue. Note that the "extra_hvm" option
exposed by the libxl library is not useful because it appends the
given options making them ineffective in this case.
Not vulnerable. This issue did not affect the versions of xen as shipped with Red Hat Enterprise Linux 5 as they did not include affected libxc library.
Created xen tracking bugs for this issue:
Affects: fedora-all [bug 1201365]