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These notes are compiled from xen-devel questions and postings that have occurred
since the inclusion of XSM. These notes are not intended to be definitive
documentation but should address many common problems that arise when
experimenting with XSM:FLASK.
Xen XSM:FLASK configuration
---------------------------
1) cd xen-unstable.hg
2) edit Config.mk in the toplevel xen directory as follows:
XSM_ENABLE ?= y
FLASK_ENABLE ?= y
ACM_SECURITY ?= n
NB: Only one security module can be selected at a time. If no module is
selected, then the default DUMMY module will be enforced. The DUMMY module
only exercises the security framework and does not enforce any security
policies. Changing the security module selection will require recompiling xen.
These settings will also configure the corresponding toolchain support.
3) make xen
4) make tools
Xen XSM:FLASK policy
--------------------
These instructions will enable the configuration and build of the sample policy.
The sample policy provides the MINIMUM policy necessary to boot a
paravirtualized dom0 and create a pv or hvm domU. Many of the
default capabilities and usages supported by dom0/domU are disallowed by the
sample policy. Further, the policy is comprised of a limited number of types and
must be adjusted to meet the specific security goals of the installation.
Modification of the policy is straightforward and is covered in a later section.
NB: The policy is not automatically built as part of the tool support because
of an external dependancy on the checkpolicy compiler. The FLASK policy uses
the same syntax and structure as SELinux and compiling the policy relies on
the SELinux policy toolchain. This toolchain is available under many
distributions as well as the following URL,
http://userspace.selinuxproject.org/trac/wiki/Releases
You will need at least libsepol and checkpolicy in order to compile a policy.
1) cd xen-unstable.hg/tools/flask/policy
2) make policy
3) make install
4) edit /etc/grub.conf, add a module line to the xen entry,
module /xenpolicy.24
NB: The .24 suffix reflects the policy format version and may differ for your
system depending on the version of checkpolicy you used to build the policy.
At the time of this writing, policy version 24 is the highest version
supported by the latest checkpolicy release and by the Xen Flask module.
You can force the policy build to a specific policy version by uncommenting
the OUTPUT_POLICY= line in the policy Makefile and setting the value as
desired (to any version supported by the Xen Flask module, presently in the
range 15-24). Make sure that your module line above matches the actual
/xenpolicy.NN file that was created in /boot by the make install.
5) reboot, and select the updated xen entry
NB: The module entry can be inserted on any line after the xen kernel line. Typical
configurations use the last module entry or the module entry that immediately
follows the xen kernel entry.
If you want to use the MLS policy, then set TYPE=xen-mls in the policy Makefile
before building the policy. Note that the MLS constraints in policy/mls
are incomplete and are only a sample.
Xen configuration of xend
-------------------------
1) cd /etc/xen
2) edit xend-config.sxp
3) uncomment the line containing the key:value pair entry,
#(xsm_module_name dummy)
4) change the value entry to 'flask'
(xsm_module_name flask)
5) restart xend
Creating policy controlled domains
----------------------------------
2) Edit the domain config file and add the following entry for a pv guest,
access_control = ["policy=,label=system_u:system_r:domU_t"]
or add the following entry for a hvm guest:
access_control = ["policy=,label=system_u:system_r:domHU_t"]
NB: The 'policy' field is not used by XSM:FLASK. The 'label' must exist in the
loaded policy. 'system_u:system_r:domU_t' is one of the existing labels from
the sample policy and shown for example purposes.
If you enabled the MLS policy, then append a MLS level (e.g. s0:c0) to the
labels, e.g.:
access_control = ["policy=,label=system_u:system_r:domU_t:s0:c0"]
2) Create the domain using the 'xm create' command.
3) Use the 'xm list --label' command to list the running
domains and their labels.
Updating the XSM:FLASK policy
-----------------------------
It is recommended that the XSM:FLASK policy be tailored to meet the specific
security goals of the platform. The policy is tailored by editing the xen.if and xen.te files in the 'policy' subdirectory.
1) cd xen-unstable.hg/tools/flask/policy
2) edit policy/modules/xen/xen.* - make changes to support platform security goals.
3) make policy
4) make install
5) reboot
Alternatively, one may reload the policy using the 'flask_loadpolicy' tool
installed by the xen tools.
1) flask_loadpolicy policy.24
NB: The sample policy permits policy reloads as well as general manipulation of
the Flask security server only from dom0. The policy can be tailored further to
restrict policy reloads and other manipulations to boot-time only, by removing
the corresponding statements from the policy.
Enforcing the XSM:FLASK policy
------------------------------
By default, XSM:FLASK is compiled and installed in permissive mode. This
configuration will allow an XSM:FLASK system to start in enforcing mode.
1) edit /etc/grub.conf
2) append the parameter 'flask_enforcing=1' to the xen kernel line.
3) reboot, and select the updated xen entry
AVC denials
-----------
XSM:Flask will emit avc: denied messages when a permission is denied
by the policy, just like SELinux. For example, if you were to use
system_u:system_r:domU_t label for a hvm guest (rather than
system_u:system_r:domHU_t), you would get a set of denials upon xm
create:
# xm dmesg | grep avc
(XEN) avc: denied { setparam } for domid=0 scontext=system_u:system_r:dom0_t tcontext=system_u:system_r:domU_t tclass=hvm
(XEN) avc: denied { getparam } for domid=0 scontext=system_u:system_r:dom0_t tcontext=system_u:system_r:domU_t tclass=hvm
(XEN) avc: denied { irqlevel } for domid=0 scontext=system_u:system_r:dom0_t tcontext=system_u:system_r:domU_t tclass=hvm
(XEN) avc: denied { pciroute } for domid=0 scontext=system_u:system_r:dom0_t tcontext=system_u:system_r:domU_t tclass=hvm
(XEN) avc: denied { setparam } for domid=4 scontext=system_u:system_r:domU_t tcontext=system_u:system_r:domU_t tclass=hvm
(XEN) avc: denied { cacheattr } for domid=0 scontext=system_u:system_r:dom0_t tcontext=system_u:system_r:domU_t tclass=hvm
(XEN) avc: denied { pcilevel } for domid=0 scontext=system_u:system_r:dom0_t tcontext=system_u:system_r:domU_t tclass=hvm
Existing SELinux tools such as audit2allow can be applied to these denials, e.g.
xm dmesg | audit2allow
The generated allow rules can then be fed back into the policy by
adding them to xen.te, although manual review is advised and will
often lead to adding parameterized rules to the interfaces in xen.if
to address the general case.
Additional notes on XSM:FLASK
-----------------------------
1) xen command line parameters
a) flask_enforcing
The default value for flask_enforcing is '0'. This parameter causes the
platform to boot in permissive mode which means that the policy is loaded
but not enforced. This mode is often helpful for developing new systems
and policies as the policy violations are reported on the xen console and
may be viewed in dom0 through 'xm dmesg'.
To boot the platform into enforcing mode, which means that the policy is
loaded and enforced, append 'flask_enforcing=1' on the grub line.
This parameter may also be changed through the flask hyercall.
b) flask_enabled
The default value for flask_enabled is '1'. This parameter causes the
platform to enable the FLASK security module under the XSM framework.
The parameter may be enabled/disabled only once per boot. If the parameter
is set to '0', only a reboot can re-enable flask. When flask_enabled is '0'
the DUMMY module is enforced.
This parameter may also be changed through the flask hypercall. But may
only be performed once per boot.
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