Xen Security Modules : XSM-FLASK

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Overview

This is intended to be a hands-on description of Xen Security Modules (XSM) as well as their structure and operation. This document has been compiled based on docs/misc/xsm-flask.txt, mailing list discussions, Xen source code, and other support material contained within the Xen source-tree.

A reader of this document should be able to: 

1. Build and install Xen with XSM enabled
2. Create and load a security policy
3. Tag virtual machines with security labels managed by the security policy
4. Utilize a simple application making use of privileges permitted by the policy

What is XSM

Xen offers a security framework called Xen Security Modules, or XSM, to enable an administrator or developer to exert fine-grained control over a Xen domain and its capabilities. Specifically, XSM makes it possible to define permissible interactions between domains, the hypervisor itself, and related resources such as memory and devices.

Via XSM, Xen implements a type of mandatory access control via a security architecture called FLASK using a module of the same name. An abbreviation for Flux Advanced Security Kernel, FLASK has its origins in several trusted operating system research projects, managed jointly between the United States’ National Security Agency, the Secure Computing Corporation, and the University of Utah. The specific intent of FLASK is to separate security enforcement from security policy, better isolating logical components of security systems.

As applied to operating system design, FLASK brings about security enhancements that improve auditing of actions and more granular control thereof. Today, it is perhaps best known through its expression in Security Enhanced Linux (SELinux).

In the same sense that SELinux is a collection of Linux security enhancements, auditing and policy mechanisms, so also is the FLASK Xen Security Module to Xen itself. Similarly, each rely upon a set of administration tools to craft security policies and guide their enforcement.

Below are some illustrative examples of what the FLASK Xen Security Module can make possible: 

- Prevent two domains from communicating via event channels or grants
- Grant a set of privileged capabilities to otherwise unprivileged domains  
- Control which domains can use device passthrough (and which devices)
- Restrict or audit operations performed by privileged domains
- Prevent a privileged domain from arbitrarily mapping pages from other domains
- Isolate hypervisor support components (such as qemu or xenstore) from each other, 
  and provide assurance that each may only perform its designated actions

Linux and Xen version requirements for use of XSM

Several important considerations exist when intending to deploy XSM. The versions of Xen and Linux are of critical importance. Below are the specific requirements:

Enabling XSM in Xen

Several steps must be performed to use XSM with Xen. First, Xen itself must be compiled with XSM. Second, a XSM FLASK policy must be created. Third and finally, the dom0 bootloader configuration must be updated to enable XSM at boot and load the policy. NOTE: these steps require a make clean, rebuild, reinstall and reboot.

Compile Xen with XSM

XSM is a compile-time option for Xen; by default, it is a disabled feature.

To use the framework, Xen must be compiled with XSM and FLASK explicitly enabled.

To do so, edit Config.mk or the .config file and set XSM_ENABLE and FLASK_ENABLE to "y"

Then, follow the standard build procedure described at Compiling_Xen_From_Source. NOTE: if a previously compiled Xen exists, be sure to 'make clean' after updating Config.mk and prior to compiling.

Compiling the sample FLASK policy

During typical Xen compilation, compilation of the FLASK sample policy is not performed due to the necessary dependency upon the SElinux compiler, otherwise known as ‘checkpolicy.’ This tool must be installed before proceeding:

Installing checkpolicy

  • apt-get install checkpolicy
  • yum install checkpolicy
  • or similar

Next, to build the sample policy itself:

  • cd xen-source-tree/
  • make -C tools/flask/policy

The demonstration policy is generated from statements defined in files within the tools/flask/policy path.. After compilation, the resultant policy will be named xenpolicy-$XEN_FULLVERSION (before 4.5 release, it's named xenpolicy.24) and reside within the directory tools/flask/policy.

Quoting from its description, this policy illustrates "most of the features of FLASK that can be used without dom0 disaggregation. The main types for domUs are: 

- domU_t is a domain that can communicate with any other domU_t
- isolated_domU_t can only communicate with dom0
- prot_domU_t is a domain type whose creation can be disabled with a boolean
- nomigrate_t is a domain that must be created via the nomigrate_t_building type, 
  and whose memory cannot be read by dom0 once created

HVM domains with stubdomain device models use two types (one per domain): 

- domHVM_t is an HVM domain that uses a stubdomain device model
- dm_dom_t is the device model for a domain with type domHVM_t"

Booting Xen with FLASK

When Xen is compiled with XSM, the following grub command-line options can be used to set FLASK properties. Please consult distribution documentation for the recommended method to update the grub configuration. Using Debian and Ubuntu as examples, these options can be placed in /etc/default/grub within the variable GRUB_CMDLINE_XEN_DEFAULT. Specifically:

  • GRUB_CMDLINE_XEN_DEFAULT="flask=<OPTION>”

When package management scripts regenerate the grub configuration (e.g., during a kernel upgrade or at the request of a sysadmin), a system configured in this manner will be guaranteed to properly embed XSM parameters within the grub configuration. For testing purposes, manually editing the grub command line at boot will also suffice.

The options for "flask=" are:

  • `permissive`: This is intended for development and is not suitable for use with untrusted guests. If a policy is provided by the bootloader, it will be loaded; errors will be reported to the ring buffer but will not prevent booting. The policy can be changed to enforcing mode using "xl setenforce".
  • `enforcing`: This requires a security policy to be provided by the bootloader and will enter enforcing mode prior to the creation of domain 0. If a valid policy is not provided, the hypervisor will not continue booting.
  • `late`: This disables loading of the security policy from the bootloader. FLASK will be enabled but will not enforce access controls until a policy is loaded by a domain using "xl loadpolicy". Once a policy is loaded, FLASK will run in enforcing mode unless "xl setenforce" has changed that setting.
  • `disabled`: This causes the XSM framework to revert to the dummy module. The dummy module provides the same security policy as is used when compiling the hypervisor without support for XSM. The xsm_op hypercall can also be used to switch to this mode after boot, but there is no way to re-enable FLASK once the dummy module is loaded.

The compiled policy must be installed into a grub-accessible location (such as /boot where kernels are also stored), as it is loaded during the Xen boot process. Otherwise, FLASK will not be enabled at runtime. It is recommended by the authors of XSM to add an extra subdirectory, such as /boot/flask, to use as a repository for XSM policies.

The policy has to be added to grub as a module after the Xen kernel multiboot line, for example as: 

 multiboot /boot/xen-VERSION.gz dom0_mem=1024M,max:1024M flask=enforcing
 module /boot/vmlinuz-X.Y-amd64 root=/dev/mapper/vg_system-root ro quiet
 module /boot/initrd.img-X.Y
 module /boot/flask/xenpolicy-VERSION

NOTE: it is critical that the ordering of the modules above be retained, as Xen expects implicitly that the kernel is the first referenced module. For Xen 4.4 and below, the policy file is always xenpolicy.24.

To automatically generate this line when running grub-update, the Xen grub update helper script can be modified (e.g., in Ubuntu/Debian by copying /etc/grub.d/20_linux_xen to 21_linux_xsm and adding the "module" line for the policy.) This helper script will differ greatly depending on installed grub version (and linux distribution).

Once Xen is booted with the FLASK policy, the policy can be reloaded using the “xl loadpolicy” command.

Creating domains with XSM security labels

DomU Configuration

Once FLASK is loaded, domains need to be created using a security label in their configuration. Without this line, the domains will be classified “unlabeled”.

Example line from a domU configuration: 

 seclabel='system_u:system_r:domU_t'

The security label’s three sections are: user, role and type (denoted by convention via _u, _r, _t suffixes, though strictly speaking, such suffixes these are not required). These will be discussed further in this document. Each component must be defined within the loaded policy to ensure proper function.

NOTE: an unlabeled domU may or may not cause a problem. If Flask is enabled, but not enforcing, the unlabelled domain is likely to work without interruption. If Flask is enabled and enforcing via a policy that explicitly permits an unlabelled domain to perform some operations, the same is also true. However, it is similarly likely that such a domain will be prevented from performing some operations (which can be problematic if the code invoking the hypercall does not tolerate failure).

Listing domUs and applied security labels

Both xl and xm utilities possess the ability to display running virtual machines with their assigned security labels.

  • For xl, use : xl list -Z
  • For xm, use : xm list --label

Example output: 

 steve@xen1:~$ sudo xl list -Z
 Name                ID    Mem VCPUs	 State    Time(s)   Security Label
 Domain-0            0    1023    24   r-----  745223.8  system_u:system_r:dom0_t
 windows-7-sp1-x86   185  1024     1   -b----   29089.3  tamas:vm_r:domU_t
 windows-xp-sp2      193   128     1   -b----   51247.2  steve:vm_r:domU_t
 centos-6-x86_64     330  2051     1   -b----    1821.8  system_u:system_r:unlabeled_t
 centos-6-i386       733   259     1   -b----     448.8  system_u:system_r:unlabeled_t

Monitoring XSM logs

XSM:Flask will emit log events "avc: denied" when a permission is denied by the policy, just like SELinux (avc being an acronym for Access Vector Cache). These log events can be viewed with xl dmesg: 

 xl dmesg | grep avc

Example output for a denied attempt by a HVM domU to invoke HVMOP_set_param: 

 (XEN) avc:  denied  { setparam } for domid=0 scontext=system_u:system_r:dom0_t
  tcontext=system_u:system_r:domU_t tclass=hvm

Within the braces is the operation that was denied. The parameters domid and scontext (source context) reference the domU that requested the operation, and tcontext (target context) describes the VM that would have been operated upon.

Familiar SELinux utilities like audit2allow can be used as a policy creation aid by processing these denied events: 

 xl dmesg | audit2allow

The output is a list of generated allow rules, which can be added to the policy via the *.te file.

FLASK Policies

The sample FLASK policy can be found in tools/flask/policy/policy/modules/xen. The two files found here are xen.te, the policy description, and xen.if, a collection of macros to be used in the policy description.

Adding new security modules

New security modules can be easily added by editing tools/flask/policy/policy/modules.conf and adding the new modules name as such: 

<module_name> = on

The module has to defined in the folder tools/flask/policy/policy/modules/<module_name> with two files: 

 <module_name>.te
 <module_name>.if

When multiple security modules are present the modules can’t contain duplicate role or type declarations.

The sample policy is recommended to be left turned on as it contains several definitions required to boot Xen.

When a new policy has been created, compile it as in the earlier section 'Compiling the sample FLASK policy.' Install the policy into its destination directory (e.g., /boot/flask), and modify the bootloader configuration as necessary.

Types, roles, users and attributes

Flask provides multiple levels of categories to abstract away from hypercalls, including types, roles, users, and attributes.

Policy Types

On the most basic level, a “type” can be defined to specify the hypercalls that type is allowed to execute and how it is allowed to execute them. When XSM is enforcing the policy, only those hypercalls will be executable that are explicitly allowed in the policy.

The sample XSM policy defines several types, for example: xen_t, dom0_t and domU_t. A new type can be defined with the “type” keyword: 

 type new_type_t;

Each type can be assigned into an attribute category, to easily reference each type having the attribute. The sample policy defines several attributes, for example: xen_type, domain_type, resource_type, etc.

A type can be assigned one or multiple attributes, for example 

 type xen_t, xen_type, mls_priv;

assigns the xen_t type the attributes xen_type and mls_priv.

Defining which hypercalls are allowed for each type is done using the “allow” keyword. Hypercalls are grouped into security classes (defined in xen/xsm/flask/policy/access_vectors) which will be discussed later. For example the following 

 allow dom0_t security_t:security check_context;

enables the dom0_t type to execute the check_context hypercall in the security class targeting a security_t type. The allow line can be generalized as follows: 

 allow <source type> <target type>:<security class> <hypercall>;

Multiple hypercalls belonging to the same security class can be defined by wrapping the hypercalls into {}, for example: 

 allow dom0_t dom0_t:resource { add remove };

Using the attributes we can quickly allow each type with that attribute access to hypercalls: 

 # Allow all domains to use (unprivileged parts of) the tmem hypercall
 allow domain_type xen_t:xen tmem_op;

Policy Roles

Roles are the second level above types, where we can define sets of types belonging to a role. While attributes are only accessible internally inside the policy (as a shortcut), a role is part of the security label. A role defines which types belong to that role, therefore it can prevent us from erroneously assigning a type to a VM when it’s role doesn’t have that type.

The sample FLASK policy defines two roles: system_r and vm_r. A role definition is as follows: 

 role system_r;
 role system_r types { xen_type domain_type };

The first line declares a new role while the second line assigns the types belonging to that role (using attributes). When we want to include all types with an attribute except some, we can exclude those types by placing “-” in front of them, like 

 role vm_r;
 role vm_r types { domain_type -dom0_t };

In the above, the vm_r role is defined as all types with the domain_type attribute, except the dom0_t type.

Policy Users

On the highest level we can define users. Users are not defined in the .te (like roles and types), rather they are defined in tools/flask/policy/policy/users. This allows us to define a generic set of users across multiple security modules. As an example, the sample FLASK policy creates three users: system_u, customer_1 and customer_2.

A user can be assigned multiple roles but by default the system_u user is assigned the system_r role, and the customer_* users are assigned the vm_r role.

A new user can be defined in the users file as 

 user <user name> roles { <role1> <role2> };

Policy Constraints

Constraints, or conditional filters, are defined in the file tools/flask/policy/policy/constraints. Constraints are a safety net to specifically disallow undesired operations that the policy may allow otherwise. The sample Flask policy defines two constraints to prevent event channels and grants between different customers. A constraints syntax is as follows: 

 constrain <security class> { <hypercall> } ( expression );

For example: 

 constrain grant { map_read map_write copy } (
     u1 == system_u or
     u2 == system_u or
     u1 == u2
 );

The constraint defines that the hypercalls belonging to the security class are only allowed when the expression is true. The expression can contain any user, role and type defined in the policy in the following format: 

 expression : ( expression )
       | not expression
       | expression and expression
       | expression or expression
       | u1 op u2
       | r1 role_op r2
       | t1 op t2
       | u1 op names
       | u2 op names
       | r1 op names
       | r2 op names
       | t1 op names
       | t2 op names

 op : == | !=
 role_op : == | != | eq | dom | domby | incomp

 names : name | { name_list }
 name_list : name | name_list name

Macros

Macros are used heavily across the sample Flask policy. For each security module a macro file can be defined as a .if file. The sample macro file (tools/flask/policy/policy/modules/xen/xen.if) contains several commonly used macros, for example for declaring a new domain type and it’s associated sub-types and to setup it’s common allow rules. Macros can call other macros. The macro .if file is required to exist for the policy to compile, even if no macros are defined for a module.

Security classes

Security classes are defined in xen/xsm/flask/policy/access_vectors and categorize each hypercall into one of the classes. Each class can hold a maximum of 32 members.

class xen

Class xen consists of operations dealing with the hypervisor itself. Unless otherwise specified, the source is the domain executing the hypercall, and the target is the xen initial sid (type xen_t).

Description Hypercall

XENPF_settime

settime

XEN_SYSCTL_tbuf_op

tbufcontrol

CONSOLEIO_read, XEN_SYSCTL_readconsole

readconsole

XEN_SYSCTL_readconsole with clear=1

clearconsole

XEN_SYSCTL_perfc_op

perfcontrol

XENPF_add_memtype

mtrr_add

XENPF_del_memtype

mtrr_del

XENPF_read_memtype

mtrr_read

XENPF_microcode_update

microcode

XEN_SYSCTL_physinfo, XEN_SYSCTL_topologyinfo, XEN_SYSCTL_numainfo

physinfo

XENPF_platform_quirk

quirk

CONSOLEIO_write

writeconsole

PHYSDEVOP_apic_read, PHYSDEVOP_alloc_irq_vector

readapic

PHYSDEVOP_apic_write

writeapic

Most XENOPROF_*

privprofile

XENOPROF_{init,enable_virq,disable_virq,get_buffer}

nonprivprofile

kexec hypercall

kexec

XENPF_firmware_info, XENPF_efi_runtime_call

firmware

XENPF_enter_acpi_sleep

sleep

XENPF_change_freq

frequency

XENPF_getidletime

getidle

XEN_SYSCTL_debug_keys

debug

XEN_SYSCTL_getcpuinfo, XENPF_get_cpu_version, XENPF_get_cpuinfo

getcpuinfo

XEN_SYSCTL_availheap

heap

XEN_SYSCTL_get_pmstat, XEN_SYSCTL_pm_op, XENPF_set_processor_pminfo,

XENPF_core_parking

pm_op

mca hypercall

mca_op

XEN_SYSCTL_lockprof_op

lockprof

XEN_SYSCTL_cpupool_op

cpupool_op

tmem hypercall (any access)

tmem_op

TMEM_CONTROL command of tmem hypercall

tmem_control

XEN_SYSCTL_scheduler_op with XEN_DOMCTL_SCHEDOP_getinfo, XEN_SYSCTL_sched_id

getscheduler

XEN_SYSCTL_scheduler_op with XEN_DOMCTL_SCHEDOP_putinfo

setscheduler

class domain and class domain2

Classes domain and domain2 consist of operations that a domain performs on another domain or on itself. Unless otherwise specified, the source is the domain executing the hypercall, and the target is the domain being operated upon(which may result in a _self or _target type). Transitions in class domain are used to produce the _self and _target types.

class domain

Description Hypercall

XEN_DOMCTL_setvcpucontext

setvcpucontext

XEN_DOMCTL_pausedomain

pause

XEN_DOMCTL_unpausedomain

unpause

XEN_DOMCTL_resumedomain

resume

XEN_DOMCTL_createdomain

create

checked in FLASK_RELABEL_DOMAIN for any relabel operation: 

 source = the old label of the domain
 target = the new label of the domain

see also the domain2 relabel{from,to,self} permissions

transition

XEN_DOMCTL_max_vcpus

max_vcpus

XEN_DOMCTL_destroydomain

destroy

XEN_DOMCTL_setvcpuaffinity

setvcpuaffinity

XEN_DOMCTL_getvcpuaffinity

getvcpuaffinity

XEN_DOMCTL_scheduler_op with XEN_DOMCTL_SCHEDOP_getinfo

getscheduler

XEN_DOMCTL_getdomaininfo, XEN_SYSCTL_getdomaininfolist

getdomaininfo

XEN_DOMCTL_getvcpuinfo

getvcpuinfo

XEN_DOMCTL_getvcpucontext

getvcpucontext

XEN_DOMCTL_max_mem

setdomainmaxmem

XEN_DOMCTL_setdomainhandle

setdomainhandle

XEN_DOMCTL_setdebugging

setdebugging

XEN_DOMCTL_hypercall_init

hypercall

XEN_DOMCTL_settimeoffset

settime

checked in XEN_DOMCTL_set_target: 

source = the new device model domain
target = the new target domain

see also the domain2 make_priv_for and set_as_target checks

set_target

SCHEDOP_remote_shutdown

shutdown

XEN_DOMCTL_set{,_machine}_address_size

setaddrsize

XEN_DOMCTL_get{,_machine}_address_size

getaddrsize

XEN_DOMCTL_sendtrigger

trigger

XEN_DOMCTL_get_ext_vcpucontext

getextvcpucontext

XEN_DOMCTL_set_ext_vcpucontext

setextvcpucontext

XEN_DOMCTL_getvcpuextstate

getvcpuextstate

XEN_DOMCTL_setvcpuextstate

setvcpuextstate

XENMEM_get_pod_target

getpodtarget

XENMEM_set_pod_target

setpodtarget

XEN_DOMCTL_subscribe, XEN_DOMCTL_disable_migrate, XEN_DOMCTL_suppress_spurious_page_faults

set_misc_info

XEN_DOMCTL_set_virq_handler

set_virq_handler


class domain2

Class domain2 is simply a continuation of class domain.

Description Hypercall

checked in FLASK_RELABEL_DOMAIN with non-DOMID_SELF: 

source = the domain making the hypercall
target = the old label of the domain being relabeled

relabelfrom

checked in FLASK_RELABEL_DOMAIN with non-DOMID_SELF: 

source = the domain making the hypercall
target = the new label of the domain being relabeled

relabelto

checked in FLASK_RELABEL_DOMAIN, only with DOMID_SELF: 

 source = the old label of the domain
 target = the new label of the domain

see also domain__transition

relabelself

checked in XEN_DOMCTL_set_target: 

 source = the domain making the hypercall
 target = the new device model domain

make_priv_for

checked in XEN_DOMCTL_set_target: 

source = the domain making the hypercall
target = the new target domain

set_as_target

XEN_DOMCTL_set_cpuid

set_cpuid

XEN_DOMCTL_gettscinfo

gettsc

XEN_DOMCTL_settscinfo

settsc

XEN_DOMCTL_scheduler_op with XEN_DOMCTL_SCHEDOP_putinfo

setscheduler


class hvm

Similar to class domain, but primarily contains domctls related to HVM domains.

Description Hypercall

XEN_DOMCTL_sethvmcontext

sethvmc

XEN_DOMCTL_gethvmcontext, XEN_DOMCTL_gethvmcontext_partial

gethvmc

HVMOP_set_param

setparam

HVMOP_get_param

getparam

HVMOP_set_pci_intx_level (also needs hvmctl)

pcilevel

HVMOP_set_isa_irq_level

irqlevel

HVMOP_set_pci_link_route

pciroute, bind_irq

XEN_DOMCTL_pin_mem_cacheattr

cacheattr

HVMOP_track_dirty_vram

trackdirtyvram

HVMOP_modified_memory, HVMOP_get_mem_type, HVMOP_set_mem_type, HVMOP_set_mem_access, HVMOP_get_mem_access, HVMOP_pagetable_dying, HVMOP_inject_trap

hvmctl

XEN_DOMCTL_set_access_required

mem_event

XEN_DOMCTL_mem_sharing_op and XENMEM_sharing_op_{share,add_physmap} with: 

 source = the domain making the hypercall
 target = domain whose memory is being shared

mem_sharing

XEN_DOMCTL_audit_p2m

audit_p2m

HVMOP_inject_msi

send_irq

checked in XENMEM_sharing_op_{share,add_physmap} with: 

 source = domain whose memory is being shared
 target = client domain

share_mem


class event

Class event describes event channels. Interdomain event channels have their own security label which is computed using a type transition between the source and target domains. Each endpoint has its own label, and the permission checks must pass on both endpoints for an event channel to be established.

Description Hypercall

when creating an interdomain event channel endpoint: 

 source = event channel label
 target = remote domain the event channel binds to. This may be a 
          _self or _target label if the endpoints are related as such.

This permission is checked when creating an unbound event channel and when the interdomain event channel is established.

bind

EVTCHNOP_send: 

 source = domain sending the event
 target = event channel label

send 

EVTCHNOP_status; same as _send

status

when creating an interdomain event channel endpoint: 

 source = the domain creating the channel (which might not be an endpoint)
 target = event channel label

create

EVTCHNOP_reset: 

 source = domain making the hypercall
 target = domain whose event channels are being reset

reset

class grant

Class grant describes pages shared by grant mappings. Pages use the security label of their owning domain.


Description Hypercall

GNTTABOP_map_grant_ref with any access

map_read

GNTTABOP_map_grant_ref with write access

map_write

GNTTABOP_unmap_grant_ref

unmap

GNTTABOP_transfer

transfer

GNTTABOP_setup_table, GNTTABOP_get_status_frames (target is commonly _self)

setup

GNTTABOP_copy

copy

GNTTABOP_query_size, GNTTABOP_get_version

query


class mmu

Class mmu describes pages of memory not accessed using grants. Permissions are checked using the domain ID used to access the page - the most common case is a domain's own ID (the _self label). Using DOMID_IO in the map command to restrict the mapping to IO memory will result in the target being domio_t, and migration uses read-only mappings with a target of DOMID_XEN (domxen_t).

Description Hypercall

checked when using mmu_update to map a page readably 

 source = domain making the hypercall (which might not own the page table)
 target = domain whose pages are being mapped

map_read

checked when using mmu_update to map a page writably 

 source = domain making the hypercall
 target = domain whose pages are being mapped

map_write

XEN_DOMCTL_getpageframeinfo*

pageinfo

XEN_DOMCTL_getmemlist

pagelist

XENMEM_{increase,decrease}_reservation, XENMEM_populate_physmap

adjust

XENMEM_{current,maximum}_reservation, XENMEM_maximum_gpfn

stat

mmu_update MMU_MACHPHYS_UPDATE

updatemp

XENMEM_add_to_physmap, XENMEM_remove_from_physmap

physmap

MMUEXT_PIN_L*_TABLE

pinpage

XENMEM_machine_memory_map (with target xen_t), XENMEM_set_memory_map (with domain target)

memorymap

checked when using mmu_update to update the page tables of another domain 

 source = domain making the hypercall
 target = domain whose page tables are being modified

remote_remap

the mmuext_op hypercall acting on the target domain

mmuext_op

XENMEM_exchange: 

 source = domain making the hypercall
 target = domain whose pages are being exchanged

exchange


class shadow

Control of the paging_domctl split by subop


Description Operation

XEN_DOMCTL_SHADOW_OP_OFF

disable

enable, get/set allocation

enable

enable, read, and clean log

logdirty


class resource

Class resource is used to describe the resources used in hardware device passthrough. Resources include: hardware IRQs, MMIO regions, x86 I/O ports, and PCI devices; see docs/misc/xsm-flask.txt for how to label them.

Access to the legacy PCI configuration space on x86 via port 0xCF8/CFC requires IS_PRIV, even with FLASK. Writes to the BARs are checked as "setup", while other reads/writes are "use"; the target is the PCI device whose configuration space is being modified. Accesses to the MMIO-based PCI express configuration space described by the ACPI MCFG table are controlled as MMIO accesses, and cannot special-case BAR writes.

The {add,remove}_{irq,ioport,iomem,device} permissions use: 

 source = domain making the hypercall
 target = resource's security label
Description Operation

checked when adding a resource to a domain: 

source = domain making the hypercall
target = domain which will have access to the resource

add

checked when removing a resource from a domain: 

source = domain making the hypercall
target = domain which will no longer have access to the resource

remove

checked when adding a resource to a domain: 

source = domain which will have access to the resource
target = resource's security label

also checked when using some core Xen devices (target xen_t)

use

PHYSDEVOP_map_pirq and ioapic writes for dom0, when acting on real IRQs 

 For GSI interrupts, the IRQ's label is indexed by the IRQ number
 For MSI interrupts, the label of the PCI device is used

add_irq

PHYSDEVOP_unmap_pirq (same as map, and only for real IRQs)

remove_irq

XEN_DOMCTL_ioport_permission, XEN_DOMCTL_ioport_mapping

add_ioport, remove_ioport

XEN_DOMCTL_iomem_permission, XEN_DOMCTL_memory_mapping

add_iomem, remove_iomem

XEN_DOMCTL_get_device_group, XEN_DOMCTL_test_assign_device: 

 source = domain making the hypercall
 target = PCI device being queried

stat_device

XEN_DOMCTL_assign_device

add_device

XEN_DOMCTL_deassign_device

remove_device

checked for PCI hot and cold-plug hypercalls, with target as the PCI device checked for CPU and memory hotplug with xen_t as the target

plug

checked for PCI hot-unplug hypercalls, with target as the PCI device checked for CPU offlining with xen_t as the target

unplug

checked for PHYSDEVOP_restore_msi* (target PCI device) checked for PHYSDEVOP_setup_gsi (target IRQ) checked for PHYSDEVOP_pci_mmcfg_reserved (target xen_t)

setup


class security

Class security describes the FLASK security server itself; these operations are accessed using the xsm_op hypercall. The source is the domain invoking the hypercall, and the target is security_t.

Any domain with access to load_policy or setenforce must be trusted, since it can bypass the rest of the security policy.

Description Operation
use the security server to compute an access check compute_av
use the security server to compute a type transition compute_create
use the security server to compute member selection compute_member
sid <-> context string conversions check_context
allow loading a new XSM/FLASK policy load_policy
use the security server to compute an object relabel compute_relabel
use the security server to list the SIDs reachable by a given user compute_user
allow switching between enforcing and permissive mode setenforce
allow changing policy booleans setbool
allow changing security server configuration parameters setsecparam
add ocontext label definitions for resources add_ocontext
remove ocontext label definitions for resources del_ocontext

XSM Limitations

XSM control over Xenstore operations

FLASK currently provides locally defined access vectors for controlling xenstore operations; however, xenstored and oxenstored do not currently provide support for managing domU access to xenstore as managed by such access vectors.

Future development is required to complete this feature.

Until this feature is available, only the much coarser xenstore permission model can be used to control domU access to xenstore data.

Xen versions prior to 4.3

Xen versions 4.2 and earlier lack full XSM support in hypercall handlers, setting a -EPERM return code for many hypercalls. This is due to XSM being considered a preview feature in development at that time. For these versions, only sysctl hypercalls and domctl hypercall XEN_DOMCTL_getdomaininfo were truly usable by non-privileged domains. The following commit was the one to remedy this situation [[1]]

Linux versions prior to 3.8

Linux versions prior to 3.8 carried a hard-coded check for xen_initial_domain() in the privcmd device handlers for dispatching hypercalls on the behalf of userspace. This caused -EPERM to be returned for all domUs other than dom0 when attempting to use a hypercall, even if that hypercall was explicitly enabled and permitted by the security label assigned to the calling domU. The relevant patch is a simple one ( [[2]] ) and is known to apply to earlier versions like 3.7.1 without modification. Back-porting should be trivial in most cases.

Hypercalls invoked directly within the kernel should not be subject to the consideration above, as it only applies to the privcmd device handler code for userspace.

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