SnappyConfinement
14277
Comment:
|
14425
|
Deletions are marked like this. | Additions are marked like this. |
Line 311: | Line 311: |
* Mediate binding to network ports (separate specification) * seccomp, network namespaces, ?? * perhaps handled via namespaces if done well |
Created: 2014-12-05
Created by: Jamie Strandboge
Contributors: Jamie Strandboge, Marc Deslauriers
Packages affected: apparmor, apparmor-easyprof-ubuntu-snappy, click-apparmor, snappy-systemd
Status: Alpha
Introduction
Snappy confinement is an evolution of the security model for Ubuntu Touch. The basic concepts for confined applications and the AppStore model pertain to snappy applications as well. In short, applications are confined using AppArmor by default and this is achieved through a simple template-based system where policy is extended through the use of policy groups.
It will be most helpful if you are familiar with:
Implementation summary
Snappy uses a simple packaging format that is an evolution of click packaging. Snappy packages use a declarative yaml syntax and defaults to using default AppArmor template and the "networking" policy group. Apps may customize the default behavior in a number of ways via the yaml syntax.
Under the hood, the security manifest is used to describe the confinement of the app. Most apps do not need to specify anything for confinement and snappy will create a security manifest like the following:
{ "policy_vendor": "ubuntu-snappy", "policy_version": 1.3, "template": "default", "policy_groups": [ "networking" ] }
The defaults provided may change as the confinement needs evolve. The template policy is found in /usr/share/apparmor/easyprof/ubuntu-snappy/templates/1.3/default and the networking policy is found in /usr/share/apparmor/easyprof/ubuntu-snappy/policy-groups/1.3/networking.
"apparmor"
Security policy in snappy is done through apparmor integration. Internally apparmor integration generates an AppArmor profile based on what is in the snappy packaging yaml. Importantly, the AppArmor profile name is unique to the application binary/service and is achieved by using the concept of an ApplicationId. The APP_ID is the composition of the package name, the service/binary name and package version. The APP_ID takes the form of <pkgname>_<appname>_<version>. For example, if this is in packaging yaml:
name: foo version: 0.1 ... services: - name: bar start: bin/bar
Then the APP_ID for the 'bar' service is foo_bar_0.1.
Applications are launched under confinement through the following:
If unspecified in the packaging yaml, snappy will choose the default profile and networking policy group (this may change as snappy involves)
Apps may choose to specify an alternate confinement binaries and services by specifying security-template and/or security-policy-groups in the yaml. Eg:
... services: - name: bar start: bin/bar security-template: nondefault security-policy-groups: networking, something, etc
For special circumstances, the packaging yaml may also use apparmor in its 'integration' section:
... services: - name: bar start: bin/bar integration: bar: apparmor: meta/bar.apparmor
aa-clickhook is run on package install and places the resulting profile in /var/lib/apparmor/profiles.
If the binary is a service, the snappy-systemd makes sure /etc/systemd/system/<service>.service has AppArmorProfile=<profilename> (in the above example, AppArmorProfile=foo_bar_0.1) such that the service starts under confinement.
If the binary is a cli binary, it can be launched under confinement via the bin-path mechanism (see below)
- apparmor policy templates and policy groups are usually shipped via the apparmor-easyprof-ubuntu-snappy package
The default template allows ELF executables, python, perl and shell (with selected corresponding utilities from /bin and /usr/bin), disallows capabilities(7) and enforces application isolation as per the snappy FHS.
"apparmor-profile"
In addition to the above, specialized, hand-crafted confinement may be done via the apparmor-profile integration section. Specifying apparmor-profile will trigger a manual review in the store, and is not needed for normal Snappy apps, but instead is provided for snappy framework and trusted snappy applications.
The aa-profile-hook works similarly to aa-clickhook:
The snappy packaging specifies to use 'apparmor-profile' in its 'integration' section, which specifies the profile. Eg:
... services: - name: bar start: bin/bar integration: bar: apparmor-profile: meta/bar.profile
the profile should reference the AppArmor variables for CLICK_DIR, APP_PKGNAME, APP_APPNAME, APP_VERSION and for profile attachment, just like the apparmor template does with click-apparmor
aa-profile-hook is run on install and places the profile in /var/lib/apparmor/profiles (prefixed with 'profile_')
If the binary is a service, the snappy-systemd makes sure /etc/systemd/system/<service>.service has AppArmorProfile=<profilename> (in the above example, AppArmorProfile=foo_bar_0.1) such that the service starts under confinement.
aa-profile-hook is shipped in click-apparmor
use of apparmor-profile requires manual review via the store.
To improve maintenace, the profile author can use AppArmor variables to avoid worrying about updating the profile name, the app name, where the app is installed or knowing the package version. Example profile:
#include <tunables/global> # Specified profile variables ###VAR### ###PROFILEATTACH### (attach_disconnected) { #include <abstractions/base> #include <abstractions/nameservice> # Read-only for the install directory @{CLICK_DIR}/@{APP_PKGNAME}/ r, @{CLICK_DIR}/@{APP_PKGNAME}/@{APP_VERSION}/ r, @{CLICK_DIR}/@{APP_PKGNAME}/@{APP_VERSION}/** mrklix, # Writable home area owner @{HOMEDIRS}/apps/@{APP_PKGNAME}/ rw, owner @{HOMEDIRS}/apps/@{APP_PKGNAME}/** mrwklix, # Read-only system area for other versions /var/lib/apps/@{APP_PKGNAME}/ r, /var/lib/apps/@{APP_PKGNAME}/** mrkix, # Writable system area only for this version. /var/lib/apps/@{APP_PKGNAME}/@{APP_VERSION}/ w, /var/lib/apps/@{APP_PKGNAME}/@{APP_VERSION}/** wl, ... specialized confinement ...
Example yaml to use the profile:
name: foo version: 0.1 ... services: - name: bar start: bin/bar integration: bar: apparmor-profile: meta/bar.profile
You may not specify the apparmor and apparmor-profile for the same <appname> (eg, specifying "apparmor-profile": "meta/appname.profile" and "apparmor": "meta/appname.apparmor" is an error).
"apparmor-policy"
apparmor-policy is provided for frameworks to extend security policy (specifically, to add templates and/or policy groups) so that apps that specify the framework may then reference these template/policy groups to access any services/binaries that the framework exposes.
Key ideas for snappy frameworks:
- Frameworks extend Ubuntu Core in useful ways for apps to use
- Frameworks are delivered via snaps
- Because frameworks require additional privilege and are considered trusted (how much depends on what the framework offers), framework authors must partner with Canonical on framework design, security policy verification, acceptance in the store, etc
- Frameworks are expected to provide significant functionality that is useful to a wide range of apps
- Multiple snappy frameworks may be installed on the same system
- The number of snappy frameworks is expected to be relatively low
See the SnappyFrameworkConfinement for more details.
"bin-path"
If the yaml specifies 'binaries' then bin-path will expose these binaries to the shell and other apps by launching apps via a specialized launcher that does the following:
- chdir()s to the app install directory
- sets up any environment
- set up mount namespace and in that mount namespace, setup an root overlay and /tmp
- sets up cgroup net_cls
- launches the app under confinement
It does this by adding a symlink to a directory in your PATH which points to the launcher. The launcher then calculates the APP_ID to launch the app under. This launcher will setup the environment, adjust the working directory and launch the binary under its AppArmor profile. For example, consider the following snappy packaging yaml:
name: foo version: 0.1 ... binaries: - name: bin/bar
bin-path will create the foo/bar symlink in a directory in your PATH (creating the 'foo' directory as needed. Also, notice this is <name>/<basename of binary name>). When executing that symlink, the launcher will:
- chdir() to /apps/foo/current
- set miscellaneous environment variables
- set up mount namespace and in that mount namespace, setup an root overlay and /tmp
- set up a cgroup for net_cls named foo_bar_0.1
run the app under the foo_bar_0.1 profile
Note, as of 2014-12-09, a shell script is temporarily being used instead of a symlink.
Normal usage
As stated, the snappy packaging yaml does not have to do anything to specify the default confinement. Eg, the following yaml:
name: foo version: 0.1 ... services: - name: bar start: bin/owncloud - name: baz start: bin/owncloud - name: norf start: bin/owncloud
will create the following AppArmor profiles with default confinement:
- foo_bar_0.1
- foo_baz_0.1
- foo_norf_0.1
Advanced usage
While the snappy packaging yaml is intentionally simple and straightforward for app developers, it can also quite flexible for those who need it. For example, consider the following yaml:
name: foo version: 0.1 ... services: - name: normal-service start: bin/normal-service - name: extra-policy-group-service start: bin/extra-policy-group-service security-policy-groups: networking, extra - name: non-default-template-service start: bin/non-default-template-service security-template: non-default binaries: - name: bin/normal-binary - name: extra-policy-group-binary security-policy-groups: networking, extra
snappy will generate the following security manifests:
normal-service:
{ "policy_vendor": "ubuntu-snappy", "policy_version": 1.3, "template": "default", "policy_groups": [ "networking" ] }
extra-policy-group-service:
{ "policy_vendor": "ubuntu-snappy", "policy_version": 1.3, "template": "default", "policy_groups": [ "networking", "extra" ] }
non-default-template-service:
{ "policy_vendor": "ubuntu-snappy", "policy_version": 1.3, "template": "non-default", "policy_groups": [ "networking" ] }
normal-binary:
{ "policy_vendor": "ubuntu-snappy", "policy_version": 1.3, "template": "default", "policy_groups": [ "networking" ] }
extra-policy-group-binary:
{ "policy_vendor": "ubuntu-snappy", "policy_version": 1.3, "template": "default", "policy_groups": [ "networking", "extra" ] }
Which in turn creates the following AppArmor profiles:
- foo_normal-service_0.1
- foo_extra-policy-group-service_0.1
- foo_non-default-template-service_0.1
- foo_normal-binary_0.1
- foo_extra-policy-group-binary_0.1
Debugging
To check to see if you have any denials:
$ sudo journalctl --no-pager -k | grep DEN
A denial will look something like:
apparmor="DENIED" operation="mkdir" profile="foo_bar_0.1" name="/var/lib/foo" pid=637 comm="bar" requested_mask="c" denied_mask="c" fsuid=0 ouid=0
If there are no denials, AppArmor shouldn't be blocking the app.
If there are denials, you can unblock yourself by:
- modifying the profile in /var/lib/apparmor/profiles that corresponds to your app
reload the profile with:
$ sudo apparmor_parser -r /var/lib/apparmor/profiles/<profile>
Do note that the local modification will not be preserved on package update. If you believe you have found a bug, please file a bug against: https://bugs.launchpad.net/ubuntu/+source/apparmor/+filebug
Helpful degugging commands
sudo aa-clickhook -f will regenerate all the apparmor profiles in /var/lib/apparmor/profiles
sudo aa-profile-hook -f will regenerate all the apparmor-profile profiles in /var/lib/apparmor/profiles
sudo aa-status will show you the profiles loaded in the kernel and what processes are running under them
ps Z, ps Z <pid> and ps auxwwZ will show you normal ps output, but with the apparmor label the profile is running under
sudo systemctl stop <app>.service and sudo systemctl start <app>.service to stop and start services from /etc/systemd/system (ie, where snappy-systemd puts the service files)
- This is often helpful when developing your app or policy for it:
In one terminal launch (tail kernel log for AppArmor denials):
$ sudo journalctl -f -k | grep DEN
In another (service name is the filename in /etc/systemd/system):
$ sudo journalctl -f -u <service name>
Then launch the app with systemctl like above, or launch manually under confinement with:
$ aa-exec -p <profile name> -- /apps/<pkgname>/<version>/...
Future work
Confining snappy apps (particularly processes requiring root) may use AppArmor-wrapped user namespaces.
- Mediate binding to network ports (separate specification)
- seccomp, network namespaces, ??
- perhaps handled via namespaces if done well
SecurityTeam/Specifications/SnappyConfinement (last edited 2016-11-17 16:29:21 by jdstrand)