Snap packages run confined under a restrictive security sandbox by default. The security policies and store policies work together to allow developers to quickly update their applications and to provide safety to end users.
This document describes the sandbox and how to configure and work with the security policies for snap packages.
Application authors should not have to know about or understand the lowlevel implementation details on how security policy is enforced. Instead, all snaps run under default security policy which can be extended through the use of interfaces, slots and plugs and the available interfaces available on the device can be seen with:
$ snap interfaces
The description of these interfaces is found in interfaces.md
.
Each command declared in apps
by the snap is tracked by the system by
assigning a security label to the command. This security label takes the form
of snap.<name>.<app>
where <name>
is the name of the snap from meta.md
and <app>
is the command name. For example, if this is in snap.yaml
:
name: foo
...
apps:
bar:
command: ...
...
then the security label for the bar
command is snap.foo.bar
. This security
label is used throughout the system including in the enforcement of security
policy by the app launcher. All snap commands declared via apps
in meta.md
are launched by the launcher and snaps run in the global (ie, default)
namespace (except where noted otherwise) to facilitate communications and
sharing between snaps and because this is more familiar for developers and
administrators. The security policy and launcher enforce application isolation
as per the snappy FHS. Under the hood, the launcher:
- Sets up various environment variables:
HOME
: set toSNAP_DATA
for daemons andSNAP_USER_DATA
for user commandsSNAP
: read-only install directorySNAP_ARCH
: the architecture of device (eg, amd64, arm64, armhf, i386, etc)SNAP_DATA
: writable area for a particular revision of the snapSNAP_COMMON
: writable area common across all revisions of the snapSNAP_LIBRARY_PATH
: additional directories added toLD_LIBRARY_PATH
SNAP_NAME
: snap name (frommeta.md
)SNAP_REVISION
: store revision of the snapSNAP_USER_DATA
: per-user writable area for a particular revision of the snapSNAP_USER_COMMON
: per-user writable area common across all revisions of the snapSNAP_VERSION
: snap version (frommeta.md
)TMPDIR
: set to/tmp
- When hardware is assigned to the snap, sets up a device cgroup with default devices (eg, /dev/null, /dev/urandom, etc) and any devices that are assigned to this snap
- Sets up a private /tmp using a per-command private mount namespace and mounting a per-command directory on /tmp
- Sets up a per-command devpts new instance
- Sets up the seccomp filter for the command
- Executes the command under the command-specific AppArmor profile under a default nice value
This combination of restrictive AppArmor profiles (which mediate file access, application execution, Linux capabilities(7), mount, ptrace, IPC, signals, coarse-grained networking), clearly defined application-specific filesystem areas, whitelist syscall filtering via seccomp, private /tmp, new instance devpts and device cgroups provides for strong application confinement and isolation.
Upon snap package install, snap.yaml
is examined and AppArmor profiles are
generated for each command to have the appropriate security label and
command-specific AppArmor rules. As mentioned, each command runs under an
app-specific default policy that may be extended through declared interfaces
which are expressed in the yaml as plugs
and slots
.
Like with AppArmor, upon snap package install, snap.yaml
is examined and
seccomp filters are generated for each command to run under a default seccomp
filter that may be extended through declared interfaces which are expressed in
the yaml as plugs
and slots
.
The snap.yaml
need not specify anything for default confinement and may
optionally specify plugs
and slots
to declare additional interfaces to use.
When an interface is connected, the snap's security policy will be updated to
allow access to use the interface. See meta.md
and interface.md
for
details.
The default AppArmor policy is deny by default and snaps are restricted to their app-specific directories, libraries, etc (enforcing ro, rw, etc). The seccomp filter is also deny by default and the default filter allows enough safe syscalls so that snaps using the default security policy should work.
Eg, consider the following:
name: foo
version: 1.0
apps:
bar:
command: bar
baz:
command: baz
daemon: simple
plugs: [network]
then:
- the security label for
bar
issnap.foo.bar
. It uses only the default policy - the security label for
baz
issnap.foo.baz
. It uses thedefault
policy plus thenetwork
interface security policy as provided by the OS snap
Security policies and store policies work together to provide flexibility, speed and safety. Because of this, use of some interfaces may trigger a manual review in the official Ubuntu store and/or may need to be connected by the user or gadget snap developer.
The interfaces available on the system and those used by snaps can be seen with
the snap interfaces
command. Eg:
$ snap interfaces
Slot Plug
:firewall-control -
:home -
:locale-control -
:log-observe snappy-debug
:mount-observe -
:network xkcd-webserver
:network-bind xkcd-webserver
:network-control -
:network-observe -
:snapd-control -
:system-observe -
:timeserver-control -
:timezone-control -
In the above it can be seen that the snappy-debug
snap has the log-observe
interface connected (and therefore the security policy from log-observe
is
added to it) and the xkcd-webserver
has the network
and network-bind
interfaces connected. An interesting quality of interfaces is that they may
either be either declared per-command or per-snap. If declared per-snap, all
the commands within the snap have the interface security policy added to the
command's security policy when the interface is connected. If declared
per-command, only the commands within the snap that declare use of the
interface have the interface security policy added to them.
Snappy may autoconnect the requested interfaces upon install or may require the
user to manually connect them. Interface connections and disconnections are
performed via the snap connect
and snap disconnect
commands. See
interfaces.md
for details.
Sometimes it is helpful when developing a snap to not have to worry about the security sandbox in order to focus on developing the snap. To support this, snappy allows installing the snap in developer mode which puts the security policy in complain mode (where violations against security policy are logged, but permitted). Eg:
$ sudo snap install --devmode <snap>
To check to see if you have any policy violations:
$ sudo grep audit /var/log/syslog
An AppArmor violation will look something like:
audit: type=1400 audit(1431384420.408:319): apparmor="DENIED" operation="mkdir" profile="snap.foo.bar" name="/var/lib/foo" pid=637 comm="bar" requested_mask="c" denied_mask="c" fsuid=0 ouid=0
If there are no AppArmor denials, AppArmor shouldn't be blocking the snap.
A seccomp violation will look something like:
audit: type=1326 audit(1430766107.122:16): auid=1000 uid=1000 gid=1000 ses=15 pid=1491 comm="env" exe="/bin/bash" sig=31 arch=40000028 syscall=983045 compat=0 ip=0xb6fb0bd6 code=0x0
The syscall=983045
can be resolved with the scmp\_sys\_resolver
command:
$ scmp_sys_resolver 983045
set_tls
If there are no seccomp violations, seccomp isn't blocking the snap.
The snappy-debug
snap can be used to help with policy violations. To use it:
$ sudo snap install snappy-debug
$ sudo /snap/bin/snappy-debug.security scanlog foo
This will:
- adjust kernel log rate limiting
- follow /var/log/syslog looking for policy violations for
foo
- resolve syscall names
- make reccomendations on how to fix violations
See snappy-debug.security help
for details.
If you believe there is a bug in the security policy or want to request a new
interface, please file a bug,
adding the snapd-interface
tag.