Without providing custom flags at installation, snaps run confined under a restrictive security sandbox. 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 snaps.
How policy is applied
Application authors should not have to know about or understand the low-level implementation details on how security policy is enforced. Instead, all snaps run under a default security policy which can be extended through the use of interfaces. Slots, plugs, and the available interfaces available on the device can be seen with:
$ snap connections
Each command declared in
apps snap metadata is tracked by the system assigning a security label to the command. This security label takes the form of
<snap> is the name of the snap, and
<app> is the application name. For example, if this is
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 during the process confinement phase when running the application.
Under the hood, the application runner:
- Sets up various environment variables:
HOME: set to
SNAP_USER_DATAfor all commands
SNAP: read-only install directory
SNAP_ARCH: the architecture of device (eg, amd64, arm64, armhf, i386, etc)
SNAP_DATA: writable area for a particular revision of the snap
SNAP_COMMON: writable area common across all revisions of the snap
SNAP_LIBRARY_PATH: additional directories which should be added to
SNAP_NAME: snap name
SNAP_INSTANCE_NAME: snap instance name incl. instance key if one is set (snapd 2.36+)
SNAP_INSTANCE_KEY: instance key if any (snapd 2.36+)
SNAP_REVISION: store revision of the snap
SNAP_USER_DATA: per-user writable area for a particular revision of the snap
SNAP_USER_COMMON: per-user writable area common across all revisions of the snap
SNAP_VERSION: snap version (from
- 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. Hardware is assigned via interface connections.
- Sets up a private mount namespace shared across all commands of the snap
- Sets up a private /tmp using a per-snap private mount namespace and mounting a per-snap 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 install, the snap metadata 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 metadata as
slots. AppArmor policy violations in strict mode snaps will be denied access and typically have errno set to
EACCES. The violation will be logged…
Similar to AppArmor, upon snap install, the snap metadata 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 metadata as
slots. Processes with seccomp policy violations will be denied access to the system call with errno set to
EPERM (snapd releases prior to 2.32 receive
SIGSYS) and the violation is logged.
Similar to AppArmor and Seccomp, upon snap install, the snap metadata is examined and udev rules are generated for each command to tag devices so they may be added/removed to the command’s device cgroup. By default, no devices are tagged and the device cgroup is not used and only AppArmor is used to mediate access. A device cgroup may be used in addition to AppArmor depending on the declared interfaces which are expressed in the metadata as
slots. Processes accessing devices not in the snap-specific device cgroup will be denied access with errno set to
EPERM. Access violations are typically not logged.
Traditional file permissions (owner, group and other as well as file ACLs) are also enforced with snaps. Processes trying to access resources which the traditional file permissions do not allow are denied access with errno typically set to
EACCES (see the man page for the operation for specifics). Access violations are typically not logged.
Working with snap security policy
The snap metadata need not specify anything for default confinement and may optionally specify
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 the snap format and interfaces for details.
The 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.
For example, consider the following:
name: foo version: 1.0 apps: bar: command: bar baz: command: baz daemon: simple plugs: [network]
- the security label for
snap.foo.bar. It uses only the default policy
- the security label for
snap.foo.baz. It uses the
defaultpolicy plus the
networkinterface security policy as provided by the core 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 connections command. Eg:
$ snap connections Interface Plug Slot Notes home wormhole:home :home - log-observe gnome-logs:log-observe :log-observe - mount-observe gnome-system-monitor:mount-observe :mount-observe - network xkcd-webserver:network :network - network-bind xkcd-webserver:network-bind :network-bind - [...]
In the above it can be seen that the
gnome-logs 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-bind interfaces connected. An interesting quality of interfaces is that they may 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 auto-connect 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 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). For example:
$ sudo snap install --devmode mysnap
To check 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
syscall=983045 can be resolved by running the
scmp_sys_resolver command on a system of the same architecture as the one with the seccomp violation:
$ scmp_sys_resolver 983045 set_tls
If there are no seccomp violations, seccomp isn’t blocking the snap. If you notice
compat=1 in the seccomp denial, then specify the correct compatibility architecture to
-a <arch>. Eg, if on an amd64 system, use
scmp_sys_resolver -a x86 191 (use
-a arm on arm64 systems).
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
- disable kernel log rate limiting (important to not miss policy denials. Can disable manually with
sudo sysctl -w kernel.printk_ratelimit=0)
- follow /var/log/syslog looking for policy violations for
- resolve syscall names (considering
- make recommendations on how to fix violations
snappy-debug.security help for details.
Traditional file permissions are enforced but denied accesses will not be logged.
Device cgroups may also block access but denied accesses will not be logged. You can see if device cgroups are in effect by:
- seeing if there are any snapd-generated udev rules in
- if rules are defined, use
udevadm info /dev/$DEVICEto see if the snap shows up in TAGS or see if the
- examine if the
/sys/fs/cgroup/snap.$SNAPNAME.$COMMANDdirectory exists and if the device is listed in
/dev/kmsgwould have ‘
c 1:11 rwm’ since
/dev/kmsgis a character device with MAJOR:MINOR as 1:11 (see
ls -l /dev/kmsg))
If you believe there is a bug in the security policy or want to request and/or contribute a new interface, please file a bug, adding the
Interface development and security policy
When participating in snappy development and implementing new interfaces for others to use, you will almost always need to write security policy for both the slots and the plugs side of the interface but keep in mind you are not expected to write perfect security policy on the first try. The review process for snapd includes a security review of the interface security policy and it is expected that the security policy will be iterated on during the review process (in other words, if you are stuck on writing security policy but the interface otherwise works, feel free to submit the interface and ask for help).
When fine-tuning AppArmor policy, it is often easiest to install the snap in strict mode then modify the AppArmor policy in place on the target system, then copying it back. Eg, these steps might be:
- build your snap
- copy your snap to your target device and install it (or use
- use the snap (perhaps using
snap run --shell <name>.<command>), monitoring /var/log/syslog for denials
/var/lib/snapd/apparmor/profiles/snap.<name>.<command>as needed (eg, adding rules before the final
sudo apparmor_parser -r /var/lib/snapd/apparmor/profiles/snap.<name>.<command>to load the policy into the kernel
sudo service snap.<name>.<command> stop/start/etcas needed for daemons
- repeat until satisfied
The same process as above holds for seccomp except the seccomp policy is in
/var/lib/snapd/seccomp/bpf/snap.<name>.<command>.src and you must generate the bpf with
sudo /usr/lib/snapd/snap-seccomp compile /var/lib/snapd/seccomp/bpf/snap.<name>.<command>.src /var/lib/snapd/seccomp/bpf/snap.<name>.<command>.bin. The
snap-confine command will load the bpf in the
.bin file for the command when you (re)launch the command or
snap run --shell. The seccomp policy language is considerably simpler and is essentially a list of allowed syscalls.
When done, copy any changes you make to
/var/lib/snapd/seccomp/bpf/snap.<name>.<command>.src to your interface code.
For device cgroups, create or modify
/etc/udev/rules.d/70-snap.$SNAPNAME.rules as necessary (eg,
KERNEL=="kmsg" TAGS+="snap_$YOURSNAPNAME_$YOURCOMMAND" would tag
/dev/kmsg for your snap), then run
sudo udevadm trigger --action=change. To undo the access, remove the file and run the
udevadm command again. When done, update the interfaces code based on your changes.
In addition to the above, here are some other useful techniques when debugging/developing policy:
- temporarily specify
@unrestrictedin the seccomp policy and this will allow all syscalls
- temporarily use a combination of bare apparmor rules to focus on only the parts you want. Eg:
file, capability, network, mount, remount, pivot_root, umount, dbus, signal, ptrace, unix,
- look at existing policy in
interfaces/builtin/*for examples of the policy language
- stracing snaps. In addition to simply stracing the app, it can also be helpful to strace the app in both devmode and strict confinement and comparing the results.
- when testing new versions of snappy-app-dev, if re-exec is enabled you will need to copy the new version to the location udev expects it (eg,
/lib/udev) and then bind mount it over where the re-exec’d snap-confine expects it (eg,
mount --bind /lib/udev/snappy-app-dev /snap/core/<version>/lib/udev/snappy-app-dev)
Installing in devmode and developing policy can also be done; you will simply focus on getting rid of logged (but otherwise allowed) policy violations.
- https://github.com/snapcore/snapd/tree/master/interfaces for existing interface code and policy
http://wiki.apparmor.net/index.php/Profiling_by_hand (but use the paths listed above and don’t use the
aa-logproftools because they are not yet snappy-aware)
- Stracing snap commands