RFC: Ubuntu Seeded Snaps

Steve Langasek steve.langasek at ubuntu.com
Thu Feb 8 23:10:08 UTC 2018

Dear developers,

As I'm sure you know, Canonical has recently been putting a lot of work into
the Snap Store[1], a repository for third-party packages that is the successor
to the past extras.ubuntu.com[2] and click packages efforts.

We are confident that snaps today represent a solid delivery vehicle for
third-party software on top of Ubuntu, and that snaps stand as a first-class
alternative to deb packages for Ubuntu users where appropriate.

Snaps are already presented alongside debs in the software catalog on the
Ubuntu Desktop, and with the 17.10 release, the Ubuntu MATE team took the
first foray into including snaps by default in an Ubuntu flavor image.  Now
in Ubuntu 18.04 LTS, we are looking at broadening the inclusion of snaps in
Ubuntu images by default.  This raises important questions about what the
policies should be for software installed by default as a snap, since the
review processes around the Ubuntu archive for universe and main don't
directly translate to the Snap Store.

I have prepared a draft which lays out what I believe the requirements
should be around snaps which we ship preinstalled, and I would greatly
appreciate the feedback of the Ubuntu Developer community around this
proposed policy:


I have also included the text of this draft below for your convenience.

= Goal =
Snaps represent a new way of building packages with reduced barriers to
entry.  By design, the snapcraft tooling imposes very little policy to avoid
also introducing friction.  As more software becomes available as snaps, we
want to take advantage of this body of packages as part of the default
Ubuntu experience, but maintaining the Ubuntu community’s commitments around
this default experience means reintroducing policy on top of snaps.  This
document is an attempt to translate existing policy for the Ubuntu archive
to the new world of the Canonical Snap Store.

= Channel availability =
Including software in the default install of Ubuntu implies a certain
commitment to handle upgrades cleanly and to provide continuity of behavior
across updates within the stable release.  The best way to ensure this
commitment holds true in the snap case is to only include snaps that come
from the stable channel.

As a side effect, since devmode snaps may not be published to the stable
channel, only strict and classic confined snaps may be included.

Snaps included in images will be installed referencing a per-Ubuntu-series
branch.  This ensures forwards-compatibility by allowing publishing to this
branch if the mainline of a snap becomes incompatible with a given Ubuntu
release, without requiring up-front maintenance of multiple snap channels.

= Maintainer =
Packages in the Ubuntu archive arrive there by one of two means: they are
synced from Debian as upstream, or they are uploaded by an Ubuntu developer. 
Similarly, to be included in an Ubuntu image, a snap should have as its
publisher either the upstream, or the Ubuntu developer community.  For the
latter, a common team should initially be created in the Snap Store whose
membership is managed by the Developer Membership Board, and kept in sync
with the ubuntu-motu team in Launchpad, with the Ubuntu Security team
additionally included.

= Source availability =
Unlike Launchpad, the Snap Store allows publishers to upload binary snaps
directly.  While a valuable option in the general case, for snaps installed
by default we should ensure that they build from source in the common
Launchpad environment.  This helps to avoid any increase to the build time
attack surface and provides a known good environment that can be similarly
duplicated if the snaps needs to be rebuilt in the future

In addition, maintainability of the product demands that the package remains
buildable if no changes have been made to the product’s source.  For .deb
packages, we enforce this by only building against other packages in the
Ubuntu distribution.  Launchpad allows snap builds to pull from third-party
repositories; this means that if those repositories change - or disappear -
the snap may no longer be functionally equivalent when rebuilt, or may not
build at all.  To address this, official Ubuntu snaps should be built only
from source that is available in Launchpad.  Snap recipe builds already
require a launchpad-hosted branch to host the snapcraft.yaml, so it is a
logical extension to require launchpad hosting for the parts also.

Both of these requirements will likely depend on changes to Launchpad and
possibly the Snap Store, to either support enforcing a different network
policy at build time or to tag builds as compliant or not with this policy.

= License =
The license policy covering Ubuntu main and restricted is documented at
https://www.ubuntu.com/about/about-ubuntu/licensing.  Snaps included by
default in Ubuntu installs should comply with this policy the same as .debs

Partner-specific images and images for community flavors may include
software that does not meet the Ubuntu main/restricted licensing policy, at
the discretion of those images’ owners, in accordance with existing

= Security Support =
Maintenance of the snap must include a clear designation of ownership of
security support.  The process for including a snap in an Ubuntu image
should include a sign-off by the Ubuntu Security Team to confirm that the
security support story is adequate.  The snap confinement model means that
in-depth code reviews should not generally be required for strict-mode snaps
that only require safe interface connections.  Classic mode snaps will
likely require more scrutiny.  The same security checks listed on
https://wiki.ubuntu.com/UbuntuMainInclusionRequirements for debs in main are
relevant to snaps.  The MIR team will be the gatekeeper for the snap
inclusion process as well and coordinate with the Security Team as
appropriate.  As an initial policy, “as appropriate” means every snap to be
installed by default in the Ubuntu image.  This policy can be revisited
after a period of one year.  Owning teams are responsible for security
support in accordance with the Ubuntu Security Team’s guidelines for
security support of Canonical-supported snaps.  A report will be provided by
the Ubuntu engineering team of the high-level CVE status across all the
snaps included in the Ubuntu image.

The Snap Store ecosystem empowers snap publishers to make their own
decisions about how and whether to backport security fixes to stable
releases vs. updating the package in the channel to a new upstream version. 
We accept this model as well for installed-by-default snaps, with the
understanding that the publisher of each of these snaps is expected to
deliver a good experience to their users.

For cases where the Ubuntu community is the maintainer of the snap rather
than upstream, it is recommended to prefer targeted backports of security
fixes where possible.


Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org

[1] https://snapcraft.io/
[2] http://wiki.ubuntu.com/AppReviewBoard/Charter
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