SNAP-CONFINE(8) snappy SNAP-CONFINE(8)
NAME
snap-confine - internal tool for confining snappy applications
SYNOPSIS
snap-confine [--classic] [--base BASE] SECURITY_TAG COMMAND [...AR-
GUMENTS]
DESCRIPTION
The snap-confine is a program used internally by snapd to construct the
execution environment for snap applications.
OPTIONS
The snap-confine program accepts two options:
--classic requests the so-called _classic_ _confinement_ in which
applications are not confined at all (like in classic systems, hence
the name). This disables the use of a dedicated, per-snap mount
namespace. The snapd service generates permissive apparmor and sec-
comp profiles that allow everything.
--base BASE directs snap-confine to use the given base snap as the
root filesystem. If omitted it defaults to the core snap. This is
derived from snap meta-data by snapd when starting the application
process.
FEATURES
Apparmor profiles
snap-confine switches to the apparmor profile $SECURITY_TAG. The pro-
file is mandatory and snap-confine will refuse to run without it.
The profile has to be loaded into the kernel prior to using snap-con-
fine. Typically this is arranged for by snapd. The profile contains
rich description of what the application process is allowed to do, this
includes system calls, file paths, access patterns, linux capabilities,
etc. The apparmor profile can also do extensive dbus mediation. Refer
to apparmor documentation for more details.
Seccomp profiles
snap-confine looks for the /var/lib/snapd/seccomp/bpf/$SECURITY_TAG.bin
file. This file is mandatory and snap-confine will refuse to run with-
out it. This file contains the seccomp bpf binary program that is
loaded into the kernel by snap-confine.
The file is generated with the /usr/lib/snapd/snap-seccomp compiler
from the $SECURITY_TAG.src file that uses a custom syntax that de-
scribes the set of allowed system calls and optionally their arguments.
The profile is then used to confine the started application.
As a security precaution disallowed system calls cause the started ap-
plication executable to be killed by the kernel. In the future this re-
striction may be lifted to return EPERM instead.
Mount profiles
snap-confine uses a helper process, snap-update-ns, to apply the mount
namespace profile to freshly constructed mount namespace. That tool
looks for the /var/lib/snapd/mount/snap.$SNAP_NAME.fstab file. If
present it is read, parsed and treated like a mostly-typical fstab(5)
file. The mount directives listed there are executed in order. All di-
rectives must succeed as any failure will abort execution.
By default all mount entries start with the following flags: bind, ro,
nodev, nosuid. Some of those flags can be reversed by an appropriate
option (e.g. rw can cause the mount point to be writable).
Certain additional features are enabled and conveyed through the use of
mount options prefixed with x-snapd-.
As a security precaution only bind mounts are supported at this time.
Sharing of the mount namespace
As of version 1.0.41 all the applications from the same snap will share
the same mount namespace. Applications from different snaps continue to
use separate mount namespaces.
ENVIRONMENT
snap-confine responds to the following environment variables
SNAP_CONFINE_DEBUG:
When defined the program will print additional diagnostic infor-
mation about the actions being performed. All the output goes to
stderr.
The following variables are only used when snap-confine is not setuid
root. This is only applicable when testing the program itself.
SNAPPY_LAUNCHER_INSIDE_TESTS:
Internal variable that should not be relied upon.
SNAPPY_LAUNCHER_SECCOMP_PROFILE_DIR:
Internal variable that should not be relied upon.
SNAP_USER_DATA:
Full path to the directory like /home/$LOG-
NAME/snap/$SNAP_NAME/$SNAP_REVISION.
This directory is created by snap-confine on startup. This is a
temporary feature that will be merged into snapd's snap-run com-
mand. The set of directories that can be created is confined
with apparmor.
FILES
snap-confine and snap-update-ns use the following files:
/var/lib/snapd/mount/snap.*.fstab:
Description of the mount profile.
/var/lib/snapd/seccomp/bpf/*.src:
Input for the /usr/lib/snapd/snap-seccomp profile compiler.
/var/lib/snapd/seccomp/bpf/*.bin:
Compiled seccomp bpf profile programs.
/run/snapd/ns/:
Directory used to keep shared mount namespaces.
snap-confine internally converts this directory to a private bind
mount. Semantically the behavior is identical to the following
mount commands:
mount --bind /run/snapd/ns /run/snapd/ns mount --make-private
/run/snapd/ns
/run/snapd/ns/.lock:
A flock(2)-based lock file acquired to create and convert
/run/snapd/ns/ to a private bind mount.
/run/snapd/ns/$SNAP_NAME.lock:
A flock(2)-based lock file acquired to create or join the mount
namespace represented as /run/snaps/ns/$SNAP_NAME.mnt.
/run/snapd/ns/$SNAP_NAME.mnt:
This file can be either:
• An empty file that may be seen before the mount namespace is pre-
served or when the mount namespace is unmounted.
• A file belonging to the nsfs file system, representing a fully
populated mount namespace of a given snap. The file is bind
mounted from /proc/self/ns/mnt from the first process in any snap.
/proc/self/mountinfo:
This file is read to decide if /run/snapd/ns/ needs to be created
and converted to a private bind mount, as described above.
Note that the apparmor profile is external to snap-confine and is
loaded directly into the kernel. The actual apparmor profile is managed
by snapd.
BUGS
Please report all bugs with https://bugs.launchpad.net/snapd/+filebug
AUTHOR
zygmunt.krynicki@canonical.com
COPYRIGHT
Canonical Ltd.
2.28 2017-09-18 SNAP-CONFINE(8)
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