doc/source/arch_sandboxing.rst - buildstream - Git at Google


 .. _sandboxing:


 Sandboxing
 ==========

 Introduction
 ------------

 BuildStream assembles each element in a *sandbox*. The sandbox is a container
 environment which serves two purposes: giving BuildStream control over
 all build aspects in order to ensure reproducibility of build results,
 and providing safety guarantees for the host system that BuildStream is
 running on.

 The exact implementation of the sandbox varies depending on which platform you
 are running BuildStream. See below for backend-specific details.

 There are several factors that affect the build output and must therefore be
 under BuildStream's control:

 * Filesystem contents and metadata
 * The user and permissions model
 * Network access
 * Device access

 Each of these is detailed below.

 For safety reasons, BuildStream also controls the following things:

 * Access to files outside of the sandbox directory
 * Access to certain kernel-specific syscalls

 Creating a sandbox can require special priviliges. This is a safety concern too
 because bugs in the `bst` program can cause damage to a host if the program is
 running with extra privileges. The exact priviliges that are required depend on
 your platform and backend.

 Element plugins can run arbitary commands within the sandbox using the
 :mod:`sandbox API <buildstream.sandbox.sandbox>`.

 What elements can and can't do in the sandbox
 ---------------------------------------------

 This section specifies how BuildStream sandboxes are intended to work. A
 specific sandbox provider may not necessarily be able to achieve all of the
 requirements listed below so be sure to read the "platform notes" section as
 well.

 Filesystem access
 ~~~~~~~~~~~~~~~~~

 The filesystem inside sandboxes should be read-only during element assembly,
 except for certain directories which element plugins can mark as being
 read/write. Most elements plugins derive from :mod:`BuildElement
 <buildstream.buildelement>`, which marks ``%{build-root}`` and
 ``%{install-root}`` as read/write.

 When running integration commands or `bst shell`, the sandbox should have a
 fully read-write filesystem. The changes made here do not need to persist
 beyond the lifetime of that sandbox, and **must not** affect the contents of
 artifacts stored in the cache.

 Certain top level directories should be treated specially in all sandboxes:

 * The ``/dev`` directory should contain device nodes, which are described in
   a separate section.

 * The ``/proc`` directory should have a UNIX 'procfs' style filesystem mounted.
   It should not expose any information about processes running outside of the
   sandbox.

 * The ``/tmp`` directory should be writable.

 Filesystem metadata
 ~~~~~~~~~~~~~~~~~~~

 The writable areas inside a BuildStream sandbox are limited in what metadata
 can be written and stored.

 * All files must be owned by UID 0 and GID 0
 * No files may have the setuid or setgid bits set
 * Extended file attributes (xattrs) cannot be written to or read.
 * Hardlinks to other files can be created, but the information about which
   files are hardlinked to each other will not be stored in the artifact
   that is created from the sandbox.

 These restrictions are due to technical limitations. In future we hope to
 support a wider range of filesystem metadata operations. See `issue #38
 <https://github.com/apache/buildstream/issues/38>`_ for more details.

 User and permissions model
 ~~~~~~~~~~~~~~~~~~~~~~~~~~

 All commands inside the sandbox run with user ID 0 and group ID 0. It should
 not be possible to become any other user ID.

 Network access
 ~~~~~~~~~~~~~~

 Builds should not be able to access the network at all from the sandbox. All
 remote resources needed to build an element must be specified in the element's
 ``sources`` list so that BuildStream is able to see when they have changed.

 A sandbox opened by `bst shell` should allow network access.

 Device access
 ~~~~~~~~~~~~~

 Builds should not be able to access any hardware devices at all.

 A few standard UNIX device files are needed, the whitelist is:

 * ``/dev/full``
 * ``/dev/null``
 * ``/dev/urandom``
 * ``/dev/random``
 * ``/dev/zero``

 It may seem odd that we have sources of randomness in the sandbox, but a lot of
 tools do expect them to exist. We take the view that it's up to integrators to
 ensure that elements do not deliberately include randomness in their output.

 A sandbox opened by `bst shell` can make any devices available. There needs to
 be a console device so that it can be used interactively.

 Platform notes
 --------------

 BuildStream delegates sandboxing for local builds to the ``buildbox-run``
 command. ``buildbox-run`` provides a platform-independent interface to execute
 commands in a sandbox based on parts of the Remote Execution API.

 Linux
 ~~~~~

 The recommended ``buildbox-run`` implementation for Linux is
 ``buildbox-run-bubblewrap``, in combination with ``buildbox-fuse``.

 These implementations use the following isolation and sandboxing primitives:

 * bind mounts
 * FUSE
 * Mount namespaces
 * Network namespaces
 * PID (process ID) namespaces
 * User namespaces (if available)
 * seccomp

 We access all of these features through a sandboxing tool named `Bubblewrap
 <https://github.com/projectatomic/bubblewrap/>`_.

 User namespaces are not enabled by default in all Linux distributions.
 BuildStream still runs on such systems but can't build projects that set
 ``build-uid`` or ``build-gid`` in the ``sandbox`` configuration.

 The Linux platform can operate as a standard user, if unprivileged user namespace
 support is available. If user namespace support is not available you have the
 option of installing bubblewrap as a setuid binary to avoid needing to run the
 entire ``bst`` process as the ``root`` user.

 FUSE is used to provide access to directories and files stored in CAS without
 having to copy or hardlink the complete input tree into a regular filesystem
 directory structure for each build job.

 Some of the operations on filesystem metadata listed above are not prohibited
 by the sandbox, but will instead be silently dropped when an artifact is
 created. For more details see `issue #38
 <https://github.com/apache/buildstream/issues/38>`_.

 Some details of the host machine are currently leaked by this platform backend.
 For more details, see `issue #262
 <https://github.com/apache/buildstream/issues/262>`_.

 Other POSIX systems
 ~~~~~~~~~~~~~~~~~~~

 On other POSIX systems ``buildbox-run-userchroot`` may be used for sandboxing.
 `userchroot <https://gitlab.com/BuildGrid/buildbox/userchroot>`_ allows regular
 users to invoke processes in a chroot environment.

 ``buildbox-run-userchroot`` stages the input tree for each build job using
 hardlinks to avoid more expensive file copies. To avoid cache corruption it is
 vital that hardlinked files cannot be overwritten. Due to this it's required
 to run ``buildbox-casd`` as a separate user, which owns the files in the local
 cache.

 Network access is not blocked in the chroot. However since there is unlikely
 to be a correct `/etc/resolv.conf` file, any network access that depends on
 name resolution will most likely fail anyway.

	.. _sandboxing:


	Sandboxing
	==========

	Introduction
	------------

	BuildStream assembles each element in a sandbox. The sandbox is a container
	environment which serves two purposes: giving BuildStream control over
	all build aspects in order to ensure reproducibility of build results,
	and providing safety guarantees for the host system that BuildStream is
	running on.

	The exact implementation of the sandbox varies depending on which platform you
	are running BuildStream. See below for backend-specific details.

	There are several factors that affect the build output and must therefore be
	under BuildStream's control:

	* Filesystem contents and metadata
	* The user and permissions model
	* Network access
	* Device access

	Each of these is detailed below.

	For safety reasons, BuildStream also controls the following things:

	* Access to files outside of the sandbox directory
	* Access to certain kernel-specific syscalls

	Creating a sandbox can require special priviliges. This is a safety concern too
	because bugs in the `bst` program can cause damage to a host if the program is
	running with extra privileges. The exact priviliges that are required depend on
	your platform and backend.

	Element plugins can run arbitary commands within the sandbox using the
	:mod:`sandbox API <buildstream.sandbox.sandbox>`.

	What elements can and can't do in the sandbox
	---------------------------------------------

	This section specifies how BuildStream sandboxes are intended to work. A
	specific sandbox provider may not necessarily be able to achieve all of the
	requirements listed below so be sure to read the "platform notes" section as
	well.

	Filesystem access
	~~~~~~~~~~~~~~~~~

	The filesystem inside sandboxes should be read-only during element assembly,
	except for certain directories which element plugins can mark as being
	read/write. Most elements plugins derive from :mod:`BuildElement
	<buildstream.buildelement>`, which marks ``%{build-root}`` and
	``%{install-root}`` as read/write.

	When running integration commands or `bst shell`, the sandbox should have a
	fully read-write filesystem. The changes made here do not need to persist
	beyond the lifetime of that sandbox, and must not affect the contents of
	artifacts stored in the cache.

	Certain top level directories should be treated specially in all sandboxes:

	* The ``/dev`` directory should contain device nodes, which are described in
	a separate section.

	* The ``/proc`` directory should have a UNIX 'procfs' style filesystem mounted.
	It should not expose any information about processes running outside of the
	sandbox.

	* The ``/tmp`` directory should be writable.

	Filesystem metadata
	~~~~~~~~~~~~~~~~~~~

	The writable areas inside a BuildStream sandbox are limited in what metadata
	can be written and stored.

	* All files must be owned by UID 0 and GID 0
	* No files may have the setuid or setgid bits set
	* Extended file attributes (xattrs) cannot be written to or read.
	* Hardlinks to other files can be created, but the information about which
	files are hardlinked to each other will not be stored in the artifact
	that is created from the sandbox.

	These restrictions are due to technical limitations. In future we hope to
	support a wider range of filesystem metadata operations. See `issue #38
	<https://github.com/apache/buildstream/issues/38>`_ for more details.

	User and permissions model
	~~~~~~~~~~~~~~~~~~~~~~~~~~

	All commands inside the sandbox run with user ID 0 and group ID 0. It should
	not be possible to become any other user ID.

	Network access
	~~~~~~~~~~~~~~

	Builds should not be able to access the network at all from the sandbox. All
	remote resources needed to build an element must be specified in the element's
	``sources`` list so that BuildStream is able to see when they have changed.

	A sandbox opened by `bst shell` should allow network access.

	Device access
	~~~~~~~~~~~~~

	Builds should not be able to access any hardware devices at all.

	A few standard UNIX device files are needed, the whitelist is:

	* ``/dev/full``
	* ``/dev/null``
	* ``/dev/urandom``
	* ``/dev/random``
	* ``/dev/zero``

	It may seem odd that we have sources of randomness in the sandbox, but a lot of
	tools do expect them to exist. We take the view that it's up to integrators to
	ensure that elements do not deliberately include randomness in their output.

	A sandbox opened by `bst shell` can make any devices available. There needs to
	be a console device so that it can be used interactively.

	Platform notes
	--------------

	BuildStream delegates sandboxing for local builds to the ``buildbox-run``
	command. ``buildbox-run`` provides a platform-independent interface to execute
	commands in a sandbox based on parts of the Remote Execution API.

	Linux
	~~~~~

	The recommended ``buildbox-run`` implementation for Linux is
	``buildbox-run-bubblewrap``, in combination with ``buildbox-fuse``.

	These implementations use the following isolation and sandboxing primitives:

	* bind mounts
	* FUSE
	* Mount namespaces
	* Network namespaces
	* PID (process ID) namespaces
	* User namespaces (if available)
	* seccomp

	We access all of these features through a sandboxing tool named `Bubblewrap
	<https://github.com/projectatomic/bubblewrap/>`_.

	User namespaces are not enabled by default in all Linux distributions.
	BuildStream still runs on such systems but can't build projects that set
	``build-uid`` or ``build-gid`` in the ``sandbox`` configuration.

	The Linux platform can operate as a standard user, if unprivileged user namespace
	support is available. If user namespace support is not available you have the
	option of installing bubblewrap as a setuid binary to avoid needing to run the
	entire ``bst`` process as the ``root`` user.

	FUSE is used to provide access to directories and files stored in CAS without
	having to copy or hardlink the complete input tree into a regular filesystem
	directory structure for each build job.

	Some of the operations on filesystem metadata listed above are not prohibited
	by the sandbox, but will instead be silently dropped when an artifact is
	created. For more details see `issue #38
	<https://github.com/apache/buildstream/issues/38>`_.

	Some details of the host machine are currently leaked by this platform backend.
	For more details, see `issue #262
	<https://github.com/apache/buildstream/issues/262>`_.

	Other POSIX systems
	~~~~~~~~~~~~~~~~~~~

	On other POSIX systems ``buildbox-run-userchroot`` may be used for sandboxing.
	`userchroot <https://gitlab.com/BuildGrid/buildbox/userchroot>`_ allows regular
	users to invoke processes in a chroot environment.

	``buildbox-run-userchroot`` stages the input tree for each build job using
	hardlinks to avoid more expensive file copies. To avoid cache corruption it is
	vital that hardlinked files cannot be overwritten. Due to this it's required
	to run ``buildbox-casd`` as a separate user, which owns the files in the local
	cache.

	Network access is not blocked in the chroot. However since there is unlikely
	to be a correct `/etc/resolv.conf` file, any network access that depends on
	name resolution will most likely fail anyway.