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RFC-0206: Deprecate stash

RFC-0206: Deprecate stash
StatusAccepted
Areas
  • Security
  • Storage
Description

Deprecate the stash service and migrate clients to use a storage capability

Issues
Gerrit change
Authors
Reviewers
Date submitted (year-month-day)2022-12-08
Date reviewed (year-month-day)2022-01-13

Summary

The stash service is unmaintained and the current implementation does not provide the properties it was initially created to deliver. This RFC proposes deprecating stash. We will migrate the clients using stash to Fuchsia's standard persistent storage capabilities then delete the three components serving the stash service.

Motivation

Stash is a simple persistence service originally created for use by platform components that need to access persistent mutable state early in the boot process, generally prior to software update.

As discussed in more detail below, three different variants of the stash protocol exist. All variants use the same underlying FIDL protocol and are implemented using the same binary. This document uses the word "stash" to cover all of these variants.

Each time Fuchsia reads persistent mutable state prior to software update is potentially an opportunity for an attacker to persist their access: If an attacker had gained full control of a device in a previous power cycle we must assume they were able to write arbitrary data into the device's persistent mutable state. A vulnerability in a code path that reads mutable data would potentially let this attacker-controlled data exploit the system in the next power cycle. When the read occurs early in the boot process the consequences can be even more severe; potentially the attacker can prevent a software update from occurring and therefore prevent the vulnerability from being patched.

Stash was intended to reduce the risk of this scenario by providing a simple and secure persistent mutable storage for use in early boot. Stash was intended to have three properties:

  1. Minimal - Stash should use a significantly smaller and simpler codebase than the full storage stack, making the implementation easier to review and reducing the risk of bugs.
  2. Easy to use - Stash should be easy for clients to use, reducing the risk of bugs in their integration with persistent mutable state.
  3. Secure - Since stash exists to improve the security of the system, its design and implementation should not introduce new security problems.

Currently stash is failing to provide most of these properties:

  1. Minimal - Stash is implemented as a FIDL server, on top of a serialization layer, on top of the standard filesystem (either fxfs or minfs on top of zxcrypt). Stash is no simpler than the standard storage stack. In theory it would be possible to migrate to a simpler storage implementation in the future without changing the FIDL interface, but there are no plans to do this.
  2. Easy to use - Stash is mainly meeting this objective. Its FIDL interface exposes a simple key-value paradigm with a limited number of data types. This keeps client code generally simple and reduces the likelihood of bugs in client code. The need to handle backwards compatibility, transactional writes, and FIDL errors does introduce some complexity and in cases clients have still needed to write their own helper libraries, e.g. wlan.
  3. Secure - The design of stash assumed component framework would provide client identity, letting a FIDL server identify its clients. Component framework does not provide client identity and there are no plans to introduce it in the foreseeable future. This means different stash clients can read and write each other's data and the protocol relies on an honor system to avoid this.

Although stash was initially intended to support only "early boot" components there is no formal definition of an early boot phase on Fuchsia or list of which components are considered "early boot". Several clients of stash do not start early in the boot process and none of the existing clients start during bootstrap.

Since 2020, Fuchsia has managed a complex system of duplication and isolation to work around the security problem noted above (see fxbug.dev/47602).

  • Three different FIDL stash protocols have been defined: fuchsia.stash.Store, fuchsia.stash.Store2, and fuchsia.stash.SecureStore.
  • Three different stash components exist, each running as a separate process and serving a separate protocol.
  • Stash clients are carefully assigned to one of the three protocols and the clients sharing a channel are assessed to ensure the risk of them reading or writing each other's data is acceptable.
  • A BUILD visibility list exists to prevent the addition of new stash clients without security review.

This situation has led to an ongoing confusion and engineering cost while its performance and security properties are barely adequate.

Stakeholders

Facilitator:

  • hjfreyer

Reviewers:

  • atait (DHCP)
  • brunodalbo (Netstack)
  • ecstone (Migration)
  • emircan (Scenic)
  • jamuraa (Bluetooth)
  • nmccracken (WLAN)
  • palmer (Security)
  • senj (Omaha Client)
  • paulfaria (Settings service)

Consulted:

  • silberst, wittrock, shayba, cgonyeo, erahm, mnck, jfsulliv

Socialization:

An early draft of this RFC was developed in collaboration with stakeholders from all impacted clients.

Design

We intend to deprecate stash and migrate most existing clients to the "data" storage capability used by other components. Storage capabilities are accessed using a standard file system API. For most components this migration will involve using a serialization library to convert the component's persistent data structures to byte streams that the component then writes to disk. Reading persistent data will involve reading files from disk then using the same library to deserialize and populate the component's persistent data structures.

Scenic is not using stash for the intended purpose (ref fxbug.dev/91585) and we will migrate their use case to structured config developer overrides.

It is always desirable to minimize the attack surface of components that start early in boot and can impact the success of a software update. The networking and SWD components that currently rely on stash should be prudent in their use of the data storage capability but we will not maintain a separate set of storage access requirements for these components. The best practices for securely persisting data to a storage capability include:

  • Minimize the amount of data that is persisted.
  • Use narrow and precisely defined data types.
  • Use a security-reviewed serialization library to pack and unpack data.

This design can be assessed against the desirable stash properties as follows:

  1. Minimal - The overall complexity is similar to the status quo: a serialization library is still used (although usage moves from a common location to each client). Fxfs (or minfs and zxcrypt) is still present. It would still be possible to back the storage capability by a simpler filesystem in the future although doing so may require changes in the clients.
  2. Easy to use - The ease of use is similar to the status quo: clients must interact with a serialization library and perform basic filesystem operations instead of managing FIDL connections, transactions, and failures. There are several existing implementations of persisting Fuchsia component state using a storage capability and these implementations could be reused.
  3. Security - Security is improved: The design guarantees isolation between components. We use existing Fuchsia technologies that have already passed security review and are already used in production.

In addition, the design improves several other properties:

  • Resource utilization is lower since we remove three component instances
  • It is easier to attribute disk utilization to the component using it

Implementation

If this proposal is accepted we will clearly document the stash component and all stash protocols as deprecated.

We will then work with each of the seven impacted client components to agree a plan and approximate timeline for migration. In some cases the team is already planning to migrate away from stash (e.g. fxbug.dev/91403), in other cases the trusted platform services team could help with the migration. This RFC does not specify a deadline for completing the migration.

Components that store critical data in stash will require a stepping stone release to complete their migration (i.e. a software release that every device must pass through during its software upgrade process). At this stepping stone release the component would be able to read its persistent state from either stash or the storage capability but would write to the storage capability. Passing through this stepping stone would ensure data is migrated to the storage capability before the component removes its code to read stash.

As we work with stash clients to plan their migrations we aim to minimize the number of platform stepping stone releases that are required.

Once all clients of a particular stash protocol have completed their migration, that protocol and the instance of stash serving it will be deleted. The stash binary will be deleted along with the last protocol.

Performance

This proposal will reduce disk, memory, and CPU utilization by deleting three component instances.

Security considerations

This proposal increases the security of Fuchsia by guaranteeing that early boot components can no longer read and write each other's persistent state and by eliminating code that is currently not maintained.

Privacy considerations

This proposal does not alter the set of user data that is collected and stored. There is a small improvement in privacy because a compromised early boot component could no longer read PII data stored by a different component.

Testing

Existing end to end tests and integration tests cover the storage and retrieval of persistent mutable state using stash. These same tests will cover the storage and retrieval of this state using the basic storage capability. Each stash client should add integration tests to validate migration of data from stash to the storage capability.

Documentation

Existing stash documentation will be deleted once migration is complete.

Alternatives Considered

Alternative 1: Use a new "basic" storage capability

The current proposal recommends using the existing "data" storage capability. A similar solution would have been to create a new "basic" storage exclusively for use by early boot components.

Using a separate storage capability lets us track components that should eventually use a simpler storage solution. Components that use the "basic" storage capability follow a set of best practices intended to reduce complexity, reduce the risk of bugs, and simplify migration to a future replacement backend. The best practices may include:

  • Use an approved serialization library to pack and unpack data. For example, serde for Rust components
  • Update the content of files atomically. For example, by writing into a temp file then renaming the temp file to the final path
  • Do not create subdirectories
  • Do not create files larger than X kB
  • Do not create more than Y files

Many of these best practices feed into simplifications that we could make in a replacement filesystem to back the "basic" storage capability. For example, it would be easier to migrate to a trivial filesystem that did not support directories if the client did not use directories.

The security team maintains a list of which components are considered "early boot". Automated tooling verifies that these components only use the "basic" storage capability and, where practical, verifies that their usage of the file system is consistent with best practices.

The initial implementation for this solution would be simple: "basic" could be backed by a new subdirectory on the existing "data" fxfs or minfs partition. However, there would be significant process and tooling cost to maintain a consistent definition of "early boot" and build the tooling to enforce data persistence patterns in these early boot components.

Fuchsia does not plan to implement a simpler filesystem to back a "basic" storage capability. Without a different filesystem there would be very little benefit in spending resources to maintain two different sets of clients backed by the same implementation, hence this solution was not selected.

Alternative 2: Write a dedicated client library for early mutable state

The current proposal recommends using existing mature libraries to serialize mutable persistent state. An alternative would be to write a new client library in each of the target languages (currently Rust and Go, potentially C++ at some point in the future).

A dedicated client library could easily enforce best practices and could be both smaller and easier to use than the existing general purpose serialization libraries like serde. However, designing and implementing these new libraries would significantly increase the engineering cost of this proposal. The existing clients use stash for different purposes with different wrappers so a single client library may not meet the expectations of all these clients. Stash is currently unstaffed and it is likely that committing to designing and implementation new client libraries would have delayed migration by several quarters.

Prior art and references