aarch64-dit: initial crate (#1102)

Adds a crate with wrappers for the Data-Independent Timing (DIT) feature
of AArch64 CPUs.

The implementation is largely a translation of Apple's guide of how to
write wrappers for enabling/disabling DIT:
https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Enable-DIT-for-constant-time-cryptographic-operations

It would be nice to wrap that all up into an RAII guard which can first
use `cpufeatures` to check for `FEAT_DIT` and, if available, enable it
for the current thread, while also first querying the processor status
register and restoring the previous state on `Drop`, which is necessary
for proper nested usage of DIT.

But for now, this just wraps the barebones functionality in an `unsafe`
API.

.github/workflows/aarch64-dit.yml

@@ -0,0 +1,37 @@
+name: aarch64-dit
+
+on:
+  pull_request:
+    paths:
+      - "aarch64-dit/**"
+      - "Cargo.*"
+  push:
+    branches: master
+
+permissions:
+  contents: read
+
+defaults:
+  run:
+    working-directory: aarch64-dit
+
+env:
+  CARGO_INCREMENTAL: 0
+  RUSTFLAGS: "-Dwarnings"
+
+jobs:
+  macos:
+    strategy:
+      matrix:
+        toolchain:
+          - 1.61.0 # MSRV
+          - stable
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: RustCrypto/actions/cargo-cache@master
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.toolchain }}
+          targets: aarch64-apple-darwin
+      - run: cargo test

.github/workflows/workspace.yml

@@ -22,7 +22,7 @@ jobs:
         with:
           toolchain: 1.72.0 # Pinned to prevent breakages
           components: clippy
-      - run: cargo clippy --all --all-features -- -D warnings
+      - run: cargo clippy --all --all-features --exclude aarch64-dit -- -D warnings
 
   rustfmt:
     runs-on: ubuntu-latest

Cargo.toml

@@ -1,6 +1,7 @@
 [workspace]
 resolver = "2"
 members = [
+    "aarch64-dit",
     "blobby",
     "block-buffer",
     "block-padding",

aarch64-dit/CHANGELOG.md

@@ -0,0 +1,5 @@
+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).

aarch64-dit/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "aarch64-dit"
+description = """
+Wrappers for enabling/disabling the Data Independent Timing (DIT) feature on AArch64 CPUs
+"""
+version = "0.0.0"
+authors = ["RustCrypto Developers"]
+license = "Apache-2.0 OR MIT"
+homepage = "https://github.com/RustCrypto/utils/tree/master/aarch64-dit"
+repository = "https://github.com/RustCrypto/utils"
+categories = ["cryptography", "hardware-support", "no-std"]
+keywords = ["crypto", "intrinsics"]
+readme = "README.md"
+edition = "2021"
+rust-version = "1.61"
+
+[dev-dependencies]
+# TODO: release `cpufeatures` with `dit` support
+cpufeatures = { path = "../cpufeatures" }

aarch64-dit/LICENSE-APACHE

@@ -0,0 +1,202 @@
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aarch64-dit/LICENSE-MIT

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2024 The RustCrypto Project Developers
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
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+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

aarch64-dit/README.md

@@ -0,0 +1,51 @@
+# [RustCrypto]: AArch64 Data-Independent Timing (DIT)
+
+[![Crate][crate-image]][crate-link]
+[![Docs][docs-image]][docs-link]
+[![Build Status][build-image]][build-link]
+![Apache 2.0/MIT Licensed][license-image]
+![MSRV][msrv-image]
+
+Wrappers for enabling/disabling the [Data-Independent Timing] feature of modern AArch64 CPUs which
+can be used to help ensure that instructions take a constant amount of time regardless of input
+data, thus preventing potential information leaks via timing sidechannels.
+
+[Documentation][docs-link]
+
+## Minimum Supported Rust Version
+
+Rust **1.61** or newer.
+
+In the future, we reserve the right to change MSRV (i.e. MSRV is out-of-scope for this crate's
+SemVer guarantees), however when we do it will be accompanied by a minor version bump.
+
+## License
+
+Licensed under either of:
+
+* [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)
+* [MIT license](http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the
+work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any
+additional terms or conditions.
+
+[//]: # (badges)
+
+[crate-image]: https://img.shields.io/crates/v/aarch64-dit.svg
+[crate-link]: https://crates.io/crates/aarch64-dit
+[docs-image]: https://docs.rs/aarch64-dit/badge.svg
+[docs-link]: https://docs.rs/aarch64-dit/
+[license-image]: https://img.shields.io/badge/license-Apache2.0/MIT-blue.svg
+[msrv-image]: https://img.shields.io/badge/rustc-1.61+-blue.svg
+[build-image]: https://github.com/RustCrypto/utils/actions/workflows/aarch64-dit.yml/badge.svg
+[build-link]: https://github.com/RustCrypto/utils/actions/workflows/aarch64-dit.yml
+
+[//]: # (links)
+
+[RustCrypto]: https://github.com/RustCrypto
+[Data-Independent Timing]: https://developer.arm.com/documentation/ddi0595/2021-06/AArch64-Registers/DIT--Data-Independent-Timing