zeroize: add support for ARM64 SIMD registers (nightly-only) (#749)

Similar to the implementation for `x86`/`x86_64`, this commit adds
nightly-only support for zeroizing ARM64 SIMD registers.

Support is gated behind an `aarch64` feature so as to avoid breaking
compilation on stable Rust. The feature is a no-op on non-`aarch64`
targets.

.github/workflows/zeroize.yml

@@ -64,4 +64,26 @@ jobs:
           override: true
           profile: minimal
       - run: cargo test --release
-      - run: cargo test --release --all-features
+      - run: cargo test --release --features alloc,derive
+
+  # Feature-gated ARM64 SIMD register support (nightly-only)
+  aarch64:
+    strategy:
+      matrix:
+        include:
+          - target: aarch64-unknown-linux-gnu
+            rust: nightly-2022-03-01
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: RustCrypto/actions/cargo-cache@master
+      - run: ${{ matrix.deps }}
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: ${{ matrix.rust }}
+          target: ${{ matrix.target }}
+          profile: minimal
+          override: true
+      - uses: RustCrypto/actions/cross-install@master
+      - run: cross test --release --target ${{ matrix.target }} --features aarch64
+      - run: cross test --release --target ${{ matrix.target }} --all-features

zeroize/Cargo.toml

@@ -21,6 +21,7 @@ zeroize_derive = { version = "1.3", path = "derive", optional = true }
 
 [features]
 default = ["alloc"]
+aarch64 = []
 alloc = []
 derive = ["zeroize_derive"]
 

zeroize/src/aarch64.rs

@@ -0,0 +1,35 @@
+//! [`Zeroize`] impls for ARM64 SIMD registers.
+//!
+//! Support for this is gated behind an `aarch64` feature because
+//! support for `core::arch::aarch64` is currently nightly-only.
+
+use crate::{atomic_fence, volatile_write, Zeroize};
+
+use core::arch::aarch64::*;
+
+macro_rules! impl_zeroize_for_simd_register {
+    ($(($type:ty, $vdupq:ident)),+) => {
+        $(
+            #[cfg_attr(docsrs, doc(cfg(target_arch = "aarch64")))]
+            #[cfg_attr(docsrs, doc(cfg(target_feature = "neon")))]
+            impl Zeroize for $type {
+                fn zeroize(&mut self) {
+                    volatile_write(self, unsafe { $vdupq(0) });
+                    atomic_fence();
+                }
+            }
+        )+
+    };
+}
+
+// TODO(tarcieri): other NEON register types?
+impl_zeroize_for_simd_register! {
+    (uint8x8_t, vdup_n_u8),
+    (uint8x16_t, vdupq_n_u8),
+    (uint16x4_t, vdup_n_u16),
+    (uint16x8_t, vdupq_n_u16),
+    (uint32x2_t, vdup_n_u32),
+    (uint32x4_t, vdupq_n_u32),
+    (uint64x1_t, vdup_n_u64),
+    (uint64x2_t, vdupq_n_u64)
+}

zeroize/src/lib.rs

@@ -238,6 +238,8 @@ extern crate alloc;
 #[cfg_attr(docsrs, doc(cfg(feature = "zeroize_derive")))]
 pub use zeroize_derive::{Zeroize, ZeroizeOnDrop};
 
+#[cfg(all(feature = "aarch64", target_arch = "aarch64"))]
+mod aarch64;
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 mod x86;