zeroize: add zeroize_stack function (#1331)

Adds a simple `optimization_barrier`-based function for stack
zeroization. It requires an explicitly provided size since erasing the
whole stack (2-8 MiB) can be expensive and it would be an overkill in
most cases. Additionally, detection of remaining stack size (e.g. using
`pthread_attr_getstack`) is platform-dependent and not available on all
targets.

Author: newpavlov

zeroize/CHANGELOG.md

@@ -7,12 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## 1.9.0 (unreleased)
 ### Added
 - `optimization_barrier` function ([#1261])
+- `zeroize_stack` function ([#1331])
 
 ### Changed
 - Edition changed to 2024 and MSRV bumped to 1.85 ([#1149])
 
 [#1149]: https://github.com/RustCrypto/utils/pull/1149
 [#1261]: https://github.com/RustCrypto/utils/pull/1261
+[#1331]: https://github.com/RustCrypto/utils/pull/1331
 
 ## 1.8.2 (2025-09-29)
 ### Changed
@@ -164,15 +166,15 @@ if you would like to support older Rust versions.
 ## 1.3.0 (2021-04-19)
 ### Added
 - impl `Zeroize` for `Box<[Z]>`
-- Clear residual space within `Option
+- Clear residual space within `Option`
 
 ### Changed
 - Ensure `Option` is `None` when zeroized
 - Bump MSRV to 1.47
 
 ## 1.2.0 (2020-12-09)
 ### Added
-- `Zeroize` support for x86(_64) SIMD registers
+- `Zeroize` support for x86(-64) SIMD registers
 
 ### Changed
 - Simplify `String::zeroize`

zeroize/src/lib.rs

@@ -253,6 +253,9 @@ mod x86;
 mod barrier;
 pub use barrier::optimization_barrier;
 
+mod stack;
+pub use stack::zeroize_stack;
+
 use core::{
     marker::{PhantomData, PhantomPinned},
     mem::{MaybeUninit, size_of},

zeroize/src/stack.rs

@@ -0,0 +1,52 @@
+/// Zeroize `N` bytes of stack space.
+///
+/// Most algorithm implementations use stack to store temporary data.
+/// Such temporaries may contain sensitive information (e.g. cryptgraphic keys)
+/// and can stay on stack after the computation is finished. If an attacker
+/// is able for some reasons to read stack data freely, it may result in
+/// leaking of the sensitive data.
+///
+/// # WARNING
+/// This function requires you to estimate how much stack space is used by your
+/// sensitive computation. This can be done by tools like [`cargo-call-stack`],
+/// but note that stack usage depends on optimization level and compiler flags.
+///
+/// [`cargo-call-stack`]: https://github.com/japaric/cargo-call-stack
+///
+/// Additionally, you must annotate your sensitive function with `#[inline(never)]`.
+///
+/// For example, the following example **DOES NOT** erase stack properly:
+/// ```
+/// pub fn encrypt_data(key: &[u8; 16], data: &mut [u8]) {
+///     leaking_encryption(key, data);
+///     zeroize::zeroize_stack::<65_536>();
+/// }
+/// # fn leaking_encryption(_: &[u8; 16], _: &mut [u8]) {}
+/// ```
+/// `leaking_encryption` may get inlined and `zeroize_stack` will erase
+/// stack memory above the stack frame reserved by `encrypt_data`, i.e.
+/// it will **NOT** erase stack memory used by `leaking_encryption`.
+///
+/// You should wrap your computation in the following way:
+/// ```
+/// #[inline(never)]
+/// fn encrypt_data_inner(key: &[u8; 16], data: &mut [u8]) {
+///     leaking_encryption(key, data);
+/// }
+///
+/// pub fn encrypt_data(key: &[u8; 16], data: &mut [u8]) {
+///     encrypt_data_inner(key, data);
+///     zeroize::zeroize_stack::<65_536>();
+/// }
+/// # fn leaking_encryption(_: &[u8; 16], _: &mut [u8]) {}
+/// ```
+/// Finally, note that `#[inline(never)]` is just a hint and may be ignored
+/// by the compiler. It works properly in practice, but such stack zeroization
+/// should be considered as "best effort" and in cases where it's not enough
+/// you should inspect the generated binary to verify that you got a desired
+/// codegen.
+#[inline(never)]
+pub fn zeroize_stack<const N: usize>() {
+    let buf = [0u8; N];
+    crate::optimization_barrier(&buf);
+}