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RustCrypto / utils

Utility crates used in RustCrypto
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aarch64-dit: initial crate #1102

Closed tarcieri closed 2 months ago

tarcieri commented 3 months ago

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.

tarcieri commented 3 months ago

Looks like this will need a higher MSRV:

error[E0658]: the target feature `dit` is currently unstable
  --> aarch64-dit/src/lib.rs:[15](https://github.com/RustCrypto/utils/actions/runs/10549036451/job/29223626727?pr=1102#step:5:16):18
   |
15 | #[target_feature(enable = "dit")]
   |                  ^^^^^^^^^^^^^^
   |
   = note: see issue #44839 <https://github.com/rust-lang/rust/issues/44839> for more information

Edit: looks like it was stabilized in 1.61

tarcieri commented 3 months ago

At some point we might consider having an ISA-independent crate for this sort of instruction pattern, provided we can actually build a portable abstraction over it: https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/best-practices/data-operand-independent-timing-isa-guidance.html