Add efi_rng opt-in backend

.github/workflows/build.yml

@@ -166,6 +166,26 @@ jobs:
       - name: Build
         run: cargo build --target ${{ matrix.target.target }} ${{ matrix.feature.feature }} -Zbuild-std=${{ matrix.feature.build-std }}
 
+  efi-rng:
+    name: UEFI RNG Protocol
+    runs-on: ubuntu-24.04
+    strategy:
+      matrix:
+        target: [
+          aarch64-unknown-uefi,
+          x86_64-unknown-uefi,
+          i686-unknown-uefi,
+        ]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly # Required to build libstd
+        with:
+          components: rust-src
+      - uses: Swatinem/rust-cache@v2
+      - env:
+          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="efi_rng"
+        run: cargo build -Z build-std=std --target=${{ matrix.target }} --features std
+
   rdrand-uefi:
     name: RDRAND UEFI
     runs-on: ubuntu-24.04

.github/workflows/workspace.yml

@@ -67,6 +67,10 @@ jobs:
       env:
         RUSTFLAGS: -Dwarnings --cfg getrandom_backend="rndr"
       run: cargo clippy -Zbuild-std=core --target aarch64-unknown-linux-gnu
+    - name: EFI RNG (efi_rng.rs)
+      env:
+        RUSTFLAGS: -Dwarnings --cfg getrandom_backend="efi_rng"
+      run: cargo clippy -Zbuild-std=std --target x86_64-unknown-uefi
     - name: Solaris (solaris.rs)
       run: cargo clippy -Zbuild-std=core --target x86_64-pc-solaris
     - name: SOLID (solid.rs)

Cargo.toml

@@ -38,6 +38,10 @@ libc = { version = "0.2.154", default-features = false }
 [target.'cfg(any(target_os = "ios", target_os = "visionos", target_os = "watchos", target_os = "tvos"))'.dependencies]
 libc = { version = "0.2.154", default-features = false }
 
+# efi_rng
+[target.'cfg(all(target_os = "uefi", getrandom_backend = "efi_rng"))'.dependencies]
+r-efi = { version = "5.1", default-features = false }
+
 # getentropy
 [target.'cfg(any(target_os = "macos", target_os = "openbsd", target_os = "vita", target_os = "emscripten"))'.dependencies]
 libc = { version = "0.2.154", default-features = false }
@@ -81,7 +85,7 @@ wasm-bindgen-test = "0.3"
 [lints.rust.unexpected_cfgs]
 level = "warn"
 check-cfg = [
-  'cfg(getrandom_backend, values("custom", "rdrand", "rndr", "linux_getrandom", "wasm_js"))',
+  'cfg(getrandom_backend, values("custom", "efi_rng", "rdrand", "rndr", "linux_getrandom", "wasm_js"))',
   'cfg(getrandom_msan)',
   'cfg(getrandom_test_linux_fallback)',
   'cfg(getrandom_test_netbsd_fallback)',

README.md

@@ -83,6 +83,7 @@ of randomness based on their specific needs:
 | `rdrand`          | x86, x86-64          | `x86_64-*`, `i686-*`     | [`RDRAND`] instruction
 | `rndr`            | AArch64              | `aarch64-*`              | [`RNDR`] register
 | `wasm_js`         | Web Browser, Node.js | `wasm32‑unknown‑unknown`, `wasm32v1-none` | [`Crypto.getRandomValues`]. Requires feature `wasm_js` ([see below](#webassembly-support)).
+| `efi_rng`         | UEFI                 | `*-unknown‑uefi`         | [`EFI_RNG_PROTOCOL`] with `EFI_RNG_ALGORITHM_RAW` (requires `std` and Nigthly compiler)
 | `custom`          | All targets          | `*`                      | User-provided custom implementation (see [custom backend])
 
 Opt-in backends can be enabled using the `getrandom_backend` configuration flag.
@@ -351,6 +352,7 @@ dual licensed as above, without any additional terms or conditions.
 [`esp_fill_random`]: https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-reference/system/random.html#functions
 [esp-idf-rng]: https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-reference/system/random.html
 [esp-trng-docs]: https://www.espressif.com/sites/default/files/documentation/esp32_technical_reference_manual_en.pdf#rng
+[`EFI_RNG_PROTOCOL`]: https://uefi.org/specs/UEFI/2.10/37_Secure_Technologies.html#efi-rng-protocol
 [`random_get`]: https://github.com/WebAssembly/WASI/blob/snapshot-01/phases/snapshot/docs.md#-random_getbuf-pointeru8-buf_len-size---errno
 [`get-random-u64`]: https://github.com/WebAssembly/WASI/blob/v0.2.1/wasip2/random/random.wit#L23-L28
 [configuration flags]: #configuration-flags

src/backends.rs

@@ -19,6 +19,9 @@ cfg_if! {
     } else if #[cfg(getrandom_backend = "rndr")] {
         mod rndr;
         pub use rndr::*;
+    } else if #[cfg(getrandom_backend = "efi_rng")] {
+        mod efi_rng;
+        pub use efi_rng::*;
     } else if #[cfg(all(getrandom_backend = "wasm_js"))] {
         cfg_if! {
             if #[cfg(feature = "wasm_js")] {

src/backends/efi_rng.rs

@@ -0,0 +1,125 @@
+//! Implementation for UEFI using EFI_RNG_PROTOCOL
+use crate::Error;
+use core::{
+    mem::MaybeUninit,
+    ptr::{self, null_mut, NonNull},
+    sync::atomic::{AtomicPtr, Ordering::Relaxed},
+};
+use r_efi::{
+    efi::{BootServices, Handle},
+    protocols::rng,
+};
+
+extern crate std;
+
+pub use crate::util::{inner_u32, inner_u64};
+
+#[cfg(not(target_os = "uefi"))]
+compile_error!("`efi_rng` backend can be enabled only for UEFI targets!");
+
+static RNG_PROTOCOL: AtomicPtr<rng::Protocol> = AtomicPtr::new(null_mut());
+
+#[cold]
+#[inline(never)]
+fn init() -> Result<NonNull<rng::Protocol>, Error> {
+    const HANDLE_SIZE: usize = size_of::<Handle>();
+
+    let boot_services = std::os::uefi::env::boot_services()
+        .ok_or(Error::BOOT_SERVICES_UNAVAILABLE)?
+        .cast::<BootServices>();
+
+    let mut handles = [ptr::null_mut(); 16];
+    // `locate_handle` operates with length in bytes
+    let mut buf_size = handles.len() * HANDLE_SIZE;
+    let mut guid = rng::PROTOCOL_GUID;
+    let ret = unsafe {
+        ((*boot_services.as_ptr()).locate_handle)(
+            r_efi::efi::BY_PROTOCOL,
+            &mut guid,
+            null_mut(),
+            &mut buf_size,
+            handles.as_mut_ptr(),
+        )
+    };
+
+    if ret.is_error() {
+        return Err(Error::TEMP_EFI_ERROR);
+    }
+
+    let handles_len = buf_size / HANDLE_SIZE;
+    let handles = handles.get(..handles_len).ok_or(Error::UNEXPECTED)?;
+
+    let system_handle = std::os::uefi::env::image_handle();
+    for &handle in handles {
+        let mut protocol: MaybeUninit<*mut rng::Protocol> = MaybeUninit::uninit();
+
+        let mut protocol_guid = rng::PROTOCOL_GUID;
+        let ret = unsafe {
+            ((*boot_services.as_ptr()).open_protocol)(
+                handle,
+                &mut protocol_guid,
+                protocol.as_mut_ptr().cast(),
+                system_handle.as_ptr(),
+                ptr::null_mut(),
+                r_efi::system::OPEN_PROTOCOL_GET_PROTOCOL,
+            )
+        };
+
+        let protocol = if ret.is_error() {
+            continue;
+        } else {
+            let protocol = unsafe { protocol.assume_init() };
+            NonNull::new(protocol).ok_or(Error::UNEXPECTED)?
+        };
+
+        // Try to use the acquired protocol handle
+        let mut buf = [0u8; 8];
+        let mut alg_guid = rng::ALGORITHM_RAW;
+        let ret = unsafe {
+            ((*protocol.as_ptr()).get_rng)(
+                protocol.as_ptr(),
+                &mut alg_guid,
+                buf.len(),
+                buf.as_mut_ptr(),
+            )
+        };
+
+        if ret.is_error() {
+            continue;
+        }
+
+        RNG_PROTOCOL.store(protocol.as_ptr(), Relaxed);
+        return Ok(protocol);
+    }
+    Err(Error::NO_RNG_HANDLE)
+}
+
+#[inline]
+pub fn fill_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    let protocol = match NonNull::new(RNG_PROTOCOL.load(Relaxed)) {
+        Some(p) => p,
+        None => init()?,
+    };
+
+    let mut alg_guid = rng::ALGORITHM_RAW;
+    let ret = unsafe {
+        ((*protocol.as_ptr()).get_rng)(
+            protocol.as_ptr(),
+            &mut alg_guid,
+            dest.len(),
+            dest.as_mut_ptr().cast::<u8>(),
+        )
+    };
+
+    if ret.is_error() {
+        Err(Error::TEMP_EFI_ERROR)
+    } else {
+        Ok(())
+    }
+}
+
+impl Error {
+    pub(crate) const BOOT_SERVICES_UNAVAILABLE: Error = Self::new_internal(10);
+    pub(crate) const NO_RNG_HANDLE: Error = Self::new_internal(11);
+    pub(crate) const TEMP_EFI_ERROR: Error = Self::new_internal(12);
+}

src/lib.rs

@@ -10,6 +10,7 @@
 #![doc = include_str!("../README.md")]
 #![warn(rust_2018_idioms, unused_lifetimes, missing_docs)]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(getrandom_backend = "efi_rng", feature(uefi_std))]
 #![deny(
     clippy::cast_lossless,
     clippy::cast_possible_truncation,