Make the RNG fall back to RtlGenRandom if BCryptGenRandom fails

Based on rust-lang/rust#96917

Cargo.toml

@@ -29,6 +29,9 @@ js-sys = { version = "0.3", optional = true }
 [target.'cfg(all(target_arch = "wasm32", target_os = "unknown"))'.dev-dependencies]
 wasm-bindgen-test = "0.3.18"
 
+[target.'cfg(windows)'.dependencies]
+once_cell = "1.13.1"
+
 [features]
 # Implement std-only traits for getrandom::Error
 std = []

src/windows.rs

@@ -6,11 +6,22 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// Implementation copied from rustc's `library/std/src/sys/windows/rand.rs`.
+// Includes a fallback to `RtlGenRandom` in case `BCryptGenRandom` fails.
+
 use crate::Error;
 use core::{ffi::c_void, num::NonZeroU32, ptr};
+use once_cell::sync::OnceCell;
 
 const BCRYPT_USE_SYSTEM_PREFERRED_RNG: u32 = 0x00000002;
 
+/// The kinds of RNG that may be available
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum Rng {
+    Preferred,
+    Fallback,
+}
+
 #[link(name = "bcrypt")]
 extern "system" {
     fn BCryptGenRandom(
@@ -21,7 +32,59 @@ extern "system" {
     ) -> u32;
 }
 
+#[link(name = "advapi32")]
+extern "system" {
+    // Forbidden when targeting UWP
+    #[link_name = "SystemFunction036"]
+    pub fn RtlGenRandom(RandomBuffer: *mut u8, RandomBufferLength: u32) -> u8;
+}
+
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    match get_rng() {
+        Rng::Preferred => {
+            preferred_rng(dest)
+        }
+        Rng::Fallback => {
+            fallback_rng(dest)
+        }
+    }
+}
+
+/// Returns the RNG that should be used
+///
+/// Panics if they are both broken
+fn get_rng() -> Rng {
+    // Assume that if the preferred RNG is broken the first time we use it, it likely means
+    // that: the DLL has failed to load, there is no point to calling it over-and-over again,
+    // and we should cache the result
+    static VALUE: OnceCell<Rng> = OnceCell::new();
+    *VALUE.get_or_init(choose_rng)
+}
+
+/// Test whether we should use the preferred or fallback RNG
+///
+/// If the preferred RNG is successful, we choose it. Otherwise, if the fallback RNG is successful,
+/// we choose that
+///
+/// Panics if both the preferred and the fallback RNG are both non-functional
+fn choose_rng() -> Rng {
+    let mut dest = [0; 1];
+
+    let preferred_error = match preferred_rng(&mut dest) {
+        Ok(_) => return Rng::Preferred,
+        Err(e) => e,
+    };
+
+    match fallback_rng(&mut dest) {
+        Ok(_) => return Rng::Fallback,
+        Err(fallback_error) => panic!(
+            "preferred RNG broken: `{}`, fallback RNG broken: `{}`",
+            preferred_error, fallback_error
+        ),
+    }
+}
+
+fn preferred_rng(dest: &mut [u8]) -> Result<(), Error> {
     // Prevent overflow of u32
     for chunk in dest.chunks_mut(u32::max_value() as usize) {
         // BCryptGenRandom was introduced in Windows Vista
@@ -47,3 +110,22 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     }
     Ok(())
 }
+
+/// Generate random numbers using the fallback RNG function (RtlGenRandom)
+#[cfg(not(target_vendor = "uwp"))]
+fn fallback_rng(dest: &mut [u8]) -> Result<(), Error> {
+    // Prevent overflow of u32
+    for chunk in dest.chunks_mut(u32::max_value() as usize) {
+        let ret = unsafe { RtlGenRandom(chunk.as_mut_ptr(), chunk.len() as u32) };
+        if ret == 0 {
+            return Err(Error::WINDOWS_RTL_GEN_RANDOM);
+        }
+    }
+    Ok(())
+}
+
+/// We can't use RtlGenRandom with UWP, so there is no fallback
+#[cfg(target_vendor = "uwp")]
+fn fallback_rng(_dest: &mut [u8]) -> Result<(), Error> {
+    Err(Error::UNSUPPORTED)
+}