Make the RNG fall back to using an algorithm handle if BCryptGenRandom fails

src/windows.rs

@@ -5,45 +5,144 @@
 // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
+#![allow(non_camel_case_types)]
 
 use crate::Error;
-use core::{ffi::c_void, mem::MaybeUninit, num::NonZeroU32, ptr};
+use core::{
+    convert::TryInto,
+    ffi::{c_long, c_void},
+    mem::MaybeUninit,
+    num::NonZeroU32,
+    ptr,
+};
 
+// same as Rust's libstd.
+type BCRYPT_ALG_HANDLE = *mut c_void;
+type NTSTATUS = c_long;
+
+// "RNG\0"
+const BCRYPT_RNG_ALGORITHM: &[u16] = &[b'R' as u16, b'N' as u16, b'G' as u16, 0];
 const BCRYPT_USE_SYSTEM_PREFERRED_RNG: u32 = 0x00000002;
 
+// Equivalent to the `NT_SUCCESS` C preprocessor macro.
+// See: https://docs.microsoft.com/en-us/windows-hardware/drivers/kernel/using-ntstatus-values
+fn nt_success(status: NTSTATUS) -> bool {
+    status >= 0
+}
+
+/// Extract error code and turn into an `Error`
+fn nt_error(status: NTSTATUS) -> Error {
+    // We zeroize the highest bit, so the error code will reside
+    // inside the range designated for OS codes.
+    let code = status as u32 ^ (1 << 31);
+    // SAFETY: the second highest bit is always equal to one,
+    // so it's impossible to get zero. Unfortunately the type
+    // system does not have a way to express this yet.
+    let code = unsafe { NonZeroU32::new_unchecked(code) };
+    Error::from(code)
+}
+
 #[link(name = "bcrypt")]
 extern "system" {
     fn BCryptGenRandom(
-        hAlgorithm: *mut c_void,
+        hAlgorithm: BCRYPT_ALG_HANDLE,
         pBuffer: *mut u8,
         cbBuffer: u32,
         dwFlags: u32,
-    ) -> u32;
+    ) -> NTSTATUS;
+    pub fn BCryptOpenAlgorithmProvider(
+        phalgorithm: *mut BCRYPT_ALG_HANDLE,
+        pszAlgId: *const u16,
+        pszimplementation: *const u16,
+        dwflags: u32,
+    ) -> NTSTATUS;
+    pub fn BCryptCloseAlgorithmProvider(hAlgorithm: BCRYPT_ALG_HANDLE, dwFlags: u32) -> NTSTATUS;
 }
 
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // Prevent overflow of u32
     for chunk in dest.chunks_mut(u32::max_value() as usize) {
-        // BCryptGenRandom was introduced in Windows Vista
+        if let Err(_) = Rng::SYSTEM.random(chunk) {
+            fallback_rng(chunk)?;
+        }
+    }
+    Ok(())
+}
+
+struct Rng {
+    algorithm: BCRYPT_ALG_HANDLE,
+    flags: u32,
+}
+
+impl Rng {
+    const SYSTEM: Self = unsafe { Self::new(ptr::null_mut(), BCRYPT_USE_SYSTEM_PREFERRED_RNG) };
+
+    /// Create the RNG from an existing algorithm handle.
+    ///
+    /// # Safety
+    ///
+    /// The handle must either be null or a valid algorithm handle.
+    const unsafe fn new(algorithm: BCRYPT_ALG_HANDLE, flags: u32) -> Self {
+        Self { algorithm, flags }
+    }
+
+    /// Open a handle to the RNG algorithm.
+    fn open() -> Result<Self, Error> {
+        use core::sync::atomic::AtomicPtr;
+        use core::sync::atomic::Ordering::{Acquire, Release};
+
+        // An atomic is used so we don't need to reopen the handle every time.
+        static HANDLE: AtomicPtr<c_void> = AtomicPtr::new(ptr::null_mut());
+
+        let mut handle = HANDLE.load(Acquire);
+        if handle.is_null() {
+            let status = unsafe {
+                BCryptOpenAlgorithmProvider(
+                    &mut handle,
+                    BCRYPT_RNG_ALGORITHM.as_ptr(),
+                    ptr::null(),
+                    0,
+                )
+            };
+            if nt_success(status) {
+                // If another thread opens a handle first then use that handle instead.
+                let result = HANDLE.compare_exchange(ptr::null_mut(), handle, Release, Acquire);
+                if let Err(previous_handle) = result {
+                    // Close our handle and return the previous one.
+                    unsafe { BCryptCloseAlgorithmProvider(handle, 0) };
+                    handle = previous_handle;
+                }
+                Ok(unsafe { Self::new(handle, 0) })
+            } else {
+                Err(nt_error(status))
+            }
+        } else {
+            Ok(unsafe { Self::new(handle, 0) })
+        }
+    }
+
+    fn random(&self, dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+        let len: u32 = dest.len().try_into().unwrap();
+        // SAFETY: dest is valid, writable buffer of length len
         let ret = unsafe {
             BCryptGenRandom(
-                ptr::null_mut(),
-                chunk.as_mut_ptr() as *mut u8,
-                chunk.len() as u32,
-                BCRYPT_USE_SYSTEM_PREFERRED_RNG,
+                self.algorithm,
+                dest.as_mut_ptr() as *mut u8,
+                len,
+                self.flags,
             )
         };
-        // NTSTATUS codes use the two highest bits for severity status.
-        if ret >> 30 == 0b11 {
-            // We zeroize the highest bit, so the error code will reside
-            // inside the range designated for OS codes.
-            let code = ret ^ (1 << 31);
-            // SAFETY: the second highest bit is always equal to one,
-            // so it's impossible to get zero. Unfortunately the type
-            // system does not have a way to express this yet.
-            let code = unsafe { NonZeroU32::new_unchecked(code) };
-            return Err(Error::from(code));
+
+        if nt_success(ret) {
+            return Ok(());
         }
+
+        Err(nt_error(ret))
     }
-    Ok(())
+}
+
+/// Generate random numbers using the fallback RNG function
+#[inline(never)]
+fn fallback_rng(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
+    Rng::open()?.random(dest)
 }