Improve RDRAND implementation (#24)

* Move sgx.rs to rdrand.rs

* Improve RDRAND implementation

.travis.yml

@@ -105,6 +105,7 @@ matrix:
         #- rustup target add x86_64-unknown-fuchsia
         - rustup target add x86_64-unknown-netbsd
         - rustup target add x86_64-unknown-redox
+        - rustup target add x86_64-fortanix-unknown-sgx
       script:
         - cargo build --target=x86_64-sun-solaris --all-features
         - cargo build --target=x86_64-unknown-cloudabi --all-features
@@ -112,6 +113,7 @@ matrix:
         #- cargo build --target=x86_64-unknown-fuchsia --all-features
         - cargo build --target=x86_64-unknown-netbsd --all-features
         - cargo build --target=x86_64-unknown-redox --all-features
+        - cargo build --target=x86_64-fortanix-unknown-sgx --all-features
         # also test minimum dependency versions are usable
         - cargo generate-lockfile -Z minimal-versions
         - cargo build --target=x86_64-sun-solaris --all-features
@@ -120,6 +122,7 @@ matrix:
         #- cargo build --target=x86_64-unknown-fuchsia --all-features
         - cargo build --target=x86_64-unknown-netbsd --all-features
         - cargo build --target=x86_64-unknown-redox --all-features
+        - cargo build --target=x86_64-fortanix-unknown-sgx --all-features
 
     # Trust cross-built/emulated targets. We must repeat all non-default values.
     - rust: stable

src/lib.rs

@@ -189,7 +189,7 @@ mod_use!(cfg(target_os = "openbsd"), openbsd_bitrig);
 mod_use!(cfg(target_os = "redox"), use_file);
 mod_use!(cfg(target_os = "solaris"), solaris_illumos);
 mod_use!(cfg(windows), windows);
-mod_use!(cfg(target_env = "sgx"), sgx);
+mod_use!(cfg(target_env = "sgx"), rdrand);
 mod_use!(cfg(target_os = "wasi"), wasi);
 
 mod_use!(

src/rdrand.rs

@@ -0,0 +1,55 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <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.
+
+//! Implementation for SGX using RDRAND instruction
+use crate::Error;
+use core::mem;
+use core::arch::x86_64::_rdrand64_step;
+use core::num::NonZeroU32;
+
+#[cfg(not(target_feature = "rdrand"))]
+compile_error!("enable rdrand target feature!");
+
+// Recommendation from "Intel® Digital Random Number Generator (DRNG) Software
+// Implementation Guide" - Section 5.2.1 and "Intel® 64 and IA-32 Architectures
+// Software Developer’s Manual" - Volume 1 - Section 7.3.17.1.
+const RETRY_LIMIT: usize = 10;
+const WORD_SIZE: usize = mem::size_of::<u64>();
+
+fn rdrand() -> Result<[u8; WORD_SIZE], Error> {
+    for _ in 0..RETRY_LIMIT {
+        unsafe {
+            // SAFETY: we've checked RDRAND support, and u64 can have any value.
+            let mut el = mem::uninitialized();
+            if _rdrand64_step(&mut el) == 1 {
+                return Ok(el.to_ne_bytes());
+            }
+        };
+    }
+    error!("RDRAND failed, CPU issue likely");
+    Err(Error::UNKNOWN)
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // We use chunks_exact_mut instead of chunks_mut as it allows almost all
+    // calls to memcpy to be elided by the compiler.
+    let mut chunks = dest.chunks_exact_mut(WORD_SIZE);
+    for chunk in chunks.by_ref() {
+        chunk.copy_from_slice(&rdrand()?);
+    }
+
+    let tail = chunks.into_remainder();
+    let n = tail.len();
+    if n > 0 {
+        tail.copy_from_slice(&rdrand()?[..n]);
+    }
+    Ok(())
+}
+
+#[inline(always)]
+pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> { None }

src/sgx.rs

@@ -1,51 +0,0 @@
-// Copyright 2018 Developers of the Rand project.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <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.
-
-//! Implementation for SGX using RDRAND instruction
-use crate::Error;
-use core::{mem, ptr};
-use core::arch::x86_64::_rdrand64_step;
-use core::num::NonZeroU32;
-
-#[cfg(not(target_feature = "rdrand"))]
-compile_error!("enable rdrand target feature!");
-
-const RETRY_LIMIT: usize = 32;
-
-fn get_rand_u64() -> Result<u64, Error> {
-    for _ in 0..RETRY_LIMIT {
-        unsafe {
-            let mut el = mem::uninitialized();
-            if _rdrand64_step(&mut el) == 1 {
-                return Ok(el);
-            }
-        };
-    }
-    Err(Error::UNKNOWN)
-}
-
-pub fn getrandom_inner(mut dest: &mut [u8]) -> Result<(), Error> {
-    while dest.len() >= 8 {
-        let (chunk, left) = {dest}.split_at_mut(8);
-        dest = left;
-        let r = get_rand_u64()?;
-        unsafe {
-            ptr::write_unaligned(chunk.as_mut_ptr() as *mut u64, r)
-        }
-    }
-    let n = dest.len();
-    if n != 0 {
-        let r = get_rand_u64()?;
-        let r: [u8; 8] = unsafe { mem::transmute(r) };
-        dest.copy_from_slice(&r[..n]);
-    }
-    Ok(())
-}
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> { None }