Add support for x86_64-fortanix-unknown-sgx target

Cargo.toml

@@ -298,6 +298,10 @@ libc = { version = "0.2.45" }
 [target.'cfg(any(target_os = "redox", all(unix, not(any(target_os = "macos", target_os = "ios")))))'.dependencies]
 lazy_static = "1.2"
 
+[target.'cfg(target_env = "sgx")'.dependencies]
+rand_core = "0.3.0"
+rdrand = "0.4.0"
+
 # Keep this in sync with `[dependencies]` in pregenerate_asm/Cargo.toml.
 [build-dependencies]
 cc = "1.0.26"

src/rand.rs

@@ -60,6 +60,8 @@ pub trait SecureRandom: sealed::Sealed {
 /// On Windows, `fill` is implemented using the platform's API for secure
 /// random number generation.
 ///
+/// On SGX, `fill()` is implemented using the `RDRAND` instruction.
+///
 /// Otherwise, `fill()` is implemented by reading from `/dev/urandom`. (This is
 /// something that should be improved for any platform that adds something
 /// better.)
@@ -93,7 +95,7 @@ impl SecureRandom for SystemRandom {
 
 impl sealed::Sealed for SystemRandom {}
 
-#[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "ios", windows)))]
+#[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "ios", windows, target_env = "sgx")))]
 use self::urandom::fill as fill_impl;
 
 #[cfg(any(
@@ -107,6 +109,10 @@ use self::sysrand_or_urandom::fill as fill_impl;
 
 #[cfg(any(target_os = "macos", target_os = "ios"))]
 use self::darwin::fill as fill_impl;
+
+#[cfg(target_env = "sgx")]
+use self::rdrandom::fill as fill_impl;
+
 use crate::sealed;
 
 #[cfg(target_os = "linux")]
@@ -275,6 +281,19 @@ mod darwin {
     }
 }
 
+#[cfg(target_env = "sgx")]
+mod rdrandom {
+    use crate::error;
+    use rdrand::RdRand;
+    use rand_core::RngCore;
+
+    pub fn fill(dest: &mut [u8]) -> Result<(), error::Unspecified> {
+        let mut rng = RdRand::new().map_err(|_| error::Unspecified)?;
+        rng.try_fill_bytes(dest).map_err(|_| error::Unspecified)?;
+        Ok(())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::rand::{self, SecureRandom};