rand: Removing use of openssl rand for crypto

According to the documentation, openssl's `rand_bytes(...)` function will be
seeded with `/dev/urandom`, which is not sound for cryptographic
application. We will be switching to using the `rdrand` crate which will
utilize the X86_64 microcode call to `RDRAND`, instead. All X86_64
instructions are protected by the Guest Hypervisor Communication Block
(GHCB), and may thus be presumed secure for cryptographic practices in
the context of confidential compute.

Signed-off-by: Larry Dewey <larry.dewey@amd.com>

Cargo.toml

@@ -38,6 +38,7 @@ doc = false
 
 [features]
 default = ["sev", "snp"]
+openssl = ["dep:openssl", "rdrand"]
 hw_tests = []
 dangerous_hw_tests = ["hw_tests"]
 sev = []
@@ -68,6 +69,7 @@ sha2 = { version = "0.10.8", optional = true }
 x509-cert = { version = "0.2.5", optional = true }
 byteorder = "1.4.3"
 base64 = "0.22.1"
+rdrand = { version = "^0.8", optional = true }
 
 [target.'cfg(target_os = "linux")'.dev-dependencies]
 kvm-ioctls = ">=0.16"

src/error.rs

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use bincode;
+#[cfg(feature = "openssl")]
+use openssl::error::ErrorStack;
 use std::{
     array::TryFromSliceError,
     convert::From,
@@ -9,6 +11,9 @@ use std::{
     io,
 };
 
+#[cfg(feature = "openssl")]
+use rdrand::ErrorCode;
+
 use std::os::raw::c_int;
 
 #[cfg(feature = "openssl")]
@@ -159,6 +164,13 @@ pub enum Indeterminate<T: Debug> {
     Unknown,
 }
 
+impl<T: Debug> From<T> for Indeterminate<T> {
+    /// Creates an easy
+    fn from(value: T) -> Self {
+        Self::Known(value)
+    }
+}
+
 #[derive(Debug)]
 /// Wrapper Error for Firmware or User API Errors
 pub enum UserApiError {
@@ -1048,3 +1060,37 @@ impl std::convert::From<LargeArrayError> for MeasurementError {
         Self::LargeArrayError(value)
     }
 }
+
+#[cfg(feature = "openssl")]
+/// Used to describe errors related to SEV-ES "Sessions".
+pub enum SessionError {
+    /// Errors which occur from using the rdrand crate.
+    RandError(ErrorCode),
+
+    /// OpenSSL Error Stack
+    OpenSSLStack(ErrorStack),
+
+    /// Errors occuring from IO operations.
+    IOError(std::io::Error),
+}
+
+#[cfg(feature = "openssl")]
+impl From<ErrorCode> for SessionError {
+    fn from(value: ErrorCode) -> Self {
+        Self::RandError(value)
+    }
+}
+
+#[cfg(feature = "openssl")]
+impl From<std::io::Error> for SessionError {
+    fn from(value: std::io::Error) -> Self {
+        Self::IOError(value)
+    }
+}
+
+#[cfg(feature = "openssl")]
+impl From<ErrorStack> for SessionError {
+    fn from(value: ErrorStack) -> Self {
+        Self::OpenSSLStack(value)
+    }
+}

src/session/key.rs

@@ -8,6 +8,8 @@ use std::{
     ptr::write_volatile,
 };
 
+use rdrand::*;
+
 use openssl::*;
 
 #[repr(transparent)]
@@ -55,9 +57,21 @@ impl Key {
         Key(vec![0u8; size])
     }
 
-    pub fn random(size: usize) -> Result<Self> {
+    /// Will attempt to create a random Key derived from the CPU RDRAND instruction.
+    pub fn random(size: usize) -> std::result::Result<Self, ErrorCode> {
+        // Create a new empty key to store the pseudo-random bytes in.
         let mut key = Key::zeroed(size);
-        rand::rand_bytes(&mut key)?;
+
+        // Instantiate a pseudo-random number generator instance to pull
+        // random data from the CPU RDRAND instruction set.
+        let mut rng = RdRand::new()?;
+
+        // Attempt to generate N-number of bytes specified by the `size`
+        // parameter, storing the bytes inside they key generated at the
+        // start of the method.
+        rng.try_fill_bytes(&mut key)?;
+
+        // Return the key when successful.
         Ok(key)
     }
 }

src/session/mod.rs

@@ -5,10 +5,14 @@
 
 mod key;
 
+use crate::error::SessionError;
+
 use super::*;
 
 use std::io::{Error, ErrorKind, Result};
 
+use rdrand::{ErrorCode, RdRand};
+
 use openssl::*;
 
 /// Represents a brand-new secure channel with the AMD SP.
@@ -43,9 +47,9 @@ impl launch::sev::Policy {
 }
 
 impl std::convert::TryFrom<launch::sev::Policy> for Session<Initialized> {
-    type Error = std::io::Error;
+    type Error = ErrorCode;
 
-    fn try_from(value: launch::sev::Policy) -> Result<Self> {
+    fn try_from(value: launch::sev::Policy) -> std::result::Result<Self, Self::Error> {
         Ok(Self {
             tek: key::Key::random(16)?,
             tik: key::Key::random(16)?,
@@ -88,7 +92,10 @@ impl Session<Initialized> {
     }
 
     /// Produces data needed to initiate the SEV launch sequence.
-    pub fn start(&self, chain: certs::sev::Chain) -> Result<launch::sev::Start> {
+    pub fn start(
+        &self,
+        chain: certs::sev::Chain,
+    ) -> std::result::Result<launch::sev::Start, SessionError> {
         use certs::sev::*;
 
         let pdh = chain.verify()?;
@@ -97,8 +104,11 @@ impl Session<Initialized> {
         let z = key::Key::new(prv.derive(pdh)?);
         let mut nonce = [0u8; 16];
         let mut iv = [0u8; 16];
-        rand::rand_bytes(&mut nonce)?;
-        rand::rand_bytes(&mut iv)?;
+
+        let mut rng: RdRand = RdRand::new()?;
+
+        rng.try_fill_bytes(&mut nonce)?;
+        rng.try_fill_bytes(&mut iv)?;
 
         Ok(launch::sev::Start {
             policy: self.policy,
@@ -109,14 +119,20 @@ impl Session<Initialized> {
 
     /// Like the above start function, yet takes PDH as input instead of deriving it from a
     /// certificate chain.
-    pub fn start_pdh(&self, pdh: certs::sev::sev::Certificate) -> Result<launch::sev::Start> {
+    pub fn start_pdh(
+        &self,
+        pdh: certs::sev::sev::Certificate,
+    ) -> std::result::Result<launch::sev::Start, SessionError> {
         let (crt, prv) = sev::Certificate::generate(sev::Usage::PDH)?;
 
         let z = key::Key::new(prv.derive(&pdh)?);
         let mut nonce = [0u8; 16];
         let mut iv = [0u8; 16];
-        rand::rand_bytes(&mut nonce)?;
-        rand::rand_bytes(&mut iv)?;
+
+        let mut rng: RdRand = RdRand::new()?;
+
+        rng.try_fill_bytes(&mut nonce)?;
+        rng.try_fill_bytes(&mut iv)?;
 
         Ok(launch::sev::Start {
             policy: self.policy,
@@ -232,9 +248,12 @@ impl Session<Verified> {
         &self,
         flags: launch::sev::HeaderFlags,
         data: &[u8],
-    ) -> Result<launch::sev::Secret> {
+    ) -> std::result::Result<launch::sev::Secret, SessionError> {
         let mut iv = [0u8; 16];
-        rand::rand_bytes(&mut iv)?;
+
+        let mut rng: RdRand = RdRand::new()?;
+
+        rng.try_fill_bytes(&mut iv)?;
 
         let ciphertext = symm::encrypt(symm::Cipher::aes_128_ctr(), &self.tek, Some(&iv), data)?;