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|
//! Memory protection.
use std::cmp::{min, Ordering};
use std::fmt;
use std::ops::{Deref, DerefMut};
use std::pin::Pin;
use memsec;
/// Protected memory.
///
/// The memory is guaranteed not to be copied around, and is cleared
/// when the object is dropped.
#[derive(Clone, Eq, Hash)]
pub struct Protected(Pin<Box<[u8]>>);
impl PartialEq for Protected {
fn eq(&self, other: &Self) -> bool {
secure_cmp(&self.0, &other.0) == Ordering::Equal
}
}
impl Protected {
/// Converts to a buffer for modification.
pub unsafe fn into_vec(self) -> Vec<u8> {
self.iter().cloned().collect()
}
}
impl Deref for Protected {
type Target = [u8];
fn deref(&self) -> &Self::Target {
&self.0
}
}
impl AsRef<[u8]> for Protected {
fn as_ref(&self) -> &[u8] {
&self.0
}
}
impl AsMut<[u8]> for Protected {
fn as_mut(&mut self) -> &mut [u8] {
&mut self.0
}
}
impl DerefMut for Protected {
fn deref_mut(&mut self) -> &mut [u8] {
&mut self.0
}
}
impl From<Vec<u8>> for Protected {
fn from(v: Vec<u8>) -> Self {
Protected(Pin::new(v.into_boxed_slice()))
}
}
impl From<Box<[u8]>> for Protected {
fn from(v: Box<[u8]>) -> Self {
Protected(Pin::new(v))
}
}
impl From<&[u8]> for Protected {
fn from(v: &[u8]) -> Self {
Vec::from(v).into()
}
}
impl Drop for Protected {
fn drop(&mut self) {
unsafe {
memsec::memzero(self.0.as_mut_ptr(), self.0.len());
}
}
}
impl fmt::Debug for Protected {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
if cfg!(debug_assertions) {
write!(f, "{:?}", self.0)
} else {
f.write_str("[<Redacted>]")
}
}
}
/// Encrypted memory.
///
/// This type encrypts sensitive data, such as secret keys, in memory
/// while they are unused, and decrypts them on demand. This protects
/// against cross-protection-boundary readout via microarchitectural
/// flaws like Spectre or Meltdown, via attacks on physical layout
/// like Rowbleed, and even via coldboot attacks.
///
/// The key insight is that these kinds of attacks are imperfect,
/// i.e. the recovered data contains bitflips, or the attack only
/// provides a probability for any given bit. Applied to
/// cryptographic keys, these kind of imperfect attacks are enough to
/// recover the actual key.
///
/// This implementation on the other hand, derives a sealing key from
/// a large area of memory, the "pre-key", using a key derivation
/// function. Now, any single bitflip in the readout of the pre-key
/// will avalanche through all the bits in the sealing key, rendering
/// it unusable with no indication of where the error occurred.
///
/// This kind of protection was pioneered by OpenSSH. The commit
/// adding it is: https://marc.info/?l=openbsd-cvs&m=156109087822676
#[derive(Clone, Debug, PartialEq, Eq, Hash)]
pub struct Encrypted {
ciphertext: Protected,
iv: Protected,
}
/// The number of pages containing random bytes to derive the prekey
/// from.
const ENCRYPTED_MEMORY_PREKEY_PAGES: usize = 4;
/// Page size.
const ENCRYPTED_MEMORY_PAGE_SIZE: usize = 4096;
/// This module contains the code that needs to access the prekey.
///
/// Code outside of it cannot access it, because `PREKEY` is private.
mod has_access_to_prekey {
use std::io::{self, Cursor, Write};
use lazy_static;
use crate::constants::{AEADAlgorithm, HashAlgorithm, SymmetricAlgorithm};
use crate::crypto::{aead, SessionKey};
use super::*;
lazy_static::lazy_static! {
static ref PREKEY: Box<[Box<[u8]>]> = {
let mut pages = Vec::new();
for _ in 0..ENCRYPTED_MEMORY_PREKEY_PAGES {
let mut page = vec![0; ENCRYPTED_MEMORY_PAGE_SIZE];
crate::crypto::random(&mut page);
pages.push(page.into());
}
pages.into()
};
}
// Algorithms used for the memory encryption.
//
// The digest of the hash algorithm must be at least as large as
// the size of the key used by the symmetric algorithm. All
// algorithms MUST be supported by the cryptographic library.
const HASH_ALGO: HashAlgorithm = HashAlgorithm::SHA256;
const SYMMETRIC_ALGO: SymmetricAlgorithm = SymmetricAlgorithm::AES256;
const AEAD_ALGO: AEADAlgorithm = AEADAlgorithm::EAX;
impl Encrypted {
/// Computes the sealing key used to encrypt the memory.
fn sealing_key() -> SessionKey {
let mut ctx = HASH_ALGO.context()
.expect("Mandatory algorithm unsupported");
PREKEY.iter().for_each(|page| ctx.update(page));
let mut sk: SessionKey = vec![0; 256/8].into();
ctx.digest(&mut sk);
sk
}
/// Encrypts the given chunk of memory.
pub fn new(p: Protected) -> Self {
let mut iv =
vec![0; AEAD_ALGO.iv_size()
.expect("Mandatory algorithm unsupported")];
crate::crypto::random(&mut iv);
let mut ciphertext = Vec::new();
{
let mut encryptor =
aead::Encryptor::new(1,
SYMMETRIC_ALGO,
AEAD_ALGO,
4096,
&iv,
&Self::sealing_key(),
&mut ciphertext)
.expect("Mandatory algorithm unsupported");
encryptor.write_all(&p).unwrap();
encryptor.finish().unwrap();
}
Encrypted {
ciphertext: ciphertext.into(),
iv: iv.into(),
}
}
/// Maps the given function over the temporarily decrypted
/// memory.
pub fn map<F, T>(&self, mut fun: F) -> T
where F: FnMut(&Protected) -> T
{
let mut plaintext = Vec::new();
let mut decryptor =
aead::Decryptor::new(1,
SYMMETRIC_ALGO,
AEAD_ALGO,
4096,
&self.iv,
&Self::sealing_key(),
Cursor::new(&self.ciphertext))
.expect("Mandatory algorithm unsupported");
io::copy(&mut decryptor, &mut plaintext)
.expect("Encrypted memory modified or corrupted");
let plaintext: Protected = plaintext.into();
fun(&plaintext)
}
}
}
/// Time-constant comparison.
pub fn secure_cmp(a: &[u8], b: &[u8]) -> Ordering {
let ord1 = a.len().cmp(&b.len());
let ord2 = unsafe {
memsec::memcmp(a.as_ptr(), b.as_ptr(), min(a.len(), b.len()))
};
let ord2 = match ord2 {
0 => Ordering::Equal,
a if a < 0
|