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Align Secret to the page size of the current system. #110

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Align Secret to the page size of the current system. #110

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3 changes: 3 additions & 0 deletions Cargo.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,9 @@ pkg-config = '0.3'
[target.'cfg(target_family = "windows")'.build-dependencies]
vcpkg = '0.2'

[build-dependencies]
page_size = '0.6'

[dev-dependencies]
libsodium-sys = '0.2'

Expand Down
12 changes: 11 additions & 1 deletion build.rs
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Expand Up @@ -62,6 +62,16 @@ fn main() {
// naïvely
println!("cargo:rustc-link-lib=dylib=sodium");
};

let out_dir = env::var("OUT_DIR").unwrap();
let out_path = std::path::Path::new(&out_dir).join("secret_pad.rs");

// probe the page size of the current system and write it out so we
// can align `Secret` using it
std::fs::write(out_path, format!(
"#[repr(align({}))]\npub(crate) struct Padding();",
page_size::get()
)).unwrap();
}

#[cfg(feature = "use-libsodium-sys")]
Expand Down Expand Up @@ -116,4 +126,4 @@ fn link(name: &str, _version: &str) -> Option<()> {
None
}
}
}
}
15 changes: 8 additions & 7 deletions src/secret.rs
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Expand Up @@ -9,6 +9,8 @@ use std::fmt::{self, Debug, Formatter};
use std::ops::{Deref, DerefMut};
use std::thread;

include!(concat!(env!("OUT_DIR"), "/secret_pad.rs"));

/// A type for protecting secrets allocated on the stack.
///
/// Stack-allocated secrets have distinct security needs from
Expand Down Expand Up @@ -78,9 +80,13 @@ use std::thread;
/// ```
///
/// [mlock]: http://man7.org/linux/man-pages/man2/mlock.2.html

pub struct Secret<T: Bytes> {
/// The internal protected memory for the [`Secret`].
data: T,

/// Additional padding to ensure we occupy a dedicated page.
_pad: Padding,
}

/// A mutable [`Deref`]-wrapper around a [`Secret`]'s internal
Expand Down Expand Up @@ -121,6 +127,7 @@ impl<T: Bytes> Secret<T> {

let mut secret = Self {
data: T::uninitialized(),
_pad: Padding(),
};

assert!(
Expand Down Expand Up @@ -203,13 +210,7 @@ impl<T: Bytes> Drop for Secret<T> {
/// Ensures that the [`Secret`]'s underlying memory is `munlock`ed
/// and zeroed when it leaves scope.
fn drop(&mut self) {
// When we call sodium_munlock on some data, it actually unlocks the entire page that
// contains the memory. If two locked items were on the same page, then the second one
// fails because it was already unlocked. On Linux, this does now throw an error. On
// Windows, it does. We'll ignore it for now, and provide a better fix later.
if unsafe { !sodium::munlock(&raw mut self.data) }
&& !(cfg!(target_family = "windows")
&& (std::io::Error::last_os_error().raw_os_error() == Some(158))) {
if unsafe { !sodium::munlock(&raw mut self.data) } {
// [`Drop::drop`] is called during stack unwinding, so we
// may be in a panic already.
assert!(
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