1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
//! Securely zero memory with a simple trait ([Zeroize]) built on stable Rust
//! primitives which guarantee the operation will not be "optimized away".
//!
//! ## About
//!
//! [Zeroing memory securely is hard] - compilers optimize for performance, and
//! in doing so they love to "optimize away" unnecessary zeroing calls. There are
//! many documented "tricks" to attempt to avoid these optimizations and ensure
//! that a zeroing routine is performed reliably.
//!
//! This crate isn't about tricks: it uses [core::ptr::write_volatile]
//! and [core::sync::atomic] memory fences to provide easy-to-use, portable
//! zeroing behavior which works on all of Rust's core number types and slices
//! thereof, implemented in pure Rust with no usage of FFI or assembly.
//!
//! - No insecure fallbacks!
//! - No dependencies!
//! - No FFI or inline assembly! **WASM friendly** (and tested)!
//! - `#![no_std]` i.e. **embedded-friendly**!
//! - No functionality besides securely zeroing memory!
//! - (Optional) Custom derive support for zeroing complex structures
//!
//! ## Minimum Supported Rust Version
//!
//! Requires Rust **1.44** or newer.
//!
//! In the future, we reserve the right to change MSRV (i.e. MSRV is out-of-scope
//! for this crate's SemVer guarantees), however when we do it will be accompanied
//! with a minor version bump.
//!
//! ## Usage
//!
//! ```
//! use zeroize::Zeroize;
//!
//! fn main() {
//!     // Protip: don't embed secrets in your source code.
//!     // This is just an example.
//!     let mut secret = b"Air shield password: 1,2,3,4,5".to_vec();
//!     // [ ... ] open the air shield here
//!
//!     // Now that we're done using the secret, zero it out.
//!     secret.zeroize();
//! }
//! ```
//!
//! The [Zeroize] trait is impl'd on all of Rust's core scalar types including
//! integers, floats, `bool`, and `char`.
//!
//! Additionally, it's implemented on slices and `IterMut`s of the above types.
//!
//! When the `alloc` feature is enabled (which it is by default), it's also
//! impl'd for `Vec<T>` for the above types as well as `String`, where it provides
//! [Vec::clear()] / [String::clear()]-like behavior (truncating to zero-length)
//! but ensures the backing memory is securely zeroed with some caveats.
//! (NOTE: see "Stack/Heap Zeroing Notes" for important `Vec`/`String` details)
//!
//! The [DefaultIsZeroes] marker trait can be impl'd on types which also
//! impl [Default], which implements [Zeroize] by overwriting a value with
//! the default value.
//!
//! ## Custom Derive Support
//!
//! This crate has custom derive support for the `Zeroize` trait,
//! gated under the `zeroize` crate's `zeroize_derive` Cargo feature,
//! which automatically calls `zeroize()` on all members of a struct
//! or tuple struct.
//!
//! Additionally it supports the following attribute:
//!
//! - `#[zeroize(drop)]`: call `zeroize()` when this item is dropped
//!
//! Example which derives `Drop`:
//!
//! ```
//! # #[cfg(feature = "derive")]
//! # {
//! use zeroize::Zeroize;
//!
//! // This struct will be zeroized on drop
//! #[derive(Zeroize)]
//! #[zeroize(drop)]
//! struct MyStruct([u8; 32]);
//! # }
//! ```
//!
//! Example which does not derive `Drop` (useful for e.g. `Copy` types)
//!
//! ```
//! #[cfg(feature = "derive")]
//! # {
//! use zeroize::Zeroize;
//!
//! // This struct will *NOT* be zeroized on drop
//! #[derive(Copy, Clone, Zeroize)]
//! struct MyStruct([u8; 32]);
//! # }
//! ```
//!
//! ## `Zeroizing<Z>`: wrapper for zeroizing arbitrary values on drop
//!
//! `Zeroizing<Z: Zeroize>` is a generic wrapper type that impls `Deref`
//! and `DerefMut`, allowing access to an inner value of type `Z`, and also
//! impls a `Drop` handler which calls `zeroize()` on its contents:
//!
//! ```
//! use zeroize::Zeroizing;
//!
//! fn main() {
//!     let mut secret = Zeroizing::new([0u8; 5]);
//!
//!     // Set the air shield password
//!     // Protip (again): don't embed secrets in your source code.
//!     secret.copy_from_slice(&[1, 2, 3, 4, 5]);
//!     assert_eq!(secret.as_ref(), &[1, 2, 3, 4, 5]);
//!
//!     // The contents of `secret` will be automatically zeroized on drop
//! }
//! ```
//!
//! ## What guarantees does this crate provide?
//!
//! This crate guarantees the following:
//!
//! 1. The zeroing operation can't be "optimized away" by the compiler.
//! 2. All subsequent reads to memory will see "zeroized" values.
//!
//! LLVM's volatile semantics ensure #1 is true.
//!
//! Additionally, thanks to work by the [Unsafe Code Guidelines Working Group],
//! we can now fairly confidently say #2 is true as well. Previously there were
//! worries that the approach used by this crate (mixing volatile and
//! non-volatile accesses) was undefined behavior due to language contained
//! in the documentation for `write_volatile`, however after some discussion
//! [these remarks have been removed] and the specific usage pattern in this
//! crate is considered to be well-defined.
//!
//! Additionally this crate leverages [compiler_fence] from
//! [core::sync::atomic] with the strictest ordering ([Ordering::SeqCst])
//! as a precaution to help ensure reads are not reordered before memory has
//! been zeroed.
//!
//! All of that said, there is still potential for microarchitectural attacks
//! (ala Spectre/Meltdown) to leak "zeroized" secrets through covert channels.
//! This crate makes no guarantees that zeroized values cannot be leaked
//! through such channels, as they represent flaws in the underlying hardware.
//!
//! ## Stack/Heap Zeroing Notes
//!
//! This crate can be used to zero values from either the stack or the heap.
//!
//! However, be aware several operations in Rust can unintentionally leave
//! copies of data in memory. This includes but is not limited to:
//!
//! - Moves and `Copy`
//! - Heap reallocation when using `Vec` and `String`
//! - Borrowers of a reference making copies of the data
//!
//! [`Pin`][pin] can be leveraged in conjunction with this crate to ensure
//! data kept on the stack isn't moved.
//!
//! The `Zeroize` impls for `Vec` and `String` zeroize the entire capacity of
//! their backing buffer, but cannot guarantee copies of the data were not
//! previously made by buffer reallocation. It's therefore important when
//! attempting to zeroize such buffers to initialize them to the correct
//! capacity, and take care to prevent subsequent reallocation.
//!
//! The `secrecy` crate provides higher-level abstractions for eliminating
//! usage patterns which can cause reallocations:
//!
//! <https://crates.io/crates/secrecy>
//!
//! ## What about: clearing registers, mlock, mprotect, etc?
//!
//! This crate is focused on providing simple, unobtrusive support for reliably
//! zeroing memory using the best approach possible on stable Rust.
//!
//! Clearing registers is a difficult problem that can't easily be solved by
//! something like a crate, and requires either inline ASM or rustc support.
//! See <https://github.com/rust-lang/rust/issues/17046> for background on
//! this particular problem.
//!
//! Other memory protection mechanisms are interesting and useful, but often
//! overkill (e.g. defending against RAM scraping or attackers with swap access).
//! In as much as there may be merit to these approaches, there are also many
//! other crates that already implement more sophisticated memory protections.
//! Such protections are explicitly out-of-scope for this crate.
//!
//! Zeroing memory is [good cryptographic hygiene] and this crate seeks to promote
//! it in the most unobtrusive manner possible. This includes omitting complex
//! `unsafe` memory protection systems and just trying to make the best memory
//! zeroing crate available.
//!
//! [Zeroize]: https://docs.rs/zeroize/latest/zeroize/trait.Zeroize.html
//! [Zeroing memory securely is hard]: http://www.daemonology.net/blog/2014-09-04-how-to-zero-a-buffer.html
//! [Vec::clear()]: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.clear
//! [String::clear()]: https://doc.rust-lang.org/std/string/struct.String.html#method.clear
//! [DefaultIsZeroes]: https://docs.rs/zeroize/latest/zeroize/trait.DefaultIsZeroes.html
//! [Default]: https://doc.rust-lang.org/std/default/trait.Default.html
//! [core::ptr::write_volatile]: https://doc.rust-lang.org/core/ptr/fn.write_volatile.html
//! [Unsafe Code Guidelines Working Group]: https://github.com/rust-lang/unsafe-code-guidelines
//! [these remarks have been removed]: https://github.com/rust-lang/rust/pull/60972
//! [core::sync::atomic]: https://doc.rust-lang.org/stable/core/sync/atomic/index.html
//! [Ordering::SeqCst]: https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html#variant.SeqCst
//! [compiler_fence]: https://doc.rust-lang.org/stable/core/sync/atomic/fn.compiler_fence.html
//! [pin]: https://doc.rust-lang.org/std/pin/struct.Pin.html
//! [good cryptographic hygiene]: https://github.com/veorq/cryptocoding#clean-memory-of-secret-data

#![no_std]
#![cfg_attr(docsrs, feature(doc_cfg))]
#![doc(html_root_url = "https://docs.rs/zeroize/1.2.0")]
#![warn(missing_docs, rust_2018_idioms, trivial_casts, unused_qualifications)]

#[cfg(feature = "alloc")]
#[cfg_attr(test, macro_use)]
extern crate alloc;

#[cfg(feature = "zeroize_derive")]
#[cfg_attr(docsrs, doc(cfg(feature = "zeroize_derive")))]
pub use zeroize_derive::Zeroize;

#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
mod x86;

use core::{ops, ptr, slice::IterMut, sync::atomic};

#[cfg(feature = "alloc")]
use alloc::{string::String, vec::Vec};

/// Trait for securely erasing types from memory
pub trait Zeroize {
    /// Zero out this object from memory using Rust intrinsics which ensure the
    /// zeroization operation is not "optimized away" by the compiler.
    fn zeroize(&mut self);
}

/// Marker trait for types whose `Default` is the desired zeroization result
pub trait DefaultIsZeroes: Copy + Default + Sized {}

impl<Z> Zeroize for Z
where
    Z: DefaultIsZeroes,
{
    fn zeroize(&mut self) {
        volatile_write(self, Z::default());
        atomic_fence();
    }
}

macro_rules! impl_zeroize_with_default {
    ($($type:ty),+) => {
        $(impl DefaultIsZeroes for $type {})+
    };
}

impl_zeroize_with_default!(i8, i16, i32, i64, i128, isize);
impl_zeroize_with_default!(u8, u16, u32, u64, u128, usize);
impl_zeroize_with_default!(f32, f64, char, bool);

/// Implement `Zeroize` on arrays of types that impl `Zeroize`
macro_rules! impl_zeroize_for_array {
    ($($size:expr),+) => {
        $(
            impl<Z> Zeroize for [Z; $size]
            where
                Z: Zeroize
            {
                fn zeroize(&mut self) {
                    self.iter_mut().zeroize();
                }
            }
        )+
     };
}

// TODO(tarcieri): const generics
impl_zeroize_for_array!(
    1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,
    27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
    51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64
);

impl<'a, Z> Zeroize for IterMut<'a, Z>
where
    Z: Zeroize,
{
    fn zeroize(&mut self) {
        for elem in self {
            elem.zeroize();
        }
    }
}

impl<Z> Zeroize for Option<Z>
where
    Z: Zeroize,
{
    fn zeroize(&mut self) {
        match self {
            Some(value) => value.zeroize(),
            None => (),
        }
    }
}

/// Impl `Zeroize` on slices of types that can be zeroized with `Default`.
///
/// This impl can eventually be optimized using an memset intrinsic,
/// such as `core::intrinsics::volatile_set_memory`. For that reason the blanket
/// impl on slices is bounded by `DefaultIsZeroes`.
///
/// To zeroize a mut slice of `Z: Zeroize` which does not impl
/// `DefaultIsZeroes`, call `iter_mut().zeroize()`.
impl<Z> Zeroize for [Z]
where
    Z: DefaultIsZeroes,
{
    fn zeroize(&mut self) {
        assert!(self.len() <= core::isize::MAX as usize);
        // Safety:
        //
        // This is safe, because the slice is well aligned and is backed by a single allocated
        // object for at least `self.len()` elements of type `Z`.
        // `self.len()` is also not larger than an `isize`, because of the assertion above.
        // The memory of the slice should not wrap around the address space.
        unsafe { volatile_set(self.as_mut_ptr(), Z::default(), self.len()) };
        atomic_fence();
    }
}

#[cfg(feature = "alloc")]
#[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
impl<Z> Zeroize for Vec<Z>
where
    Z: Zeroize,
{
    /// "Best effort" zeroization for `Vec`.
    ///
    /// Ensures the entire capacity of the `Vec` is zeroed. Cannot ensure that
    /// previous reallocations did not leave values on the heap.
    fn zeroize(&mut self) {
        self.iter_mut().zeroize();

        // Zero the capacity of the `Vec` that is not initialized.
        {
            // Safety:
            //
            // This is safe, because `Vec` never allocates more than `isize::MAX` bytes.
            // This exact use case is even mentioned in the documentation of `pointer::add`.
            let extra_capacity_start = unsafe { self.as_mut_ptr().add(self.len()) as *mut u8 };
            let extra_capacity_len = self.capacity().saturating_sub(self.len());

            // Safety:
            // The memory pointed to by `extra_capacity_start` is valid for `extra_capacity_len *
            // mem::size_of::<Z>()` bytes, because the allocation of the `Vec` has enough reported
            // capacity for elements of type `Z`.
            // It is also properly aligned, because the `T` here is `u8`, which has an alignment of
            // `1`.
            // `extra_capacity_len` is not larger than an `isize`, because `Vec` never allocates
            // more than `isize::MAX` bytes.
            // The `Vec` allocation also guarantees to never wrap around the address space.
            unsafe {
                volatile_set(
                    extra_capacity_start,
                    0,
                    extra_capacity_len * core::mem::size_of::<Z>(),
                )
            };
            atomic_fence();
        }

        self.clear();
    }
}

#[cfg(feature = "alloc")]
#[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
impl Zeroize for String {
    fn zeroize(&mut self) {
        unsafe { self.as_mut_vec() }.zeroize();
    }
}

/// Fallible trait for representing cases where zeroization may or may not be
/// possible.
///
/// This is primarily useful for scenarios like reference counted data, where
/// zeroization is only possible when the last reference is dropped.
pub trait TryZeroize {
    /// Try to zero out this object from memory using Rust intrinsics which
    /// ensure the zeroization operation is not "optimized away" by the
    /// compiler.
    #[must_use]
    fn try_zeroize(&mut self) -> bool;
}

/// `Zeroizing` is a a wrapper for any `Z: Zeroize` type which implements a
/// `Drop` handler which zeroizes dropped values.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Zeroizing<Z: Zeroize>(Z);

impl<Z> Zeroizing<Z>
where
    Z: Zeroize,
{
    /// Move value inside a `Zeroizing` wrapper which ensures it will be
    /// zeroized when it's dropped.
    pub fn new(value: Z) -> Self {
        value.into()
    }
}

impl<Z> From<Z> for Zeroizing<Z>
where
    Z: Zeroize,
{
    fn from(value: Z) -> Zeroizing<Z> {
        Zeroizing(value)
    }
}

impl<Z> ops::Deref for Zeroizing<Z>
where
    Z: Zeroize,
{
    type Target = Z;

    fn deref(&self) -> &Z {
        &self.0
    }
}

impl<Z> ops::DerefMut for Zeroizing<Z>
where
    Z: Zeroize,
{
    fn deref_mut(&mut self) -> &mut Z {
        &mut self.0
    }
}

impl<Z> Zeroize for Zeroizing<Z>
where
    Z: Zeroize,
{
    fn zeroize(&mut self) {
        self.0.zeroize();
    }
}

impl<Z> Drop for Zeroizing<Z>
where
    Z: Zeroize,
{
    fn drop(&mut self) {
        self.0.zeroize()
    }
}

/// Use fences to prevent accesses from being reordered before this
/// point, which should hopefully help ensure that all accessors
/// see zeroes after this point.
#[inline]
fn atomic_fence() {
    atomic::compiler_fence(atomic::Ordering::SeqCst);
}

/// Perform a volatile write to the destination
#[inline]
fn volatile_write<T: Copy + Sized>(dst: &mut T, src: T) {
    unsafe { ptr::write_volatile(dst, src) }
}

/// Perform a volatile `memset` operation which fills a slice with a value
///
/// Safety:
/// The memory pointed to by `dst` must be a single allocated object that is valid for `count`
/// contiguous elements of `T`.
/// `count` must not be larger than an `isize`.
/// `dst` being offset by `mem::size_of::<T> * count` bytes must not wrap around the address space.
/// Also `dst` must be properly aligned.
#[inline]
unsafe fn volatile_set<T: Copy + Sized>(dst: *mut T, src: T, count: usize) {
    // TODO(tarcieri): use `volatile_set_memory` when stabilized
    for i in 0..count {
        // Safety:
        //
        // This is safe because there is room for at least `count` objects of type `T` in the
        // allocation pointed to by `dst`, because `count <= isize::MAX` and because
        // `dst.add(count)` must not wrap around the address space.
        let ptr = dst.add(i);
        // Safety:
        //
        // This is safe, because the pointer is valid and because `dst` is well aligned for `T` and
        // `ptr` is an offset of `dst` by a multiple of `mem::size_of::<T>()` bytes.
        ptr::write_volatile(ptr, src);
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    #[cfg(feature = "alloc")]
    use alloc::boxed::Box;

    #[test]
    fn zeroize_byte_arrays() {
        let mut arr = [42u8; 64];
        arr.zeroize();
        assert_eq!(arr.as_ref(), [0u8; 64].as_ref());
    }

    #[cfg(feature = "alloc")]
    #[test]
    fn zeroize_vec() {
        let mut vec = vec![42; 3];
        vec.zeroize();
        assert!(vec.is_empty());
    }

    #[cfg(feature = "alloc")]
    #[test]
    fn zeroize_vec_entire_capacity() {
        #[derive(Clone)]
        struct PanicOnNonZeroDrop(u64);

        impl Zeroize for PanicOnNonZeroDrop {
            fn zeroize(&mut self) {
                self.0 = 0;
            }
        }

        impl Drop for PanicOnNonZeroDrop {
            fn drop(&mut self) {
                if self.0 != 0 {
                    panic!("dropped non-zeroized data");
                }
            }
        }

        // Ensure that the entire capacity of the vec is zeroized and that no unitinialized data
        // is ever interpreted as initialized
        let mut vec = vec![PanicOnNonZeroDrop(42); 2];

        unsafe {
            vec.set_len(1);
        }

        vec.zeroize();

        unsafe {
            vec.set_len(2);
        }

        drop(vec);
    }

    #[cfg(feature = "alloc")]
    #[test]
    fn zeroize_string() {
        let mut string = String::from("Hello, world!");
        string.zeroize();
        assert!(string.is_empty());
    }

    #[cfg(feature = "alloc")]
    #[test]
    fn zeroize_string_entire_capacity() {
        let mut string = String::from("Hello, world!");
        string.truncate(5);

        string.zeroize();

        // convert the string to a vec to easily access the unused capacity
        let mut as_vec = string.into_bytes();
        unsafe { as_vec.set_len(as_vec.capacity()) };

        assert!(as_vec.iter().all(|byte| *byte == 0));
    }

    #[cfg(feature = "alloc")]
    #[test]
    fn zeroize_box() {
        let mut boxed_arr = Box::new([42u8; 3]);
        boxed_arr.zeroize();
        assert_eq!(boxed_arr.as_ref(), &[0u8; 3]);
    }
}