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
//! 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! //! //! ## 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 `std` feature is enabled (which it is by default), it's also impl'd //! for `Vec`s of 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, which //! automatically calls `zeroize()` on all members of a struct or tuple struct, //! and adds a `Drop` impl which calls `zeroize()` when the item is dropped: //! //! ``` //! use zeroize::Zeroize; //! //! // This struct will be zeroized on drop //! #[derive(Zeroize)] //! struct MyStruct([u8; 64]); //! ``` //! //! If, for some reason, you only want `Zeroize` to be derived but *don't* //! want an automatic `Drop` impl, you can add the `zeroize(no_drop)` //! attribute: //! //! ``` //! use zeroize::Zeroize; //! //! // This struct will *NOT* be zeroized on drop //! #[derive(Zeroize)] //! #[zeroize(no_drop)] //! struct MyStruct([u8; 64]); //! ``` //! //! If you prefer explicitness, you can add the `#[zeroize(drop)]` //! attribute to signal intent to zeroize values on `Drop`. However note this //! syntax is not necessary as the `Drop` handler is added by default: //! //! ``` //! use zeroize::Zeroize; //! //! // This struct will be zeroized on drop //! #[derive(Zeroize)] //! #[zeroize(drop)] //! struct MyStruct([u8; 64]); //! ``` //! //! ## `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? //! //! Ideally a secure memory-zeroing function would guarantee the following: //! //! 1. Ensure the zeroing operation can't be "optimized away" by the compiler. //! 2. Ensure all subsequent reads to the memory following the zeroing operation //! will always see zeroes. //! //! This crate guarantees #1 is true: LLVM's volatile semantics ensure it. //! //! The story around #2 is much more complicated. In brief, it should be true //! that LLVM's current implementation does not attempt to perform //! optimizations which would allow a subsequent (non-volatile) read to see the //! original value prior to zeroization. However, this is not a guarantee, but //! rather an LLVM implementation detail, a.k.a. *undefined behavior*. //! It provides what we believe to be the best implementation possible on //! stable Rust, but we cannot yet make guarantees it will work reliably //! 100% of the time (particularly on exotic CPU architectures). //! //! For more background, we can look to the [core::ptr::write_volatile] //! documentation: //! //! > Volatile operations are intended to act on I/O memory, and are guaranteed //! > to not be elided or reordered by the compiler across other volatile //! > operations. //! > //! > Memory accessed with `read_volatile` or `write_volatile` should not be //! > accessed with non-volatile operations. //! //! Uhoh! This crate does not guarantee all reads to the memory it operates on //! are volatile, and the documentation for [core::ptr::write_volatile] //! explicitly warns against mixing volatile and non-volatile operations. //! Perhaps we'd be better off with something like a `VolatileCell` //! type which owns the associated data and ensures all reads and writes are //! volatile so we don't have to worry about the semantics of mixing volatile and //! non-volatile accesses. //! //! While that's a strategy worth pursuing (and something we may investigate //! separately from this crate), it comes with some onerous API requirements: //! it means any data that we might ever desire to zero is owned by a //! `VolatileCell`. However, this does not make it possible for this crate //! to act on references, which severely limits its applicability. In fact //! a `VolatileCell` can only act on values, i.e. to read a value from it, //! we'd need to make a copy of it, and that's literally the opposite of //! what we want. //! //! It's worth asking what the precise semantics of mixing volatile and //! non-volatile reads actually are, and whether a less obtrusive API which //! can act entirely on mutable references is possible, safe, and provides the //! desired behavior. //! //! Unfortunately, that's a tricky question, because //! [Rust does not have a formally defined memory model][memory-model], //! and the behavior of mixing volatile and non-volatile memory accesses is //! therefore not rigorously specified and winds up being an LLVM //! implementation detail. The semantics were discussed extensively in this //! thread, specifically in the context of zeroing secrets from memory: //! //! <https://internals.rust-lang.org/t/volatile-and-sensitive-memory/3188/24> //! //! Some notable details from this thread: //! //! - Rust/LLVM's notion of "volatile" is centered around data *accesses*, not //! the data itself. Specifically it maps to flags in LLVM IR which control //! the behavior of the optimizer, and is therefore a bit different from the //! typical C notion of "volatile". //! - As mentioned earlier, LLVM does not presently contain optimizations which //! would reorder a non-volatile read to occur before a volatile write. //! However, there is nothing precluding such optimizations from being added. //! LLVM presently appears to exhibit the desired behavior for point //! #2 above, but there is nothing preventing future versions of Rust //! and/or LLVM from changing that. //! //! To help mitigate concerns about reordering potentially exposing values //! after they have been zeroed, this crate leverages the [core::sync::atomic] //! memory fence functions including [compiler_fence] and [fence] (which uses //! the CPU's native fence instructions). These fences are leveraged with the //! strictest ordering guarantees, [Ordering::SeqCst], which ensures no //! accesses are reordered. Without a formally defined memory model we can't //! guarantee these will be effective, but we hope they will cover most cases. //! //! Concretely the threat of leaking "zeroized" secrets (via reordering by //! LLVM and/or the CPU via out-of-order or speculative execution) would //! require a non-volatile access to be reordered ahead of the following: //! //! 1. before an [Ordering::SeqCst] compiler fence //! 2. before an [Ordering::SeqCst] runtime fence //! 3. before a volatile write //! //! This seems unlikely, but our usage of mixed non-volatile and volatile //! accesses is technically undefined behavior, at least until guarantees //! about this particular mixture of operations is formally defined in a //! Rust memory model. //! //! Furthermore, given the recent history of microarchitectural attacks //! (Spectre, Meltdown, etc), there is also potential for "zeroized" secrets //! to be leaked through covert channels (e.g. memory fences have been used //! as a covert channel), so we are wary to make guarantees unless they can //! be made firmly in terms of both a formal Rust memory model and the //! generated code for a particular CPU architecture. //! //! In conclusion, this crate guarantees the zeroize operation will not be //! elided or "optimized away", makes a "best effort" to ensure that //! memory accesses will not be reordered ahead of the "zeroize" operation, //! but **cannot** yet guarantee that such reordering will not occur. //! //! In the future it might be possible to guarantee such behavior using //! [LLVM's "unordered" atomic mode][unordered], which is documented as //! being free of undefined behavior. There's an open issue to //! [expose atomic memcpy/memset in core/std][llvm-atomic] //! in which case this crate could leverage them to provide well-defined //! guarantees that zeroization will always occur. //! //! ## 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. //! //! This crate does not intend to implement higher-level abstractions to //! eliminate these risks, instead it merely makes a best effort to clear the //! memory it's aware of. //! //! Crates which are built on `zeroize` and provide higher-level abstractions //! for strategically avoiding these problems would certainly be interesting! //! (and something we may consider developing in the future) //! //! ## What about: clearing registers, mlock, mprotect, etc? //! //! This crate is laser-focused on being a simple, unobtrusive crate for zeroing //! memory in as reliable a manner as is 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 //! [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 //! [fence]: https://doc.rust-lang.org/stable/core/sync/atomic/fn.fence.html //! [memory-model]: https://github.com/nikomatsakis/rust-memory-model //! [unordered]: https://llvm.org/docs/Atomics.html#unordered //! [llvm-atomic]: https://github.com/rust-lang/rust/issues/58599 //! [pin]: https://doc.rust-lang.org/std/pin/struct.Pin.html //! [good cryptographic hygiene]: https://cryptocoding.net/index.php/Coding_rules#Clean_memory_of_secret_data #![no_std] #![deny( warnings, missing_docs, trivial_casts, trivial_numeric_casts, unused_import_braces, unused_qualifications )] #![doc(html_root_url = "https://docs.rs/zeroize/0.7.0")] #[cfg(any(feature = "std", test))] #[cfg_attr(test, macro_use)] extern crate std; #[cfg(feature = "zeroize_derive")] #[allow(unused_imports)] #[macro_use] extern crate zeroize_derive; #[cfg(feature = "zeroize_derive")] #[doc(hidden)] pub use zeroize_derive::*; use core::{ops, ptr, slice::IterMut, sync::atomic}; #[cfg(all(feature = "alloc", not(feature = "std")))] use alloc::{string::String, vec::Vec}; #[cfg(feature = "std")] use std::{string::String, vec::Vec}; /// Trait for securely erasing types from memory pub trait Zeroize { /// Zero out this object from memory (using Rust or OS intrinsics which /// ensure the zeroization operation is not "optimized away") 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 $type {})+ }; } impl_zeroize_with_default!(i8, i16, i32, i64, i128, isize); impl_zeroize_with_default!(u8, u16, u32, u64, u128, usize); impl_zeroize_with_default!(f32, f64, char, bool); /// Implement `Zeroize` on arrays of types that can be zeroized with `Default`. /// /// This impl can eventually be optimized using an atomic memset intrinsic. /// See notes for the blanket impl of `Zeroize` on `[Z]`. macro_rules! impl_zeroize_for_array { ($($size:expr),+) => { $( impl<Z> Zeroize for [Z; $size] where Z: DefaultIsZeroes { fn zeroize(&mut self) { self.as_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(); } } } /// Implement `Zeroize` on slices of types that can be zeroized with `Default`. /// /// This impl can eventually be optimized using an atomic memset intrinsic, /// such as `llvm.memset.element.unordered.atomic`. For that reason the blanket /// impl on slices is bounded by `DefaultIsZeroes`. See: /// /// <https://github.com/rust-lang/rust/issues/58599> /// /// 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) { volatile_set(self, Z::default()); atomic_fence(); } } #[cfg(feature = "alloc")] impl<Z> Zeroize for Vec<Z> where Z: DefaultIsZeroes, { fn zeroize(&mut self) { self.resize(self.capacity(), Default::default()); self.as_mut_slice().zeroize(); self.clear(); } } #[cfg(feature = "alloc")] impl Zeroize for String { fn zeroize(&mut self) { unsafe { self.as_bytes_mut() }.zeroize(); debug_assert!(self.as_bytes().iter().all(|b| *b == 0)); self.clear(); } } /// `Zeroizing` is a a wrapper for any `Z: Zeroize` type which implements a /// `Drop` handler which zeroizes dropped values. pub struct Zeroizing<Z: Zeroize>(Z); impl<Z> Zeroizing<Z> where Z: Zeroize, { /// Wrap a value in `Zeroizing`, ensuring it's zeroized on drop. pub fn new(value: Z) -> Self { 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(); } } // We could `derive(Zeroize)` for this, but doing it by hand allows `Zeroizing` // to function regardless of whether the `zeroize_derive` feature is enabled // or not. 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::fence(atomic::Ordering::SeqCst); atomic::compiler_fence(atomic::Ordering::SeqCst); } /// Perform a volatile write to the destination // TODO(tarcieri): replace this with atomic writes when they're stable #[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 // TODO(tarcieri): use `llvm.memset.element.unordered.atomic` // See: https://github.com/rust-lang/rust/issues/58599 #[inline] fn volatile_set<T: Copy + Sized>(dst: &mut [T], src: T) { // TODO(tarcieri): use `volatile_set_memory` on nightly? for elem in dst { volatile_write(elem, src); } } #[cfg(test)] mod tests { use super::*; #[cfg(all(feature = "alloc", not(feature = "std")))] use alloc::boxed::Box; #[cfg(feature = "std")] use std::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_past_len() { let mut vec = Vec::with_capacity(5); for i in 0..4 { vec.push(10 + i); } vec.clear(); // safe if: new_len <= capacity AND elements "were initialised" unsafe { vec.set_len(1); } assert_eq!(10, vec[0], "clear() hasn't erased our push()es"); vec.clear(); vec.zeroize(); unsafe { vec.set_len(4); } for i in 0..4 { assert_eq!(0, vec[i], "it's been zero'd"); } } #[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_box() { let mut boxed_arr = Box::new([42u8; 3]); boxed_arr.zeroize(); assert_eq!(boxed_arr.as_ref(), &[0u8; 3]); } }