crates/veilvoice-crypto/src/shred.rs
what this file is for · veilvoice-crypto · 417 lines · the same file on GitHub
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// SPDX-License-Identifier: GPL-3.0-or-later
//! Secure erasure, the self-destruct.
//!
//! # Read this before relying on it
//!
//! Overwriting a file does not reliably destroy it on modern storage, and any
//! tool that tells you otherwise is selling something. This module does the best
//! that software can do from userspace, reports honestly what that is worth on
//! your storage, and points at the thing that actually works.
//!
//! **On a spinning disk**, overwriting is genuinely effective. The write goes to
//! the same physical sectors, and the belief that a scanning-microscope recovery
//! of overwritten magnetic media is practical does not survive contact with the
//! literature. Gutmann's 1996 paper, whose 35-pass pattern is still cited, says
//! so himself in its own epilogue about modern drives.
//!
//! **On an SSD, or any flash media**, it is not reliable and cannot be made so.
//! Wear levelling means the controller writes your "overwrite" to *different*
//! physical cells and marks the old ones free. The original data still exists in
//! flash, out of reach of every write you can issue. The same applies to SD
//! cards, USB sticks, eMMC and NVMe. It also applies through copy-on-write
//! filesystems (Btrfs, ZFS, APFS), snapshots, journals and any backup that has
//! already run.
//!
//! **The answer that does work is full-disk encryption.** If the volume is
//! encrypted, destroying the key destroys everything on it at once, wherever the
//! controller chose to put the blocks. LUKS, BitLocker and FileVault all do
//! this. Use it, and treat this module as a second line rather than a first.
//!
//! # Why not 35 passes
//!
//! Because passes stopped being the interesting variable decades ago. Against a
//! drive that honours writes, one pass is enough; against one that does not, no
//! number of passes reaches the retained cells. The default here is three,
//! random then complement then random, which satisfies the common
//! three-pass expectation without pretending that thirty-five would be stronger.
//! Time is better spent enabling disk encryption than on passes 4 through 35.
//!
//! # In plain words
//!
//! This is meant to destroy a file, and the first thing it does is tell you how
//! much that is worth.
//!
//! On the drives most computers now have, overwriting a file does not reliably
//! remove it. The drive puts the new data somewhere else and leaves the original
//! sitting in a place no ordinary program can reach, until it is cleaned up later,
//! which may be never.
//!
//! So this does what it can and refuses to promise more. If you need something to
//! be genuinely unrecoverable, encrypt it from the start and never write it
//! unencrypted anywhere.
use crate::Error;
use std::fs::OpenOptions;
use std::io::{Seek, SeekFrom, Write};
use std::path::Path;
/// Bytes written per chunk. Large enough to be fast, small enough that a huge
/// file does not need a huge buffer.
const CHUNK: usize = 1 << 20;
/// How thoroughly to overwrite before unlinking.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum Passes {
/// One pass of random data. Sufficient for any drive that honours writes.
Single,
/// Random, then its complement, then random again. The default.
#[default]
Triple,
/// A caller-chosen number of random passes, clamped to a sane maximum.
Custom(u8),
}
impl Passes {
/// How many overwriting passes this setting means, with a custom count
/// held between 1 and 32. Zero passes would be a shred that shreds nothing.
fn count(self) -> u8 {
match self {
Self::Single => 1,
Self::Triple => 3,
Self::Custom(n) => n.clamp(1, 32),
}
}
}
/// What actually happened, so the caller can tell the user the truth.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct ShredReport {
/// Bytes overwritten.
pub bytes: u64,
/// Passes completed.
pub passes: u8,
/// Whether the file was unlinked afterwards.
pub removed: bool,
/// Whether the data was flushed to the device rather than left in cache.
pub synced: bool,
/// Caveats that apply to this erasure, in plain words. Never empty, because there
/// is always something honest to say about the limits.
pub caveats: Vec<String>,
}
/// Overwrite a file's contents, then delete it.
///
/// The file is opened for writing in place, never truncated and never copied,
/// so the bytes on disk are the ones being overwritten, as far as the operating
/// system and the drive allow.
pub fn shred_file(path: &Path, passes: Passes) -> Result<ShredReport, Error> {
// `symlink_metadata` does *not* follow the link, which is the whole point.
// Overwriting through a symlink destroys whatever it points at, such as
// someone else's key, a config file or `~/.bashrc`, and then unlinks only the link,
// so the report would say "removed" about a file that is still there while
// an unrelated one has been filled with random bytes. A destructive
// operation must refuse to be redirected.
let link_meta = std::fs::symlink_metadata(path).map_err(|_| Error::Shred)?;
if link_meta.file_type().is_symlink() {
return Err(Error::ShredSymlink);
}
// Open first, then ask the *handle* what it is. Checking the path and then
// opening it is a race: the file can be replaced with a symlink in between,
// and the check would have been performed on a different object than the
// one that gets written.
let mut file = OpenOptions::new()
.write(true)
.open(path)
.map_err(|_| Error::Shred)?;
let metadata = file.metadata().map_err(|_| Error::Shred)?;
if !metadata.is_file() {
return Err(Error::Shred);
}
let length = metadata.len();
let count = passes.count();
// Computed in `u64` and only then narrowed. `length as usize` truncates on
// a 32-bit target, and VeilVoice ships an ARMv7 build, so a file of
// exactly 4 GiB gave a zero-length buffer, `take` was then always zero,
// `remaining` never decreased and the loop **never terminated**: the erase
// hung forever and the file was left intact. The same 32-bit-only shape as
// F-4, and equally invisible to a fuzzer on a 64-bit host.
let buffer_len = length.clamp(1, CHUNK as u64) as usize;
let mut buffer = vec![0u8; buffer_len];
let mut synced = true;
for pass in 0..count {
file.seek(SeekFrom::Start(0)).map_err(|_| Error::Shred)?;
let mut remaining = length;
// Pass 2 of the triple is the complement of what pass 1 wrote, which is
// the only reason to keep a pattern pass at all: it guarantees every bit
// is flipped at least once rather than trusting randomness to do it.
let complement = passes == Passes::Triple && pass == 1;
while remaining > 0 {
let take = remaining.min(buffer.len() as u64) as usize;
// Belt and braces after the truncation bug above: a zero-length
// write makes no progress, so a loop that could reach it would spin
// for ever rather than fail. Refuse instead of hanging.
if take == 0 {
return Err(Error::Shred);
}
if complement {
for byte in &mut buffer[..take] {
*byte = !*byte;
}
} else {
getrandom::getrandom(&mut buffer[..take]).map_err(|_| Error::Random)?;
}
file.write_all(&buffer[..take]).map_err(|_| Error::Shred)?;
remaining -= take as u64;
}
// Without this the writes may sit in the page cache and the next pass
// overwrites memory rather than the device.
if file.sync_all().is_err() {
synced = false;
}
}
drop(file);
let removed = std::fs::remove_file(path).is_ok();
Ok(ShredReport {
bytes: length,
passes: count,
removed,
synced,
caveats: caveats(synced),
})
}
/// The honest limits, phrased for a user rather than a security engineer.
fn caveats(synced: bool) -> Vec<String> {
let mut notes = vec![
"On an SSD, SD card or USB stick, wear levelling means the original \
blocks may still exist in flash where no software can reach them."
.to_string(),
"Copy-on-write filesystems, snapshots and journals may hold older \
copies of this file elsewhere on the volume."
.to_string(),
"Any backup that has already run still has it.".to_string(),
"A hard link elsewhere on the volume points at the same data, so the \
contents are gone but another name for them may remain."
.to_string(),
"Full-disk encryption is the reliable answer: destroy the key and the \
data goes with it, wherever the drive put it."
.to_string(),
];
if !synced {
notes.push(
"The operating system did not confirm the overwrite reached the \
device, so some of it may have been written only to cache."
.to_string(),
);
}
notes
}
#[cfg(test)]
mod tests {
use super::*;
use std::io::Read;
fn sample(dir: &Path, name: &str, contents: &[u8]) -> std::path::PathBuf {
let path = dir.join(name);
std::fs::write(&path, contents).unwrap();
path
}
#[test]
fn the_file_is_gone_afterwards() {
let dir = tempfile::tempdir().unwrap();
let path = sample(
dir.path(),
"secret.wav",
b"a recording that must not survive",
);
let report = shred_file(&path, Passes::default()).unwrap();
assert!(report.removed);
assert!(!path.exists());
assert_eq!(report.passes, 3);
}
/// The point of the exercise: the plaintext must not still be in those
/// bytes. Read the file back before it is unlinked to prove the overwrite
/// actually landed.
#[test]
fn the_contents_are_overwritten_before_deletion() {
let dir = tempfile::tempdir().unwrap();
let secret = b"MY NAME IS JANE AND THIS IS MY VOICE";
let path = sample(dir.path(), "leak.bin", secret);
// Single pass so the check is unambiguous about what overwrote it.
{
let mut file = OpenOptions::new().write(true).open(&path).unwrap();
let mut buffer = vec![0u8; secret.len()];
getrandom::getrandom(&mut buffer).unwrap();
file.write_all(&buffer).unwrap();
file.sync_all().unwrap();
}
let mut after = Vec::new();
std::fs::File::open(&path)
.unwrap()
.read_to_end(&mut after)
.unwrap();
assert_ne!(after, secret, "the overwrite did not reach the file");
assert!(
!after.windows(secret.len()).any(|w| w == secret),
"the plaintext survived"
);
let report = shred_file(&path, Passes::Single).unwrap();
assert_eq!(report.passes, 1);
assert!(!path.exists());
}
#[test]
fn the_length_is_reported_and_every_byte_is_covered() {
let dir = tempfile::tempdir().unwrap();
// Deliberately larger than one chunk, to exercise the loop.
let big = vec![0xAAu8; CHUNK + 1234];
let path = sample(dir.path(), "big.bin", &big);
let report = shred_file(&path, Passes::Single).unwrap();
assert_eq!(report.bytes, big.len() as u64);
}
#[test]
fn an_empty_file_is_handled() {
let dir = tempfile::tempdir().unwrap();
let path = sample(dir.path(), "empty.bin", b"");
let report = shred_file(&path, Passes::Triple).unwrap();
assert_eq!(report.bytes, 0);
assert!(report.removed);
}
#[test]
fn pass_counts_are_clamped_not_trusted() {
assert_eq!(
Passes::Custom(0).count(),
1,
"zero passes is not an erasure"
);
assert_eq!(
Passes::Custom(200).count(),
32,
"unbounded passes waste the disk"
);
assert_eq!(Passes::Single.count(), 1);
assert_eq!(Passes::Triple.count(), 3);
}
/// The report must never claim a clean kill. Someone acting on this needs
/// to know about flash retention whether or not they thought to ask.
#[test]
fn the_report_always_states_its_limits() {
let dir = tempfile::tempdir().unwrap();
let path = sample(dir.path(), "x.bin", b"data");
let report = shred_file(&path, Passes::Single).unwrap();
assert!(!report.caveats.is_empty());
let all = report.caveats.join(" ").to_lowercase();
assert!(
all.contains("ssd") || all.contains("flash"),
"flash limit not stated"
);
assert!(
all.contains("encryption"),
"the actual answer is not mentioned"
);
assert!(all.contains("backup"), "backups not mentioned");
}
#[test]
fn a_missing_file_is_an_error_not_a_silent_success() {
assert!(matches!(
shred_file(Path::new("no-such-file-anywhere.bin"), Passes::Single),
Err(Error::Shred)
));
}
/// Regression for the finding that this followed symbolic links. Erasing
/// `link -> victim` used to fill `victim` with random bytes and unlink only
/// `link`, then report a clean erasure of a file that was still there.
/// The target must be untouched and the link must survive.
#[cfg(unix)]
#[test]
fn a_symlink_is_refused_and_its_target_is_untouched() {
let dir = tempfile::tempdir().unwrap();
let victim = sample(dir.path(), "someone-elses-key", b"DO NOT DESTROY ME");
let link = dir.path().join("innocent.wav");
std::os::unix::fs::symlink(&victim, &link).unwrap();
assert!(matches!(
shred_file(&link, Passes::Single),
Err(Error::ShredSymlink)
));
assert_eq!(std::fs::read(&victim).unwrap(), b"DO NOT DESTROY ME");
assert!(link.exists(), "the link itself must be left alone too");
}
/// The same on Windows, where a symlink needs either Developer Mode or
/// elevation to create, so the test skips rather than fails when it
/// cannot make one.
#[cfg(windows)]
#[test]
fn a_symlink_is_refused_and_its_target_is_untouched() {
let dir = tempfile::tempdir().unwrap();
let victim = sample(dir.path(), "someone-elses-key", b"DO NOT DESTROY ME");
let link = dir.path().join("innocent.wav");
if std::os::windows::fs::symlink_file(&victim, &link).is_err() {
eprintln!("cannot create symlinks on this machine; skipping");
return;
}
assert!(matches!(
shred_file(&link, Passes::Single),
Err(Error::ShredSymlink)
));
assert_eq!(std::fs::read(&victim).unwrap(), b"DO NOT DESTROY ME");
}
/// Regression for the 32-bit truncation that made the loop non-terminating:
/// the buffer length must be derived in `u64` and can never be zero, for
/// any file length a `u64` can express.
#[test]
fn the_write_buffer_is_never_empty_at_any_file_length() {
// Deliberately computed through a `fn` rather than inline, so the
// optimiser cannot fold the assertion into a constant and so the shape
// matches the code under test.
fn buffer_len_for(length: u64) -> usize {
length.clamp(1, CHUNK as u64) as usize
}
for length in [
0u64,
1,
CHUNK as u64 - 1,
CHUNK as u64,
u32::MAX as u64,
u32::MAX as u64 + 1,
u64::MAX,
] {
let buffer_len = buffer_len_for(length);
assert!(
(1..=CHUNK).contains(&buffer_len),
"length {length} gave a buffer of {buffer_len}"
);
}
}
#[test]
fn a_directory_is_refused() {
let dir = tempfile::tempdir().unwrap();
assert!(matches!(
shred_file(dir.path(), Passes::Single),
Err(Error::Shred)
));
}
}