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rust-bitcoin-unsafe-fast
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[hashes] Disable fixed-time equality cmp when building for fuzzers 

Fuzzers want to break memcmp calls into separate comparisons for
coverage monitoring, allowing them to not-quite-brute-force find
inputs that fully match. Thus, we disable our fancy fixed-time
comparison when built with the `hashes_fuzz` cfg.
This commit is contained in:
Matt Corallo 2025-03-26 18:45:13 +00:00
parent 158240c3c9
commit 7ac7273013
1 changed files with 38 additions and 29 deletions
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67
hashes/src/cmp.rs
View File

@ -15,41 +15,50 @@
/// As of rust 1.31.0 disassembly looks completely within reason for this, see
/// <https://godbolt.org/z/mMbGQv>.
pub fn fixed_time_eq(a: &[u8], b: &[u8]) -> bool {
assert!(a.len() == b.len());
let count = a.len();
let lhs = &a[..count];
let rhs = &b[..count];
#[cfg(hashes_fuzz)]
{
// Fuzzers want to break memcmp calls into separate comparisons for coverage monitoring,
// so we avoid our fancy fixed-time comparison below for fuzzers.
a == b
}
#[cfg(not(hashes_fuzz))]
{
assert!(a.len() == b.len());
let count = a.len();
let lhs = &a[..count];
let rhs = &b[..count];
let mut r: u8 = 0;
for i in 0..count {
let mut rs = unsafe { core::ptr::read_volatile(&r) };
rs |= lhs[i] ^ rhs[i];
unsafe {
core::ptr::write_volatile(&mut r, rs);
let mut r: u8 = 0;
for i in 0..count {
let mut rs = unsafe { core::ptr::read_volatile(&r) };
rs |= lhs[i] ^ rhs[i];
unsafe {
core::ptr::write_volatile(&mut r, rs);
}
}
}
{
let mut t = unsafe { core::ptr::read_volatile(&r) };
t |= t >> 4;
unsafe {
core::ptr::write_volatile(&mut r, t);
{
let mut t = unsafe { core::ptr::read_volatile(&r) };
t |= t >> 4;
unsafe {
core::ptr::write_volatile(&mut r, t);
}
}
}
{
let mut t = unsafe { core::ptr::read_volatile(&r) };
t |= t >> 2;
unsafe {
core::ptr::write_volatile(&mut r, t);
{
let mut t = unsafe { core::ptr::read_volatile(&r) };
t |= t >> 2;
unsafe {
core::ptr::write_volatile(&mut r, t);
}
}
}
{
let mut t = unsafe { core::ptr::read_volatile(&r) };
t |= t >> 1;
unsafe {
core::ptr::write_volatile(&mut r, t);
{
let mut t = unsafe { core::ptr::read_volatile(&r) };
t |= t >> 1;
unsafe {
core::ptr::write_volatile(&mut r, t);
}
}
unsafe { (::core::ptr::read_volatile(&r) & 1) == 0 }
}
unsafe { (::core::ptr::read_volatile(&r) & 1) == 0 }
}
#[cfg(test)]