Merge rust-bitcoin/rust-bitcoin#4289: [hashes] Disable fixed-time equality cmp when building for fuzzers, use fixed-time equality for `Hmac`

2d9e240fb6 [hashes] Use `fixed_time_eq` for `Hmac::eq` (Matt Corallo)
7ac7273013 [hashes] Disable fixed-time equality cmp when building for fuzzers (Matt Corallo)

Pull request description:

  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.

ACKs for top commit:
  Kixunil:
    ACK 2d9e240fb6
  apoelstra:
    ACK 2d9e240fb6ae13e6139713f9bb8ccb51e5dc0bff; successfully ran local tests

Tree-SHA512: 372e344ae5497a10ca03a50baadb9d2e8a6dac914bbc4ca91fd5c9b839fb036f42c8b47c252ca3466c15286e889a6f5b51390cc0d938ba24dc50b13e8b863463

hashes/src/cmp.rs

@@ -15,6 +15,14 @@
 /// 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 {
+    #[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];
@@ -51,6 +59,7 @@ pub fn fixed_time_eq(a: &[u8], b: &[u8]) -> bool {
         }
         unsafe { (::core::ptr::read_volatile(&r) & 1) == 0 }
     }
+}
 
 #[cfg(test)]
 mod tests {

hashes/src/hmac/mod.rs

@@ -15,8 +15,9 @@ use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use crate::{Hash, HashEngine};
 
 /// A hash computed from a RFC 2104 HMAC. Parameterized by the underlying hash function.
-#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialOrd, Ord, Hash)]
 #[repr(transparent)]
+#[allow(clippy::derived_hash_with_manual_eq)]
 pub struct Hmac<T: Hash>(T);
 
 impl<T: Hash + str::FromStr> str::FromStr for Hmac<T> {
@@ -24,6 +25,14 @@ impl<T: Hash + str::FromStr> str::FromStr for Hmac<T> {
     fn from_str(s: &str) -> Result<Self, Self::Err> { Ok(Hmac(str::FromStr::from_str(s)?)) }
 }
 
+impl<T: Hash> PartialEq for Hmac<T> {
+    fn eq(&self, other: &Self) -> bool {
+        crate::cmp::fixed_time_eq(self.as_ref(), other.as_ref())
+    }
+}
+
+impl<T: Hash> Eq for Hmac<T> {}
+
 /// Pair of underlying hash engines, used for the inner and outer hash of HMAC.
 #[derive(Debug, Clone)]
 pub struct HmacEngine<T: HashEngine> {