Restrict MaybeUninit trait impls to fix soundness (#308)

Previously, we implemented `FromZeroes` and `FromBytes` for
`MaybeUninit<T>` with no bound on `T`. This resulted in a soundness hole
in which `T` - and thus `MaybeUninit<T>` - could contain an
`UnsafeCell`, which is a violation of the contracts of `FromZeroes` and
`FromBytes`.

This is a breaking change, but it's very unlikely to be one that code is
currently relying on, especially given that the 0.7.x release train was
published very recently. Thus, in this commit, we publish 0.7.3, and we
will yank 0.7.{0,1,2} as soon as 0.7.3 is published.

Fixes #299

Cargo.toml

@@ -11,7 +11,7 @@
 [package]
 edition = "2021"
 name = "zerocopy"
-version = "0.7.2"
+version = "0.7.3"
 authors = ["Joshua Liebow-Feeser <joshlf@google.com>"]
 description = "Utilities for zero-copy parsing and serialization"
 license = "BSD-2-Clause"
@@ -55,7 +55,7 @@ simd-nightly = ["simd"]
 __internal_use_only_features_that_work_on_stable = ["alloc", "simd"]
 
 [dependencies]
-zerocopy-derive = { version = "=0.7.2", path = "zerocopy-derive", optional = true }
+zerocopy-derive = { version = "=0.7.3", path = "zerocopy-derive", optional = true }
 
 [dependencies.byteorder]
 version = "1.3"
@@ -66,7 +66,7 @@ optional = true
 # zerocopy-derive remain equal, even if the 'derive' feature isn't used.
 # See: https://github.com/matklad/macro-dep-test
 [target.'cfg(any())'.dependencies]
-zerocopy-derive = { version = "=0.7.2", path = "zerocopy-derive" }
+zerocopy-derive = { version = "=0.7.3", path = "zerocopy-derive" }
 
 [dev-dependencies]
 rand = "0.6"
@@ -78,4 +78,4 @@ static_assertions = "1.1"
 # CI test failures.
 trybuild = "=1.0.80"
 # In tests, unlike in production, zerocopy-derive is not optional
-zerocopy-derive = { version = "=0.7.2", path = "zerocopy-derive" }
+zerocopy-derive = { version = "=0.7.3", path = "zerocopy-derive" }

src/lib.rs

@@ -996,13 +996,23 @@ safety_comment! {
     //
     /// SAFETY:
     /// - `FromZeroes`, `FromBytes`: `MaybeUninit<T>` has no restrictions on its
-    ///   contents.
+    ///   contents. Unfortunately, in addition to bit validity, `FromZeroes` and
+    ///   `FromBytes` also require that implementers contain no `UnsafeCell`s.
+    ///   Thus, we require `T: FromZeroes` and `T: FromBytes` in order to ensure
+    ///   that `T` - and thus `MaybeUninit<T>` - contains to `UnsafeCell`s.
+    ///   Thus, requiring that `T` implement each of these traits is sufficient
     /// - `Unaligned`: `MaybeUninit<T>` is guaranteed by its documentation [1]
     ///   to have the same alignment as `T`.
     ///
-    /// [1] https://doc.rust-lang.org/nightly/core/mem/union.MaybeUninit.html#layout-1
-    unsafe_impl!(T => FromZeroes for MaybeUninit<T>);
-    unsafe_impl!(T => FromBytes for MaybeUninit<T>);
+    /// [1]
+    /// https://doc.rust-lang.org/nightly/core/mem/union.MaybeUninit.html#layout-1
+    ///
+    /// TODO(https://github.com/google/zerocopy/issues/251): If we split
+    /// `FromBytes` and `RefFromBytes`, or if we introduce a separate
+    /// `NoCell`/`Freeze` trait, we can relax the trait bounds for `FromZeroes`
+    /// and `FromBytes`.
+    unsafe_impl!(T: FromZeroes => FromZeroes for MaybeUninit<T>);
+    unsafe_impl!(T: FromBytes => FromBytes for MaybeUninit<T>);
     unsafe_impl!(T: Unaligned => Unaligned for MaybeUninit<T>);
     assert_unaligned!(MaybeUninit<()>, MaybeUninit<u8>);
 }
@@ -4059,7 +4069,7 @@ mod tests {
         assert_impls!(ManuallyDrop<[NotZerocopy]>: !FromZeroes, !FromBytes, !AsBytes, !Unaligned);
 
         assert_impls!(MaybeUninit<u8>: FromZeroes, FromBytes, Unaligned, !AsBytes);
-        assert_impls!(MaybeUninit<NotZerocopy>: FromZeroes, FromBytes, !AsBytes, !Unaligned);
+        assert_impls!(MaybeUninit<NotZerocopy>: !FromZeroes, !FromBytes, !AsBytes, !Unaligned);
 
         assert_impls!(Wrapping<u8>: FromZeroes, FromBytes, AsBytes, Unaligned);
         assert_impls!(Wrapping<NotZerocopy>: !FromZeroes, !FromBytes, !AsBytes, !Unaligned);

zerocopy-derive/Cargo.toml

@@ -5,7 +5,7 @@
 [package]
 edition = "2021"
 name = "zerocopy-derive"
-version = "0.7.2"
+version = "0.7.3"
 authors = ["Joshua Liebow-Feeser <joshlf@google.com>"]
 description = "Custom derive for traits from the zerocopy crate"
 license = "BSD-2-Clause"