Merge #559: Backport and bump 0.24.2

5c6225e Bump version to 0.24.2 (Tobin C. Harding)
0a696b2 Add saftey docs for PreallocatedContext trait (Tobin C. Harding)
dd194b6 context: introduce unsafe `PreallocatedContext` trait (Andrew Poelstra)

Pull request description:

  I believe this is what tcharding meant by #558

ACKs for top commit:
  apoelstra:
    utACK 5c6225e

Tree-SHA512: 54cac9bd146e9dd32cf28f3a914053c7a68e99756e29431e0324691b2ef803dac044bb7002183c2773e042ac5f34f3a43646e46823bc1349ffb34f2eaa4c42a6

CHANGELOG.md

@@ -1,3 +1,8 @@
+
+# 0.24.2 - 2022-12-05
+
+* Backport [fix soundness issue with `preallocated_gen_new`](https://github.com/rust-bitcoin/rust-secp256k1/pull/548)
+
 # 0.24.1 - 2022-10-25
 
 * [Fix broken deserialization logic of `KeyPair`](https://github.com/rust-bitcoin/rust-secp256k1/issues/491) that previously always panicked. After the patch deserialization only panics if neither the `global-context` nor the `alloc` (default) feature is active.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "secp256k1"
-version = "0.24.1"
+version = "0.24.2"
 authors = [ "Dawid Ciężarkiewicz <dpc@ucore.info>",
             "Andrew Poelstra <apoelstra@wpsoftware.net>" ]
 license = "CC0-1.0"

src/context.rs

@@ -300,8 +300,22 @@ unsafe impl<'buf> Context for AllPreallocated<'buf> {
     }
 }
 
-impl<'buf, C: Context + 'buf> Secp256k1<C> {
-    /// Lets you create a context with preallocated buffer in a generic manner(sign/verify/all)
+/// Trait marking that a particular context object internally points to
+/// memory that must outlive `'a`
+///
+/// # Safety
+///
+/// This trait is used internally to gate which context markers can safely
+/// be used with the `preallocated_gen_new` function. Do not implement it
+/// on your own structures.
+pub unsafe trait PreallocatedContext<'a> {}
+
+unsafe impl<'buf> PreallocatedContext<'buf> for AllPreallocated<'buf> {}
+unsafe impl<'buf> PreallocatedContext<'buf> for SignOnlyPreallocated<'buf> {}
+unsafe impl<'buf> PreallocatedContext<'buf> for VerifyOnlyPreallocated<'buf> {}
+
+impl<'buf, C: Context + PreallocatedContext<'buf>> Secp256k1<C> {
+    /// Lets you create a context with a preallocated buffer in a generic manner (sign/verify/all).
     pub fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Secp256k1<C>, Error> {
         #[cfg(target_arch = "wasm32")]
         ffi::types::sanity_checks_for_wasm();