RFR: 8365203: defineClass with direct buffer can cause use-after-free

[Alan Bateman]

On Mon, 11 Aug 2025 11:33:19 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> ### Description
> This PR proposes to update the `ClassLoader` implementation to properly guard access to the provided `ByteBuffer` when defining a class using `defineClass(String, ByteBuffer, ...)`. Specifically, calls to `SharedSecrets.getJavaNioAccess().acquireSession(ByteBuffer)` and `releaseSession(ByteBuffer)` have been introduced to ensure safe and consistent buffer access throughout the native class definition process, even in the case of a `ByteBuffer` is backed by a `MemorySegment`.
> 
> ### Impact
> This modification is internal to the `ClassLoader` implementation and does not affect the public API.
> Improves the robustness and security of class loading from buffers.
> 
> ### Testing
> Tier 1, 2, and 3 JDK tests pass on multiple platforms.

test/jdk/java/lang/ClassLoader/defineClass/GuardByteBuffer.java line 58:

> 56:         };
> 57:         final List<Thread> threads = new ArrayList<>();
> 58:         for (int i = 0; i < Runtime.getRuntime().availableProcessors(); i++) {

This means all cores spinning for 20s - we'll have to see if it causes any side effects and slow down of other tests that happen to run at the same time in other agent VMs (make run-test uses concurrency by default).

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PR Review Comment: https://git.openjdk.org/jdk/pull/26724#discussion_r2267040749