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

[Per Minborg]

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.

src/java.base/share/classes/java/lang/ClassLoader.java line 1054:

> 1052:         String source = defineClassSourceLocation(protectionDomain);
> 1053: 
> 1054:         SharedSecrets.getJavaNioAccess().acquireSession(b);

Now that the fields in `SharedSecrets` are `@Stable`, we do not have to make a local copy in a `static final` field.

test/jdk/java/lang/ClassLoader/defineClass/GuardByteBuffer.java line 43:

> 41: 
> 42:     @Test
> 43:     void guardCrash() throws InterruptedException {

I was not able to reproduce the crash using this test on a Mac. The original reproducer worked on a Windows machine.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26724#discussion_r2266596901
PR Review Comment: https://git.openjdk.org/jdk/pull/26724#discussion_r2266592416