perf/core: Fix WARN_ON(!ctx) in __free_event() for partial init

In the Linux kernel, the following vulnerability has been resolved:

perf/core: Fix WARN_ON(!ctx) in __free_event() for partial init

Move the get_ctx(child_ctx) call and the child_event->ctx assignment to

occur immediately after the child event is allocated. Ensure that

child_event->ctx is non-NULL before any subsequent error path within

inherit_event calls free_event(), satisfying the assumptions of the

cleanup code.

Details:

There's no clear Fixes tag, because this bug is a side-effect of

multiple interacting commits over time (up to 15 years old), not

a single regression.

The code initially incremented refcount then assigned context

immediately after the child_event was created. Later, an early

validity check for child_event was added before the

refcount/assignment. Even later, a WARN_ON_ONCE() cleanup check was

added, assuming event->ctx is valid if the pmu_ctx is valid.

The problem is that the WARN_ON_ONCE() could trigger after the initial

check passed but before child_event->ctx was assigned, violating its

precondition. The solution is to assign child_event->ctx right after

its initial validation. This ensures the context exists for any

subsequent checks or cleanup routines, resolving the WARN_ON_ONCE().

To resolve it, defer the refcount update and child_event->ctx assignment

directly after child_event->pmu_ctx is set but before checking if the

parent event is orphaned. The cleanup routine depends on

event->pmu_ctx being non-NULL before it verifies event->ctx is

non-NULL. This also maintains the author's original intent of passing

in child_ctx to find_get_pmu_context before its refcount/assignment.

[ mingo: Expanded the changelog from another email by Gabriel Shahrouzi. ]