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

Overview

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. ]

Affected

• Linux >= 7ef5aa081f989ecfecc1df02068a80aebbd3ec31 < 90dc6c1e3b200812da8d0aa030e1b7fda8226d0e
• Linux >= 1209b0b29fd472e7dbd2b06544b019dd9f9b7e51 < cb56cd11feabf99e08bc18960700a53322ffcea7
• Linux >= c70ca298036c58a88686ff388d3d367e9d21acf0 < 0ba3a4ab76fd3367b9cb680cad70182c896c795c
• Linux 315a50c6b1c6ce191f19f3372935d8e2ed9b53a6
• Linux >= 6.13.12 < 6.14
• Linux >= 6.12.24 < 6.12.26
• Linux >= 6.14.3 < 6.14.5

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.