CVE-2026-23248: perf/core: Fix refcount bug and potential UAF in perf_mmap

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

perf/core: Fix refcount bug and potential UAF in perfmmap

Syzkaller reported a refcountt: addition on 0; use-after-free warning in perfmmap.

The issue is caused by a race condition between a failing mmap() setup and a concurrent mmap() on a dependent event (e.g., using output redirection).

In perfmmap(), the ringbuffer (rb) is allocated and assigned to event->rb with the mmapmutex held. The mutex is then released to perform maprange().

If maprange() fails, perfmmapclose() is called to clean up. However, since the mutex was dropped, another thread attaching to this event (via inherited events or output redirection) can acquire the mutex, observe the valid event->rb pointer, and attempt to increment its reference count. If the cleanup path has already dropped the reference count to zero, this results in a use-after-free or refcount saturation warning.

Fix this by extending the scope of mmapmutex to cover the maprange() call. This ensures that the ring buffer initialization and mapping (or cleanup on failure) happens atomically effectively, preventing other threads from accessing a half-initialized or dying ring buffer.