Re: [PATCH] perf/core: Fix refcount bug and potential UAF in perf_mmap

[Peter Zijlstra]

On Mon, Feb 02, 2026 at 03:44:35PM +0800, Haocheng Yu wrote:
> Syzkaller reported a refcount_t: addition on 0; use-after-free warning
> in perf_mmap.
> 
> The issue is caused by a race condition between mmap() and event
> teardown. In perf_mmap(), the ring_buffer (rb) is accessed via
> map_range() after the mmap_mutex is released. If another thread
> closes the event or detaches the buffer during this window, the
> reference count of rb can drop to zero, leading to a UAF or
> refcount saturation when map_range() or subsequent logic attempts
> to use it.

So you're saying this is something like:

	Thread-1		Thread-2

	mmap(fd)
				close(fd) / ioctl(fd, IOC_SET_OUTPUT)


I don't think close() is possible, because mmap() should have a
reference on the struct file from fget(), no?

That leaves the ioctl(), let me go have a peek.