Re: perf: use-after-free in perf_release

[Oleg Nesterov]

On 03/14, Peter Zijlstra wrote:
>
> Yes, this looks buggy. But I cannot explain how that would result in the
> observed use-after-free.

Yes...

Suppose that copy_process() fails after perf_event_init_task(). In this
case perf_event_free_task() does put_ctx(), but if this ctx has another
reference (ctx->refcount > 1) then ctx->task will point to the already
freed task, copy_process() does free_task() at the end of error path.
And we can't replace it with put_task_struct().

I am looking at TASK_TOMBSTONE, perhaps perf_event_free_task() should
use it too?

Oleg.