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Date: Tue, 14 Mar 2017 15:03:02 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        syzkaller <syzkaller@googlegroups.com>
Subject: Re: perf: use-after-free in perf_release
Message-ID: <20170314140302.GA28146@redhat.com>
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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.