From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752012AbdCNOc3 (ORCPT <rfc822;w@1wt.eu>);
        Tue, 14 Mar 2017 10:32:29 -0400
Received: from mx1.redhat.com ([209.132.183.28]:37618 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1750909AbdCNOcH (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 14 Mar 2017 10:32:07 -0400
Date: Tue, 14 Mar 2017 15:30:11 +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>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        syzkaller <syzkaller@googlegroups.com>
Subject: Re: perf: use-after-free in perf_release
Message-ID: <20170314143010.GA30351@redhat.com>
References: <CACT4Y+bgMfF=jUjNcVSSrBZy0xbyCPeNiDsW-c-S9sHe-Vxc=g@mail.gmail.com> <20170306131459.GC6515@twins.programming.kicks-ass.net> <20170307140414.GA31678@redhat.com> <CACT4Y+bzNOrMHmfP_cv3Jzaqs0zB3qHY9eu0y4JdKvVw0mgveA@mail.gmail.com> <20170307165131.GA6097@redhat.com> <20170314125508.GK3343@twins.programming.kicks-ass.net> <20170314140302.GA28146@redhat.com> <20170314140754.GG3328@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170314140754.GG3328@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.5.18 (2008-05-17)
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Tue, 14 Mar 2017 14:32:07 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 03/14, Peter Zijlstra wrote:
>
> On Tue, Mar 14, 2017 at 03:03:02PM +0100, Oleg Nesterov wrote:
> > 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?
>
> The idea was that the task isn't visible when we use
> perf_event_free_task(). But I'll have a look.

I can be easily wrong, I do not understans this code.

But. perf_event_init_task() adds child_event to parent_event->child_list.

If perf_event_release_kernel(parent_event) is called before copy_process()
does perf_event_free_task() which (in particular) removes it from child_list,
perf_event_release_kernel() can find this child_event and do get_ctx(ctx)
(under the list_for_each_entry(child, &event->child_list, child_list) loop).

Then it does put_ctx(ctx), but ctx->task can be already freed by
copy_process()->free_task() in this case.

No?

Oleg.