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From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
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
Date: Thu, 16 Mar 2017 13:05:32 +0100	[thread overview]
Message-ID: <20170316120532.oo3lbz5nphziffnj@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170315164302.GA17317@redhat.com>

On Wed, Mar 15, 2017 at 05:43:02PM +0100, Oleg Nesterov wrote:
> On 03/14, Peter Zijlstra wrote:
> >
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -10346,6 +10346,17 @@ void perf_event_free_task(struct task_struct *task)
> >  			continue;
> >
> >  		mutex_lock(&ctx->mutex);
> > +		raw_spin_lock_irq(&ctx->lock);
> > +		/*
> > +		 * Destroy the task <-> ctx relation and mark the context dead.
> > +		 *
> > +		 * This is important because even though the task hasn't been
> > +		 * exposed yet the context has been (through child_list).
> > +		 */
> > +		RCU_INIT_POINTER(task->perf_event_ctxp[ctxn], NULL);
> > +		WRITE_ONCE(ctx->task, TASK_TOMBSTONE);
> > +		put_task_struct(task); /* cannot be last */
> > +		raw_spin_unlock_irq(&ctx->lock);
> 
> Agreed, this is what I had in mind. Although you know, I spent 3
> hours looking at your patch and I still can't convince myself I am
> really sure it closes all races ;)

Ha; yes I know that feeling. I used to have a few sheets of paper filled
with diagrams. Sadly I could not find them again. Must've been over
eager cleaning my desk at some point.

> 
> OK, I believe this is correct. And iiuc both RCU_INIT_POINTER(NULL)
> and put_task_struct() are not strictly necessary? At least until we
> add WARN_ON(tsk->usage != 2) before free_task() in copy process().

Right; I just kept the code similar to the other location. I even
considered making a helper function to not duplicate, but in the end
decided against it.

> ---------------------------------------------------------------------
> This is off-topic, but to me list_for_each_entry(event->child_list)
> in perf_event_release_kernel() looks very confusing and misleading.
> And list_first_entry_or_null(), we do not really need NULL if list
> is empty, tmp == child should be F even if we use list_first_entry().
> And given that we already have list_is_last(), it would be nice to
> add list_is_first() and cleanup perf_event_release_kernel() a bit:
> 

Agreed; its a bit of a weird one.

Let me go write proper patches for the things we have so far though.

  reply	other threads:[~2017-03-16 12:05 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-06  9:57 Dmitry Vyukov
2017-03-06 12:13 ` Peter Zijlstra
2017-03-06 12:17   ` Dmitry Vyukov
2017-03-06 12:23     ` Peter Zijlstra
2017-03-06 12:27       ` Dmitry Vyukov
2017-03-06 12:47         ` Peter Zijlstra
2017-03-06 13:14 ` Peter Zijlstra
2017-03-06 13:34   ` Dmitry Vyukov
2017-03-07  9:08     ` Peter Zijlstra
2017-03-07  9:26       ` Dmitry Vyukov
2017-03-07  9:37         ` Peter Zijlstra
2017-03-07  9:43           ` Dmitry Vyukov
2017-03-07 10:00             ` Peter Zijlstra
2017-03-07 13:16   ` Peter Zijlstra
2017-03-07 13:27     ` Peter Zijlstra
2017-03-07 14:04   ` Oleg Nesterov
2017-03-07 14:17     ` Dmitry Vyukov
2017-03-07 16:51       ` Oleg Nesterov
2017-03-07 17:29         ` Peter Zijlstra
2017-03-14 12:55         ` Peter Zijlstra
2017-03-14 13:24           ` Oleg Nesterov
2017-03-14 13:47             ` Peter Zijlstra
2017-03-14 14:03           ` Oleg Nesterov
2017-03-14 14:07             ` Peter Zijlstra
2017-03-14 14:30               ` Oleg Nesterov
2017-03-14 15:02                 ` Peter Zijlstra
2017-03-14 15:07                   ` Peter Zijlstra
2017-03-14 15:37                     ` Oleg Nesterov
2017-03-14 15:46                       ` Peter Zijlstra
2017-03-14 15:19                   ` Oleg Nesterov
2017-03-14 15:26                     ` Peter Zijlstra
2017-03-14 15:59                       ` Peter Zijlstra
2017-03-15 16:43                         ` Oleg Nesterov
2017-03-16 12:05                           ` Peter Zijlstra [this message]
2017-03-16 13:57                           ` Peter Zijlstra
2017-03-16 16:41                             ` Oleg Nesterov
2017-03-16 12:47 [PATCH 0/4] perf patches Peter Zijlstra
2017-03-16 12:47 ` [PATCH 1/4] perf: Fix use-after-free in perf_release() Peter Zijlstra
2017-03-16 15:19   ` [tip:perf/urgent] perf/core: " tip-bot for Peter Zijlstra
2017-03-16 12:47 ` [PATCH 2/4] perf: Fix event inheritance on fork() Peter Zijlstra
2017-03-16 15:19   ` [tip:perf/urgent] perf/core: " tip-bot for Peter Zijlstra
2017-03-16 12:47 ` [PATCH 3/4] perf: Simplify perf_event_free_task() Peter Zijlstra
2017-03-16 15:20   ` [tip:perf/urgent] perf/core: " tip-bot for Peter Zijlstra
2017-03-16 12:47 ` [PATCH 4/4] perf: Better explain the inherit magic Peter Zijlstra
2017-03-16 15:21   ` [tip:perf/urgent] perf/core: " tip-bot for Peter Zijlstra
2017-03-16 13:20 ` [PATCH 0/4] perf patches Ingo Molnar

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