linux-kernel - Re: Q: perf_event && task->ptrace

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Message-ID: <20110117203459.GA32700@redhat.com>
Date:	Mon, 17 Jan 2011 21:34:59 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Q: perf_event && task->ptrace_bps[]

On 11/08, Oleg Nesterov wrote:
>
> I am trying to understand the usage of hw-breakpoints in arch_ptrace().
> ptrace_set_debugreg() and related code looks obviously racy. Nothing
> protects us against flush_ptrace_hw_breakpoint() called by the dying
> tracee. Afaics we can leak perf_event or use the already freed memory
> or both.
>
> Am I missed something?
>
> Looking into the git history, I don't even know which patch should be
> blamed (if I am right), there were too many changes. I noticed that
> 2ebd4ffb6d0cb877787b1e42be8485820158857e "perf events: Split out task
> search into helper" moved the PF_EXITING check from find_get_context().
> This check coould help if sys_ptrace() races with SIGKILL, but it was
> racy anyway.

Ping.

Any idea how to fix this cleanly? May be we can reuse perf_event_mutex,
but this looks soooo ugly. And do_exit()->flush_ptrace_hw_breakpoint()
has the strange "FIXME:" comment which doesn't help me to understand
what can we do.

Probably the best fix is to change this code so that the tracer owns
->ptrace_bps[], not the tracee. But this is not trivial, and needs a
lot of changes in ptrace code.


I am reading perf_event.c, but all I found so far is a couple of trivial
methods to crash the kernel via sys_perf_event_open(), will report
tomorrow...

Oleg.

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