linux-kernel - Q: perf_install_in_context/perf_event

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110120193033.GA13924@redhat.com>
Date:	Thu, 20 Jan 2011 20:30:33 +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: Q: perf_install_in_context/perf_event_enable are racy?

On 01/19, Oleg Nesterov wrote:
>
> Also. I believe there are more problems in perf_install_event(), but
> I need to recheck.

Help! I can't believe it can be so trivially wrong, but otoh I can't
understand how this can be correct.

So, ignoring details and !task case, __perf_install_in_context() does:

	if (cpuctx->task_ctx || ctx->task != current)
		return;

	cpuctx->task_ctx = ctx;
	event_sched_in(event);

Stupid question, what if this task has already passed
perf_event_exit_task() and thus it doesn't have ->perf_event_ctxp[] ?
Given that perf_event_context_sched_out() does nothing if !ctx, who
will event_sched_out() this event?

OK, even if I am right this is trivial, we just need the additional
check.

But, it seems, there is another problem. Forget about the exiting,
I can't understand why we can trust current in the code above.
With __ARCH_WANT_INTERRUPTS_ON_CTXSW schedule() does:

	// sets cpuctx->task_ctx = NULL
	perf_event_task_sched_out();

	// enables irqs
	prepare_lock_switch();

	// updates current_task
	switch_to();

What if IPI comes in the window before switch_to() ?

(the same questions for __perf_event_enable).

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/