linux-kernel - Re: suspicious RCU usage (perf)

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Message-ID: <20130830155936.GT31370@twins.programming.kicks-ass.net>
Date:	Fri, 30 Aug 2013 17:59:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Dave Jones <davej@...hat.com>, paulmck@...ux.vnet.ibm.com,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>
Subject: Re: suspicious RCU usage (perf)

On Fri, Aug 30, 2013 at 05:52:43PM +0200, Frederic Weisbecker wrote:
> On Tue, Aug 27, 2013 at 02:16:29PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 26, 2013 at 03:03:04PM -0400, Steven Rostedt wrote:
> > > > Is there some path through sys_perf_open_event that might be
> > > > missing a capability check perhaps ?
> > > > 
> > > 
> > > That's a question for Ingo, Peter or Jiri.
> > 
> > Its not something I've looked at recently, git blames Jiri and fweisbec
> > for most of that code.
> > 
> > Permission checks appear to live in
> > kernel/trace/trace_event_perf.c:perf_trace_event_perm().
> 
> Actually the following condition is weird:
> 
> 	 /* The ftrace function trace is allowed only for root. */
> 	 if (ftrace_event_is_function(tp_event) &&
> 	     perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
>      			    return -EPERM;
> 

That says: If its its a function-event and we're paranoid but we don't
have root, bail.

> We probably intended to do:
> 
>    /* The ftrace function trace is allowed only for root. */
>    if (ftrace_event_is_function(tp_event) ||
>        perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
>        			      return -EPERM;
> 
> Can somebody confirm?

That would always disallow function-events, no?
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