linux-kernel - Re: [patch] Performance Counters for Linux, v4

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Date:	Tue, 16 Dec 2008 14:03:02 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Pavel Machek <pavel@...e.cz>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephane Eranian <eranian@...glemail.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Robert Richter <robert.richter@....com>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Anvin <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>,
	perfctr-devel@...ts.sourceforge.net
Subject: Re: [patch] Performance Counters for Linux, v4


* Pavel Machek <pavel@...e.cz> wrote:

> On Tue 2008-12-16 13:50:00, Ingo Molnar wrote:
> > 
> > * Pavel Machek <pavel@...e.cz> wrote:
> > 
> > > Hmm, if I timec some setuid program, what happens?
> > 
> > yes, i already had a quick look at that a few days ago when i implemented 
> > counter inheritance (for different reasons) and couldnt find the cleanest 
> > place to put the exec() flushing into so i procrastinated that a bit :)
> > 
> > > Performance counters seem like great tool to pull secret keys out of 
> > > other processes :-).
> > 
> > if you worry about _that_ angle you also have to:
> > 
> >  - turn off the cycle counter
> > 
> >  - turn off precise utimes
> 
> Probably good idea, yes.
> 
> >  - plus you have to forbid SMT CPUs as well. On HT a task could
> >    co-schedule with your setuid task and observe its timing
> >    characteristics via its _own_ behavior. (which is impacted by whatever
> >    is running on another SMT/HT thread.)
> 
> Yes, SMT is evil.

HT got added back to Nehalem, so SMT is coming to you in every future x86 
CPU. It brings a serious performance win, so nobody will turn off SMT 
threading in practice. If SMT worries you, it needs explicit partitioning 
of security-relevant processing to different physical CPUs, via 
cgroups/cpusets/etc.

> > the real exec() worry are: active, IRQ driven samples/events. Not possible 
> > yet via the current iteration of counter inheritance (hence my 
> > procrastination) - but it makes sense and that's why i was looking at the 
> > exec() angle.
> > 
> > and that will flush simple counters too, removing your theoretical attack 
> > angle as well.
> > 
> > So how about the patch below?
> 
> Thanks!
> 
> > Subject: perfcounters: flush on setuid exec
> > From: Ingo Molnar <mingo@...e.hu>
> > Date: Tue Dec 16 13:40:44 CET 2008
> > 
> > Pavel Machek pointed out that performance counters should be flushed
> > when crossing protection domains on setuid execution.
> > 
> > Reported-by: Pavel Machek <pavel@...e.cz>
> > Signed-off-by: Ingo Molnar <mingo@...e.hu>
> 
> Acked-by: Pavel Machek <pavel@...e.cz>

find below the final commit, thanks Pavel.

	Ingo

------------>
>From f65cb45cba63f249458b669aa67069eabc37b2f5 Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@...e.hu>
Date: Tue, 16 Dec 2008 13:40:44 +0100
Subject: [PATCH] perfcounters: flush on setuid exec

Pavel Machek pointed out that performance counters should be flushed
when crossing protection domains on setuid execution.

Reported-by: Pavel Machek <pavel@...e.cz>
Acked-by: Pavel Machek <pavel@...e.cz>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 fs/exec.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index ec5df9a..d5165d8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -33,6 +33,7 @@
 #include <linux/string.h>
 #include <linux/init.h>
 #include <linux/pagemap.h>
+#include <linux/perf_counter.h>
 #include <linux/highmem.h>
 #include <linux/spinlock.h>
 #include <linux/key.h>
@@ -1017,6 +1018,13 @@ int flush_old_exec(struct linux_binprm * bprm)
 		set_dumpable(current->mm, suid_dumpable);
 	}
 
+	/*
+	 * Flush performance counters when crossing a
+	 * security domain:
+	 */
+	if (!get_dumpable(current->mm))
+		perf_counter_exit_task(current);
+
 	/* An exec changes our domain. We are no longer part of the thread
 	   group */
 
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