Re: [patch] Performance Counters for Linux, v4

[Pavel Machek <pavel@suse.cz>]

On Tue 2008-12-16 13:50:00, Ingo Molnar wrote:
> 
> * Pavel Machek <pavel@suse.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.

> 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@elte.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@suse.cz>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>

Acked-by: Pavel Machek <pavel@suse.cz>

> @@ -1015,6 +1016,13 @@ int flush_old_exec(struct linux_binprm *
>  		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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