linux-kernel - Re: [PATCH 2/2] perf/x86/intel/bts: Disallow use by unprivileged users on paranoid systems

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Date:	Fri, 28 Aug 2015 07:31:30 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH 2/2] perf/x86/intel/bts: Disallow use by unprivileged
 users on paranoid systems


* Alexander Shishkin <alexander.shishkin@...ux.intel.com> wrote:

> BTS leaks kernel addresses even in userspace-only mode due to imprecise IP
> sampling, so sometimes syscall entry points or page fault handler addresses
> end up in a userspace trace.
> 
> Now, intel_bts driver exports trace data zero-copy, it does not scan through
> it to filter out the kernel addresses and it's would be a O(n) job.
> 
> To work around this situation, this patch forbids the use of intel_bts
> driver by unprivileged users with paranoid setting higher than 1, which
> forbids kernel tracing.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel_bts.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_bts.c b/arch/x86/kernel/cpu/perf_event_intel_bts.c
> index 80df16e020..4f6daff92d 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_bts.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_bts.c
> @@ -495,6 +495,16 @@ static int bts_event_init(struct perf_event *event)
>  	if (x86_add_exclusive(x86_lbr_exclusive_bts))
>  		return -EBUSY;
>  
> +	/*
> +	 * BTS leaks kernel addresses even when CPL0 tracing is
> +	 * disabled, so disallow intel_bts driver for unprivileged
> +	 * users on paranoid systems since it provides trace data
> +	 * to the user in a zero-copy fashion.
> +	 */
> +	if (event->attr.exclude_kernel && perf_paranoid_kernel() &&
> +	    !capable(CAP_SYS_ADMIN))
> +		return -EACCES;

I.e. it's disabled by default as well, with default paranoia settings?

Thanks,

	Ingo
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