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 12:40:10 +0300
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>
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

Ingo Molnar <mingo@...nel.org> writes:

> * 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?

Actually no, the kernel's default is 1, which allows kernel profiling
for unprivileged users. Distros might be more strict though.

Regards,
--
Alex
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