linux-kernel - Re: [PATCH 07/12] perf_events: add LBR software filter support for Intel X86

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Date:	Fri, 7 Oct 2011 12:40:48 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu,
	acme@...hat.com, ming.m.lin@...el.com, robert.richter@....com,
	ravitillo@....gov
Subject: Re: [PATCH 07/12] perf_events: add LBR software filter support for
 Intel X86

On Fri, Oct 7, 2011 at 12:38 PM, Stephane Eranian <eranian@...gle.com> wrote:
> On Thu, Oct 6, 2011 at 5:32 PM, Andi Kleen <andi@...stfloor.org> wrote:
>>> +     kernel_insn_init(&insn, kaddr);
>>> +     insn_get_opcode(&insn);
>>
>> This makes me uncomfortable. AFAIK that's the first use of the opcode
>> decoder being used directly for user space. It has a quite large attack
>> surface. Who says it cannot be exploited?
>>
> This is not new, it's already used for the PEBS fixups and that includes
> user level fixups, if possible.
>
> We are not executing the instruction here, just decoding it to filter it out
> from a buffer if necessary.
>
I would add that in this particular usage, the source address is coming
straight from LBR, it's not made up my SW. That means it corresponds
to a point where there was a control flow change. But it can certainly
be any x86 opcode (not just branches). LBR captures control flow changes
due to traps, faults, interrupts.
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