Re: [PATCH 3/3] perf, x86, lbr: Demand proper privileges forPERF_SAMPLE_BRANCH_KERNEL

From: Peter Zijlstra
Date: Wed May 15 2013 - 10:30:43 EST


On Wed, May 15, 2013 at 03:37:22PM +0200, Stephane Eranian wrote:
> On Fri, May 3, 2013 at 2:11 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> > We should always have proper privileges when requesting kernel data.
> >
> > Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> > Cc: eranian@xxxxxxxxxx
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > Link: http://lkml.kernel.org/n/tip-v0x9ky3ahzr6nm3c6ilwrili@xxxxxxxxxxxxxx
> > ---
> > arch/x86/kernel/cpu/perf_event_intel_lbr.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> > +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> > @@ -318,8 +318,11 @@ static void intel_pmu_setup_sw_lbr_filte
> > if (br_type & PERF_SAMPLE_BRANCH_USER)
> > mask |= X86_BR_USER;
> >
> > - if (br_type & PERF_SAMPLE_BRANCH_KERNEL)
> > + if (br_type & PERF_SAMPLE_BRANCH_KERNEL) {
> > + if (perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
> > + return -EACCES;
> > mask |= X86_BR_KERNEL;
> > + }
> >
> This will prevent regular users from capturing kernel -> kernel branches.
> But it won't prevent users from getting kernel -> user branches. Thus
> some kernel address will still be captured. I guess they could be eliminated
> by the sw_filter.
>
> When using LBR priv level filtering, the filter applies to the branch target
> only.

Ah, indeed. I'll try and whip up a patch.
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