Re: [PATCH v1 0/2] perf: Drop leaked kernel samples

Kyle Huey
2018-06-15T03:35:53Z

I strongly object to this patch as written. As I said when I
originally reported[0] the regression introduced by the previous
version of this patch a year ago.

"It seems like this change should, at a bare minimum, be limited to
counters that actually perform sampling of register state when the
interrupt fires. In our case, with the retired conditional branches
counter restricted to counting userspace events only, it makes no
difference that the PMU interrupt happened to be delivered in the
kernel."

This means identifying which values of `perf_event_attr::sample_type`
are security concerns (presumably PERF_SAMPLE_IP is, and
PERF_SAMPLE_TIME is not, and someone needs to go through and decide on
all of them) and filtering on those values for this new behavior.

And because rr sets its sample_type to 0, once you do that, the sysctl
will not be necessary.

- Kyle

On Fri, Jun 15, 2018 at 3:03 AM, Jin Yao <yao.jin@linux.intel.com> wrote:
> On workloads that do a lot of kernel entry/exits we see kernel
> samples, even though :u is specified. This is due to skid existing.
>
> This might be a security issue because it can leak kernel addresses even
> though kernel sampling support is disabled.
>
> One patch "perf/core: Drop kernel samples even though :u is specified"
> was posted in last year but it was reverted because it introduced a
> regression issue that broke the rr-project.
>
> Now this patch set uses sysctl to control the dropping of leaked
> kernel samples.
>
> /sys/devices/cpu/perf_allow_sample_leakage:
>
> 0 - default, drop the leaked kernel samples.
> 1 - don't drop the leaked kernel samples.
>
> For rr it can write 1 to /sys/devices/cpu/perf_allow_sample_leakage to
> keep original system behavior.
>
> Jin Yao (2):
> perf/core: Use sysctl to turn on/off dropping leaked kernel samples
> perf Documentation: Introduce the sysctl perf_allow_sample_leakage
>
> kernel/events/core.c | 58 ++++++++++++++++++++++++++++++++
> tools/perf/Documentation/perf-record.txt | 14 ++++++++
> 2 files changed, 72 insertions(+)
>
> --
> 2.7.4
>

[0] https://lkml.org/lkml/2017/6/27/1159