Re: [PATCH v1 1/2] perf/core: Use sysctl to turn on/off dropping leaked kernel samples

Stephane Eranian
2018-06-15T06:03:10Z

On Thu, Jun 14, 2018 at 7:10 PM Jin Yao <yao.jin@linux.intel.com> wrote:
>
> When doing sampling, for example:
>
> perf record -e cycles:u ...
>
> 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, which used sampling
> events to receive a signal on overflow. These signals were critical
> to the correct operation of rr.
>
> See '6a8a75f32357 ("Revert "perf/core: Drop kernel samples even
> though :u is specified"")' for detail.
>
> Now the idea is to use 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.
>
> For example,
>
> root@skl:/tmp# cat /sys/devices/cpu/perf_allow_sample_leakage
> 0
> root@skl:/tmp# perf record -e cycles:u ./div
> root@skl:/tmp# perf report --stdio
>
> ........ ....... ............. ................
>
> 47.01% div div [.] main
> 20.74% div libc-2.23.so [.] __random_r
> 15.59% div libc-2.23.so [.] __random
> 8.68% div div [.] compute_flag
> 4.48% div libc-2.23.so [.] rand
> 3.50% div div [.] rand@plt
> 0.00% div ld-2.23.so [.] do_lookup_x
> 0.00% div ld-2.23.so [.] memcmp
> 0.00% div ld-2.23.so [.] _dl_start
> 0.00% div ld-2.23.so [.] _start
>
> There is no kernel symbol reported.
>
> root@skl:/tmp# echo 1 > /sys/devices/cpu/perf_allow_sample_leakage
> root@skl:/tmp# cat /sys/devices/cpu/perf_allow_sample_leakage
> 1
> root@skl:/tmp# perf record -e cycles:u ./div
> root@skl:/tmp# perf report --stdio
>
> ........ ....... ................ .............
>
> 47.53% div div [.] main
> 20.62% div libc-2.23.so [.] __random_r
> 15.32% div libc-2.23.so [.] __random
> 8.66% div div [.] compute_flag
> 4.53% div libc-2.23.so [.] rand
> 3.34% div div [.] rand@plt
> 0.00% div [kernel.vmlinux] [k] apic_timer_interrupt
> 0.00% div libc-2.23.so [.] intel_check_word
> 0.00% div ld-2.23.so [.] brk
> 0.00% div [kernel.vmlinux] [k] page_fault
> 0.00% div ld-2.23.so [.] _start
>
> We can see the kernel symbols apic_timer_interrupt and page_fault.
>
> Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
> ---
> kernel/events/core.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 58 insertions(+)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 80cca2b..7867541 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7721,6 +7721,28 @@ int perf_event_account_interrupt(struct perf_event *event)
> return __perf_event_account_interrupt(event, 1);
> }
>
> +static int perf_allow_sample_leakage __read_mostly;
> +
> +static bool sample_is_allowed(struct perf_event *event, struct pt_regs *regs)
> +{
> + int allow_leakage = READ_ONCE(perf_allow_sample_leakage);
> +
> + if (allow_leakage)
> + return true;
> +
> + /*
> + * Due to interrupt latency (AKA "skid"), we may enter the
> + * kernel before taking an overflow, even if the PMU is only
> + * counting user events.
> + * To avoid leaking information to userspace, we must always
> + * reject kernel samples when exclude_kernel is set.
> + */
> + if (event->attr.exclude_kernel && !user_mode(regs))
> + return false;
> +
And how does that filter PEBS or LBR records?

> + return true;
> +}
> +
> /*
> * Generic event overflow handling, sampling.
> */
> @@ -7742,6 +7764,12 @@ static int __perf_event_overflow(struct perf_event *event,
> ret = __perf_event_account_interrupt(event, throttle);
>
> /*
> + * For security, drop the skid kernel samples if necessary.
> + */
> + if (!sample_is_allowed(event, regs))
> + return ret;
> +
> + /*
> * XXX event_limit might not quite work as expected on inherited
> * events
> */
> @@ -9500,9 +9528,39 @@ perf_event_mux_interval_ms_store(struct device *dev,
> }
> static DEVICE_ATTR_RW(perf_event_mux_interval_ms);
>
> +static ssize_t
> +perf_allow_sample_leakage_show(struct device *dev,
> + struct device_attribute *attr, char *page)
> +{
> + int allow_leakage = READ_ONCE(perf_allow_sample_leakage);
> +
> + return snprintf(page, PAGE_SIZE-1, "%d\n", allow_leakage);
> +}
> +
> +static ssize_t
> +perf_allow_sample_leakage_store(struct device *dev,
> + struct device_attribute *attr,
> + const char *buf, size_t count)
> +{
> + int allow_leakage, ret;
> +
> + ret = kstrtoint(buf, 0, &allow_leakage);
> + if (ret)
> + return ret;
> +
> + if (allow_leakage != 0 && allow_leakage != 1)
> + return -EINVAL;
> +
> + WRITE_ONCE(perf_allow_sample_leakage, allow_leakage);
> +
> + return count;
> +}
> +static DEVICE_ATTR_RW(perf_allow_sample_leakage);
> +
> static struct attribute *pmu_dev_attrs[] = {
> &dev_attr_type.attr,
> &dev_attr_perf_event_mux_interval_ms.attr,
> + &dev_attr_perf_allow_sample_leakage.attr,
> NULL,
> };
> ATTRIBUTE_GROUPS(pmu_dev);
> --
> 2.7.4
>