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

[Stephane Eranian]

On Thu, Jun 14, 2018 at 7:10 PM Jin Yao <yao.jin@xxxxxxxxxxxxxxx> 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.
>
These kernel symbols do not match your description here. How much skid
do you think you have here?
You're saying you are at the user level, you get a counter overflow,
and the interrupted IP lands in the kernel
because you where there by the time the interrupt is delivered. How
many instructions does it take to get
from user land to apic_timer_interrupt() or page_fault()? These
functions are not right at the kernel entry,
I believe. So how did you get there, the skid must have been VERY big
or symbolization has a problem.

> Signed-off-by: Jin Yao <yao.jin@xxxxxxxxxxxxxxx>
> ---
>  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;
> +
> +       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
>