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

Stephane Eranian
2018-06-19T16:50:46Z

On Fri, Jun 15, 2018 at 12:15 AM Jin, Yao <yao.jin@linux.intel.com> wrote:
>
>
>
> On 6/15/2018 1:59 PM, Stephane Eranian wrote:
> > 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.
> >>
> > 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.
> >
>
> I'm testing with the latest perf/core branch (4.17+). Again I test with
> Linux's vmstat (not test with my application).
>
> perf record -e cycles:u vmstat 1
> perf script -F ip
>
> 7f84e2b0bc30
> 7f84e2b0bc30
> 7f84e2b0bc30
> 7f84e2b0bc30
> ffffffffb7a01070
> 7f84e2b243f0
> 7f84e2b11891
> 7f84e2b27f5e
> 7f84e25a3b26
> 7f84e25680f5
>
> cat /proc/kallsyms | grep page_fault
> ....
> ffffffffb7a01070 T page_fault
> ffffffffb7a010a0 T async_page_fault
> ....
>
> So one sample (ip ffffffffb7a01070) hits on page_fault.
>
> Maybe you can have a try too. :)
>
Ok, I tried and checked! These symbols are all in entry_64.S, these
are the first instructions executed on the timer intr or fault.
So it looks normal that they show up due to the interrupt skid, even
if the skid is 1 cycle.
PEBS, especially when using precise=1 could also show these symbols.

>
> Thanks
> Jin Yao
>
> >> 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;
> >> +
> >> + 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
> >>