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