[PATCH v2 2/4] time: make getboottime64 aware of time namespace

["Michael Weiß" <michael.weiss@aisec.fraunhofer.de>]

getboottime64() provides the time stamp of system boot. In case of
time namespaces, the offset to the boot time stamp was not applied
earlier. However, getboottime64 is used e.g., in /proc/stat to print
the system boot time to userspace. In container runtimes which utilize
time namespaces to virtualize boottime of a container, this leaks
information about the host system boot time.

Therefore, we make getboottime64() to respect the time namespace offset
for boottime by subtracting the boottime offset.

Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>
---
 kernel/time/timekeeping.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 4c47f388a83f..67530cdb389e 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -17,6 +17,7 @@
 #include <linux/clocksource.h>
 #include <linux/jiffies.h>
 #include <linux/time.h>
+#include <linux/time_namespace.h>
 #include <linux/tick.h>
 #include <linux/stop_machine.h>
 #include <linux/pvclock_gtod.h>
@@ -2154,6 +2155,8 @@ void getboottime64(struct timespec64 *ts)
 {
 	struct timekeeper *tk = &tk_core.timekeeper;
 	ktime_t t = ktime_sub(tk->offs_real, tk->offs_boot);
+	/* shift boot time stamp according to the timens offset */
+	t = timens_ktime_to_host(CLOCK_BOOTTIME, t);
 
 	*ts = ktime_to_timespec64(t);
 }
-- 
2.20.1