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timekeeping/vsyscall: Prevent math overflow in BOOTTIME update

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The VDSO update for CLOCK_BOOTTIME has a overflow issue as it shifts the
nanoseconds based boot time offset left by the clocksource shift. That
overflows once the boot time offset becomes large enough. As a consequence
CLOCK_BOOTTIME in the VDSO becomes a random number causing applications to
misbehave.

Fix it by storing a timespec64 representation of the offset when boot time
is adjusted and add that to the MONOTONIC base time value in the vdso data
page. Using the timespec64 representation avoids a 64bit division in the
update code.

Fixes: 44f57d788e ("timekeeping: Provide a generic update_vsyscall() implementation")
Reported-by: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1908221257580.1983@nanos.tec.linutronix.de
This commit is contained in:
Thomas Gleixner
2019-08-22 13:00:15 +02:00
parent 59c36bc8d3
commit b99328a60a
3 changed files with 23 additions and 9 deletions
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5
include/linux/timekeeper_internal.h
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@@ -57,6 +57,7 @@ struct tk_read_base {
* @cs_was_changed_seq: The sequence number of clocksource change events
* @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
* @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
* @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
* @cycle_interval: Number of clock cycles in one NTP interval
* @xtime_interval: Number of clock shifted nano seconds in one NTP
* interval.
@@ -84,6 +85,9 @@ struct tk_read_base {
*
* wall_to_monotonic is no longer the boot time, getboottime must be
* used instead.
*
* @monotonic_to_boottime is a timespec64 representation of @offs_boot to
* accelerate the VDSO update for CLOCK_BOOTTIME.
*/
struct timekeeper {
struct tk_read_base tkr_mono;
@@ -99,6 +103,7 @@ struct timekeeper {
u8 cs_was_changed_seq;
ktime_t next_leap_ktime;
u64 raw_sec;
struct timespec64 monotonic_to_boot;
/* The following members are for timekeeping internal use */
u64 cycle_interval;