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

[Andrei Vagin <avagin@gmail.com>]

On Fri, Oct 09, 2020 at 03:28:15PM +0200, Christian Brauner wrote:
> On Thu, Oct 08, 2020 at 07:39:42AM +0200, Michael Weiß wrote:
> > 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);
> 
> Note that getbootime64() is mostly used in net/sunrpc and I don't know
> if this change has any security implications for them.

I would prefer to not patch kernel internal functions if they are used
not only to expose time to the userspace.

I think when kernel developers sees the getboottime64 function, they
will expect that it returns the real time of kernel boot. They will
not expect that it is aware of time namespaces and a returned time will
depend on a task in which context it will be called.

IMHO, as a minimum, we need to update the documentation for this function or
even adjust a function name.

And I think we need to consider an option to not change getbootime64 and
apply a timens offset right in the show_stat(fs/proc/stat.c) function.

Thanks,
Andrei