Re: [PATCH] psi: fix integer overflow on unsigned int multiply on 32 bit systems

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Nov 4, 2021 at 9:55 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> CC Suren

Thanks!

>
> Sorry, this fell through the cracks.
>
> On Mon, Sep 13, 2021 at 02:21:35PM +0530, Ravi Singh wrote:
> > psi accepts window sizes upto WINDOW_MAX_US(10000000). In the case
> > where window_us is larger than 4294967, the result of an
> > multiplication overflows an unsigned int/long(4 bytes on 32 bit
> > system).
> >
> > For example, this can happen when the window_us is 5000000 so 5000000
> > * 1000 (NSEC_PER_USEC) will result in 5000000000 which is greater than
> > UINT_MAX(4294967295). Due to this overflow, 705032704 is stored in
> > t->win.size instead of 5000000000. Now psi will be monitoring the
> > window size of 705 msecs instead of 5 secs as expected by user.
> >
> > Fix this by type casting the first term of the mutiply to a u64.

Looks reasonable to me.

> >
> > Issue doesnot occur on 64 bit systems because NSEC_PER_USEC is of type
> > long which is 8 bytes on 64 bit systems.
> >
> > Signed-off-by: Ravi Singh <ravi.singh1@samsung.com>
>
> Fixes: 0e94682b73bf psi: introduce psi monitor
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

>
> Peter would you mind taking this through -tip?
>
> > ---
> >  kernel/sched/psi.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > index 1652f2bb5..a2cc33dc2 100644
> > --- a/kernel/sched/psi.c
> > +++ b/kernel/sched/psi.c
> > @@ -1145,7 +1145,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
> >       t->group = group;
> >       t->state = state;
> >       t->threshold = threshold_us * NSEC_PER_USEC;
> > -     t->win.size = window_us * NSEC_PER_USEC;
> > +     t->win.size = (u64)window_us * NSEC_PER_USEC;
> >       window_reset(&t->win, 0, 0, 0);
> >
> >       t->event = 0;
> > --
> > 2.17.1