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

[Johannes Weiner]

CC Suren

On Mon, Sep 06, 2021 at 05:55:24PM +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.
> 
> 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@xxxxxxxxxxx>
> Signed-off-by: Vishal Goel <vishal.goel@xxxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

> ---
>  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
>