Re: [PATCH] mm/mempolicy: prevent the risk of division by 0

[Gregory Price <gourry@gourry.net>]

On Sun, Sep 07, 2025 at 09:08:29AM -0700, Chelsy Ratnawat wrote:
> If no bits are set in the policy's node mask, then nodes will be 0.
> This patch adds a check if nodes == 0 before dividing.
> 
> Signed-off-by: Chelsy Ratnawat <chelsyratnawat2001@gmail.com>
> ---
>  mm/mempolicy.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index eb83cff7db8c..faacc604fc16 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2530,6 +2530,8 @@ static unsigned long alloc_pages_bulk_interleave(gfp_t gfp,
>  	unsigned long total_allocated = 0;
>  
>  	nodes = nodes_weight(pol->nodes);
> +	if (nodes == 0)
> +		return 0;

3 second look at this code would tell you the only way this can happen
is a mempolicy where MPOL_INTERLEAVE is set and the nodemask is empty

The only way this can occur is

1) someone setting that explicitly, which is invalid and protected by
   a line in mpol_new:

   else if (nodes_empty(*nodes))
	return ERR_PTR(-EINVAL);

2) or a rebind event occurring due to a cgroup task migration or
   cpuset change - which is protected by a check in the rebind:

   if (nodes_empty(tmp))
	tmp = *nodes;


I doubt this bug was observed in action.

>  	nr_pages_per_node = nr_pages / nodes;
>  	delta = nr_pages - nodes * nr_pages_per_node;
>  
> -- 
> 2.47.3
> 
>