Re: [PATCH v3] mm/hugetlb: Fix unsigned overflow in __nr_hugepages_store_common()

[Mike Kravetz <mike.kravetz@oracle.com>]

On 2/22/19 6:01 AM, Jing Xiangfeng wrote:
Thanks, just a couple small changes.

> User can change a node specific hugetlb count. i.e.
> /sys/devices/system/node/node1/hugepages/hugepages-2048kB

Please make that,
/sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages

> the calculated value of count is a total number of huge pages. It could
> be overflow when a user entering a crazy high value. If so, the total
> number of huge pages could be a small value which is not user expect.
> We can simply fix it by setting count to ULONG_MAX, then it goes on. This
> may be more in line with user's intention of allocating as many huge pages
> as possible.
> 
> Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
> ---
>  mm/hugetlb.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index afef616..18fa7d7 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2423,7 +2423,10 @@ static ssize_t __nr_hugepages_store_common(bool obey_mempolicy,
>  		 * per node hstate attribute: adjust count to global,
>  		 * but restrict alloc/free to the specified node.
>  		 */
> +		unsigned long old_count = count;
>  		count += h->nr_huge_pages - h->nr_huge_pages_node[nid];

Also, adding a comment here about checking for overflow would help people
reading the code.  Something like,

		/*
		 * If user specified count causes overflow, set to
		 * largest possible value.
		 */

-- 
Mike Kravetz

> +		if (count < old_count)
> +			count = ULONG_MAX;
>  		init_nodemask_of_node(nodes_allowed, nid);
>  	} else
>  		nodes_allowed = &node_states[N_MEMORY];
>