Re: [PATCH v4] mm/hugetlb: Fix unsigned overflow in __nr_hugepages_store_common()

[David Rientjes <rientjes@google.com>]

On Sun, 24 Feb 2019, Mike Kravetz wrote:

> > User can change a node specific hugetlb count. i.e.
> > /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>
> 
> Thank you.
> 
> Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
> > ---
> >  mm/hugetlb.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index afef616..6688894 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2423,7 +2423,14 @@ 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];
> > +		/*
> > +		 * If user specified count causes overflow, set to
> > +		 * largest possible value.
> > +		 */
> > +		if (count < old_count)
> > +			count = ULONG_MAX;
> >  		init_nodemask_of_node(nodes_allowed, nid);
> >  	} else
> >  		nodes_allowed = &node_states[N_MEMORY];
> > 

Looks like this fixes the overflow issue, but isn't there already a 
possible underflow since we don't hold hugetlb_lock?  Even if 
count == 0, what prevents h->nr_huge_pages_node[nid] being greater than 
h->nr_huge_pages here?  I think the per hstate values need to be read with 
READ_ONCE() and stored on the stack to do any sane bounds checking.