Re: [PATCH v3] mm/hugetlb: Fix null nodemask in alloc_fresh_hugetlb_folio

[Andrew Morton <akpm@linux-foundation.org>]

On Sun,  5 Jul 2026 17:51:19 +0000 Sourav Panda <souravpanda@google.com> wrote:

> alloc_buddy_hugetlb_folio_with_mpol() can pass a NULL nodemask to
> alloc_fresh_hugetlb_folio() as a fallback to allocate from all
> nodes. If order is gigantic, alloc_fresh_hugetlb_folio() propagates
> the NULL nodemask down to hugetlb_cma_alloc_frozen_folio() which blindly
> dereferences it in for_each_node_mask(), leading to a null pointer
> dereference.
> 
> Similarly, if the CMA allocation fails, the fallback
> alloc_contig_frozen_pages() is also called with a NULL nodemask,
> which may cause issues.
> 
> Fix this by explicitly checking if nodemask is NULL in
> alloc_fresh_hugetlb_folio() and defaulting to
> cpuset_current_mems_allowed. This ensures that both the CMA and
> contiguous allocators receive a valid nodemask safely using a seqcount
> loop to prevent torn reads.
> 
> From a userspace perspective, this bug allows an unprivileged user to
> crash the kernel (trigger a panic) by requesting a gigantic hugepage
> allocation with MPOL_PREFERRED_MANY on a system where CMA is only
> configured on a subset of NUMA nodes.

ow.

> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1864,6 +1864,18 @@ static struct folio *alloc_fresh_hugetlb_folio(struct hstate *h,
>  		gfp_t gfp_mask, int nid, nodemask_t *nmask)
>  {
>  	struct folio *folio;
> +	nodemask_t local_node_mask;
> +
> +	if (!nmask) {
> +		unsigned int cpuset_mems_cookie;
> +
> +		do {
> +			cpuset_mems_cookie = read_mems_allowed_begin();
> +			local_node_mask = cpuset_current_mems_allowed;
> +		} while (read_mems_allowed_retry(cpuset_mems_cookie));
> +
> +		nmask = &local_node_mask;
> +	}

For my edification, is there anything which prevents
cpuset_current_mems_allowed from changing after it has been read?  So
we end up with a folio on an inappropriate node?  Or does that not
really matter much.


cpuset_current_mems_allowed is a nasty thing.  It looks like a simple
global variable, but appearances can be deceptive:

#ifdef ...
#define cpuset_current_mems_allowed (current->mems_allowed)
#else
#define cpuset_current_mems_allowed (node_states[N_MEMORY])
#endif


Thanks, I'll queue this for a bit of testing and shall await reviewer
input.