Re: [PATCH v7] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

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

On Tue, 11 Aug 2026 05:29:09 +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() via
> alloc_gigantic_frozen_folio().
> 
> Additionally, hugetlb_cma_alloc_frozen_folio() previously attempted
> allocation on hugetlb_cma[nid] without verifying if nid is included in
> the caller's nodemask. Adding a node_isset(nid, *nodemask) check ensures
> the initial preferred node allocation honors the memory policy / nodemask.
> 
> However, hugetlb_cma_alloc_frozen_folio() dereferences the nodemask in
> node_isset(nid, *nodemask) and for_each_node_mask(node, *nodemask),
> leading to a null pointer dereference kernel panic when nodemask is NULL.
> 
> Fix this by checking if nodemask is NULL in
> hugetlb_cma_alloc_frozen_folio() and defaulting it to
> cpuset_current_mems_allowed. Enclose the allocation attempts within
> the cpuset seqcount retry loop so that if the cpuset changes concurrently
> during allocation, the attempts are retried using the updated nodemask.
> This ensures that the initial node check and fallback loop safely honor
> the task's cpuset without violating cpuset constraints or causing NULL
> pointer dereferences or unexpected allocation failures.
> 
> 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.

Thanks.  Sashio might have found another thing with MPOL_PREFERRED_MANY
and CMA:

	https://sashiko.dev/#/patchset/20260811052909.475635-1-souravpanda@google.com



We're days away from 7.2 and I do dislike sending hotfixes upstream at
such a late stage.  I expect I'll upstream this and a few other
hotfixes after 7.2 is released.  It'll all end up in the same place.

I would still upstream hotfixes which address added-in-this-cycle bugs,
but there aren't any of those at present.