Re: [PATCH v6] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Muchun Song]

> On Aug 11, 2026, at 07:08, Sourav Panda <souravpanda@xxxxxxxxxx> 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.
> 
> This can be reproduced by booting a VM with two NUMA nodes, restricting
> CMA to Node 1 (e.g., hugetlb_cma=1:1G default_hugepagesz=1G
> hugepagesz=1G hugepages=0), and running a program that allocates a
> 1GB hugepage area without reserving, restricts allocation to Node 0
> using mbind() with MPOL_PREFERRED_MANY, and triggers a page fault:
> 
>  void *ptr = mmap(NULL, 1UL << 30, PROT_READ | PROT_WRITE,
>                   MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB |
>                   MAP_HUGE_1GB | MAP_NORESERVE, -1, 0);
>  unsigned long nodemask = 1; /* Node 0 */
>  mbind(ptr, 1UL << 30, MPOL_PREFERRED_MANY, &nodemask,
>        sizeof(nodemask) * 8, 0);
>  memset(ptr, 0, 1UL << 30); /* Trigger fault */
> 
> This results in a NULL pointer dereference:
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000000
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  Oops: Oops: 0000 [#1] SMP NOPTI
>  RIP: 0010:hugetlb_cma_alloc_frozen_folio+0x75/0x120
>  Call Trace:
>   <TASK>
>   only_alloc_fresh_hugetlb_folio.isra.0+0x2c/0x160
>   alloc_surplus_hugetlb_folio+0x6d/0x100
>   alloc_hugetlb_folio+0x3c5/0x660
>   hugetlb_no_page+0x3d9/0x650
> 
> Fixes: eb02f14c4a2b ("mm/hugetlb: allow overcommitting gigantic hugepages")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sourav Panda <souravpanda@xxxxxxxxxx>
> ---
> Changes in v6:
> - Enclosed the CMA allocation attempts within the cpuset seqcount retry loop,
>  retrying allocation upon cpuset mems_allowed updates to prevent unexpected
>  allocation failures as suggested by Muchun Song.
> - v5: https://lore.kernel.org/linux-mm/20260809043250.2917406-1-souravpanda@xxxxxxxxxx/
> - v4: https://lore.kernel.org/linux-mm/20260726072935.3513996-1-souravpanda@xxxxxxxxxx/
> - v3: https://lore.kernel.org/linux-mm/20260705175119.440599-1-souravpanda@xxxxxxxxxx/
> - v2: https://lore.kernel.org/linux-mm/20260704174930.2885785-1-souravpanda@xxxxxxxxxx/
> - v1: https://lore.kernel.org/linux-mm/20260702215713.627941-1-souravpanda@xxxxxxxxxx/
> 
> mm/hugetlb_cma.c | 23 ++++++++++++++++++-----
> 1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
> index 39344d6c78d8..9debf033d4fd 100644
> --- a/mm/hugetlb_cma.c
> +++ b/mm/hugetlb_cma.c
> @@ -3,6 +3,7 @@
> #include <linux/mm.h>
> #include <linux/cma.h>
> #include <linux/compiler.h>
> +#include <linux/cpuset.h>
> #include <linux/mm_inline.h>
> 
> #include <asm/page.h>
> @@ -30,15 +31,27 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
> 	int node;
> 	struct folio *folio;
> 	struct page *page = NULL;
> + 	const nodemask_t *nmask;
> + 	nodemask_t local_node_mask;
> + 	unsigned int cpuset_mems_cookie;
> 
> 	if (!hugetlb_cma_size)
> 		return NULL;
> 
> - 	if (hugetlb_cma[nid])
> +		retry_cpuset:
> + 	if (!nodemask) {
> + 		cpuset_mems_cookie = read_mems_allowed_begin();
> + 		local_node_mask = cpuset_current_mems_allowed;
> + 		nmask = &local_node_mask;

I don't think we need a local_node_mask, nmask = &cpuset_current_mems_allowed
is enough. nodemask_t could be a huge array, so it might consume a lot of memory
on the stack. I don't think we need to do that unless we see a clear necessity.

Otherwise, LGTM.

Thanks.

> + 	} else {
> + 		nmask = nodemask;
> + 	}
> +
> + 	if (hugetlb_cma[nid] && node_isset(nid, *nmask))
> 		page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);
> 
> 	if (!page && !(gfp_mask & __GFP_THISNODE)) {
> - 	for_each_node_mask(node, *nodemask) {
> + 	for_each_node_mask(node, *nmask) {
> 		if (node == nid || !hugetlb_cma[node])
> 			continue;
> 
> @@ -48,8 +61,12 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
> 		}
> 	}
> 
> - 	if (!page)
> + 	if (!page) {
> + 		if (!nodemask &&
> + 		    unlikely(read_mems_allowed_retry(cpuset_mems_cookie)))
> + 			goto retry_cpuset;
> 		return NULL;
> + 	}
> 
> 	folio = page_folio(page);
> 	folio_set_hugetlb_cma(folio);
> -- 
> 2.55.0