Re: [PATCH v5] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Muchun Song <muchun.song@linux.dev>]

> On Aug 9, 2026, at 12:32, 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 (safely read using a seqcount retry loop).
> 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.
> 
> 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@vger.kernel.org
> Signed-off-by: Sourav Panda <souravpanda@google.com>
> ---
> Changes in v5:
> - Replaced defaulting nodemask to &node_states[N_MEMORY] with safely reading
>  cpuset_current_mems_allowed using a seqcount retry loop, ensuring fallback
>  allocations comply with task hardwall cpusets as suggested by Usama Arif.
> - v4: https://lore.kernel.org/linux-mm/20260726072935.3513996-1-souravpanda@google.com/
> - v3: https://lore.kernel.org/linux-mm/20260705175119.440599-1-souravpanda@google.com/
> - v2: https://lore.kernel.org/linux-mm/20260704174930.2885785-1-souravpanda@google.com/
> - v1: https://lore.kernel.org/linux-mm/20260702215713.627941-1-souravpanda@google.com/
> 
> mm/hugetlb_cma.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
> index 39344d6c78d8..3ae9347078e9 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>
> @@ -30,11 +31,23 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
> 	int node;
> 	struct folio *folio;
> 	struct page *page = NULL;
> + 	nodemask_t local_node_mask;
> 
> 	if (!hugetlb_cma_size)
> 		return NULL;
> 
> - 	if (hugetlb_cma[nid])
> + 	if (!nodemask) {
> + 		unsigned int cpuset_mems_cookie;
> +
> + 		do {
> + 			cpuset_mems_cookie = read_mems_allowed_begin();
> + 			local_node_mask = cpuset_current_mems_allowed;

I think it only makes sense to move cma_alloc_frozen_compound() inside the
loop. Otherwise, during subsequent memory allocations, we could still face
unexpected allocation failures.

Muchun,
Thanks.

> + 		} while (read_mems_allowed_retry(cpuset_mems_cookie));
> +
> + 		nodemask = &local_node_mask;
> + 	}
> +
> + 	if (hugetlb_cma[nid] && node_isset(nid, *nodemask))
> 		page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);
> 
> 	if (!page && !(gfp_mask & __GFP_THISNODE)) {
> -- 
> 2.55.0