Re: [PATCH v4] mm/hugetlb_cma: Fix null nodemask dereference in hugetlb_cma_alloc_frozen_folio

[Usama Arif <usama.arif@linux.dev>]

On Sun, 26 Jul 2026 07:29:34 +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().
> 
> hugetlb_cma_alloc_frozen_folio() blindly dereferences the nodemask in
> node_isset(nid, *nodemask) and for_each_node_mask(node, *nodemask),
> leading to a null pointer dereference kernel panic.
> 
> Fix this by checking if nodemask is NULL in
> hugetlb_cma_alloc_frozen_folio() and defaulting it to
> node_states[N_MEMORY]. This allows hugetlb_cma allocations to fall
> back to any node with memory, keeping behavior consistent with
> alloc_contig_frozen_pages() and alloc_buddy_frozen_folio().
> 
> >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 v4:
> - Reverted the alloc_fresh_hugetlb_folio() cpuset snapshot approach from v3.
>   As Muchun Song pointed out, snapshotting cpuset_current_mems_allowed does
>   not prevent false-positive allocation failures without complex retry loops,
>   and alloc_contig_frozen_pages() / alloc_buddy_frozen_folio() already handle
>   NULL nodemasks safely internally.
> - Handled NULL nodemask directly inside hugetlb_cma_alloc_frozen_folio()
>   by defaulting nodemask to node_states[N_MEMORY] (Option 2), keeping
>   HugeTLB allocators clean and consistent.
> - 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 | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
> index 39344d6c78d8..5744de0ceeb7 100644
> --- a/mm/hugetlb_cma.c
> +++ b/mm/hugetlb_cma.c
> @@ -34,7 +34,10 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
>  	if (!hugetlb_cma_size)
>  		return NULL;
>  
> -	if (hugetlb_cma[nid])
> +	if (!nodemask)
> +		nodemask = &node_states[N_MEMORY];

Hi Sourav,

hmm what if cpuset.mems only allows allocation from node 0, and hugetlb CMA
is only available on node 1. This will no now allow allocating a gigantic page
on node 1 when its explicitly not allowed?

Would it not be better to check cpuset as well?

Thanks,
Usama

> +
> +	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
>