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

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

On 06/08/2026 06:33, Sourav Panda wrote:
> On Wed, Aug 5, 2026 at 7:47 PM Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>> On Wed, Aug 05, 2026 at 01:24:42PM -0700, Usama Arif wrote:
>>> 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?
>>
>> That's fair point but should not the user be also responsible in provding a right
>> nodemask containing CMA memory if it prefers avoiding node_states[N_MEMORY] based
>> fallback mechanism in kernel ?
>>

I don't think the task can be made responsible for providing a CMA-aware fallback mask
here.  Userspace supplies the MPOL_PREFERRED_MANY preference, but the kernel itself
changes the second attempt to a NULL nodemask.  That permits fallback outside the
preferred policy nodes; it does not permit fallback outside the task's hardwall cpuset.

> 
> We have precedence for three options, which makes selecting one difficult :)
> 
> 1) nodemask = &cpuset_current_mems_allowed is currently being used in:
>        only_alloc_fresh_hugetlb_folio --> alloc_buddy_frozen_folio -->
> __alloc_frozen_pages_noprof --> prepare_alloc_pages.

Here I think using &cpuset_current_mems_allowed is part of the page allocator's wider cpuset handling.

> 2) Some places use cookies with cpuset_current_mems_allowed to prevent
> torn writes. But this adds complexity.
>        An example would be dequeue_hugetlb_folio_nodemask()
> 3) node_states[N_MEMORY] is sprinkled all over the kernel (simplest).
> 

I would prefer option 2.  A NULL mempolicy mask should fall back to
cpuset_current_mems_allowed, not all N_MEMORY nodes. 


>>>
>>> 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
>>>>