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

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

> On Aug 11, 2026, at 13:26, Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
> On Tue, Aug 11, 2026 at 10:48:31AM +0800, Muchun Song wrote:
>> 
>> 
>>> On Aug 11, 2026, at 07:08, 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.
>>> 
>>> 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 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@google.com/
>>> - 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 | 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.
> 
> But does not local_node_mask capture current snapshot for cpuset_current_mems_allowed
> after first taking a reference via read_mems_allowed_begin() which ensures that nmask
> has a constant value there after. Otherwise nmask could just change value by the time
> it actually gets used.

However, this value can change while you are copying it, so a snapshot itself is not
a consistent value. Right?

> 
>> 
>> 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