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

On Sun, Aug 9, 2026 at 7:24 PM Muchun Song <[email protected]> wrote:
>
>
>
> > On Aug 10, 2026, at 04:11, Sourav Panda <[email protected]> wrote:
> >
> > On Sat, Aug 8, 2026 at 11:58 PM Muchun Song <[email protected]> wrote:
> >>
> >>
> >>
> >>> On Aug 9, 2026, at 12:32, Sourav Panda <[email protected]> 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: [email protected]
> >>> Signed-off-by: Sourav Panda <[email protected]>
> >>> ---
> >>> 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/[email protected]/
> >>> - v3: https://lore.kernel.org/linux-mm/[email protected]/
> >>> - v2: https://lore.kernel.org/linux-mm/[email protected]/
> >>> - v1: https://lore.kernel.org/linux-mm/[email protected]/
> >>>
> >>> 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.
> >
> > Hi Muchun!
>
> Hi,
>
> >
> > What do you think of the following diff? This way
> > cma_alloc_frozen_compound is also protected from stale nodemasks.
> >
> >         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;
> >    +    } 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;
> >
> >    @@ -48,6 +61,9 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int
> > order, gfp_t gfp_mask,
> >             }
> >         }
> >
> >    +    if (!page && !nodemask &&
> > unlikely(read_mems_allowed_retry(cpuset_mems_cookie)))
> >    +        goto retry_cpuset;
> >    +
> >         if (!page)
> >             return NULL;
>
> The overall looks good to me. But can we change it to this here?
>
>         if (!page) {
>                 if (!nodemask &&
>                     unlikely(read_mems_allowed_retry(cpuset_mems_cookie)))
>                         goto retry_cpuset;
>                 return NULL;
>         }
>
> That way, we can remove the 'if (!page)' check here.

Sure thing! Thanks! I shall send this tomorrow noon (PST) since I am
OOO starting Thursday.

  @@ -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;
  + } 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;
  + }


>
> Muchun,
> Thanks.
>
> >
> > If this looks good to you, I'll send out v6 with this change.
> >
> > With regards,
> > Sourav Panda
> >
> >>
> >>> +             } 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
>
>