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

[Sourav Panda]

On Sat, Aug 8, 2026 at 11:58 PM Muchun Song <muchun.song@xxxxxxxxx> wrote:
>
>
>
> > On Aug 9, 2026, at 12:32, Sourav Panda <souravpanda@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Sourav Panda <souravpanda@xxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxx/
> > - v3: https://lore.kernel.org/linux-mm/20260705175119.440599-1-souravpanda@xxxxxxxxxx/
> > - v2: https://lore.kernel.org/linux-mm/20260704174930.2885785-1-souravpanda@xxxxxxxxxx/
> > - v1: https://lore.kernel.org/linux-mm/20260702215713.627941-1-souravpanda@xxxxxxxxxx/
> >
> > 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!

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;

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
>