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

[Sourav Panda]

On Wed, Aug 5, 2026 at 7:47 PM Anshuman Khandual
<anshuman.khandual@xxxxxxx> 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@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().
> > >
> > > 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@xxxxxxxxxxxxxxx
> > > Signed-off-by: Sourav Panda <souravpanda@xxxxxxxxxx>
> > > ---
> > > 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@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 | 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 ?
>

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.
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).

> >
> > 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
> > >