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

[Anshuman Khandual]

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 ?

> 
> 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
> >