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

[Anshuman Khandual]

On Tue, Aug 11, 2026 at 10:48:31AM +0800, Muchun Song wrote:
> 
> 
> > On Aug 11, 2026, at 07:08, 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. 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Sourav Panda <souravpanda@xxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxx/
> > - 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 | 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.

> 
> 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
>