+ mm-hugetlb_cma-fix-null-nodemask-dereference-in-hugetlb_cma_alloc_frozen_folio.patch added to mm-hotfixes-unstable branch

The patch titled
     Subject: mm/hugetlb_cma: dix null nodemask dereference in hugetlb_cma_alloc_frozen_folio
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-hugetlb_cma-fix-null-nodemask-dereference-in-hugetlb_cma_alloc_frozen_folio.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb_cma-fix-null-nodemask-dereference-in-hugetlb_cma_alloc_frozen_folio.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Sourav Panda <[email protected]>
Subject: mm/hugetlb_cma: dix null nodemask dereference in hugetlb_cma_alloc_frozen_folio
Date: Sun, 26 Jul 2026 07:29:34 +0000

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

Link: https://lore.kernel.org/[email protected]
Fixes: eb02f14c4a2b ("mm/hugetlb: allow overcommitting gigantic hugepages")
Signed-off-by: Sourav Panda <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Usama Arif <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Frank van der Linden <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 mm/hugetlb_cma.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/hugetlb_cma.c~mm-hugetlb_cma-fix-null-nodemask-dereference-in-hugetlb_cma_alloc_frozen_folio
+++ a/mm/hugetlb_cma.c
@@ -34,7 +34,10 @@ struct folio *hugetlb_cma_alloc_frozen_f
 	if (!hugetlb_cma_size)
 		return NULL;
 
-	if (hugetlb_cma[nid])
+	if (!nodemask)
+		nodemask = &node_states[N_MEMORY];
+
+	if (hugetlb_cma[nid] && node_isset(nid, *nodemask))
 		page = cma_alloc_frozen_compound(hugetlb_cma[nid], order);
 
 	if (!page && !(gfp_mask & __GFP_THISNODE)) {
_

Patches currently in -mm which might be from [email protected] are

mm-hugetlb_cma-fix-null-nodemask-dereference-in-hugetlb_cma_alloc_frozen_folio.patch
mm-hugetlb_cma-support-percentage-based-hugetlb_cma-reservation.patch