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

[Sourav Panda <souravpanda@google.com>]

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@vger.kernel.org
Signed-off-by: Sourav Panda <souravpanda@google.com>
---
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@google.com/
- v2: https://lore.kernel.org/linux-mm/20260704174930.2885785-1-souravpanda@google.com/
- v1: https://lore.kernel.org/linux-mm/20260702215713.627941-1-souravpanda@google.com/

 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];
+
+	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