[PATCH v5] 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().

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 (safely read using a seqcount retry loop).
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.

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 v5:
- Replaced defaulting nodemask to &node_states[N_MEMORY] with safely reading
  cpuset_current_mems_allowed using a seqcount retry loop, ensuring fallback
  allocations comply with task hardwall cpusets as suggested by Usama Arif.
- v4: https://lore.kernel.org/linux-mm/20260726072935.3513996-1-souravpanda@google.com/
- 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 | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
index 39344d6c78d8..3ae9347078e9 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>
@@ -30,11 +31,23 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
 	int node;
 	struct folio *folio;
 	struct page *page = NULL;
+	nodemask_t local_node_mask;
 
 	if (!hugetlb_cma_size)
 		return NULL;
 
-	if (hugetlb_cma[nid])
+	if (!nodemask) {
+		unsigned int cpuset_mems_cookie;
+
+		do {
+			cpuset_mems_cookie = read_mems_allowed_begin();
+			local_node_mask = cpuset_current_mems_allowed;
+		} while (read_mems_allowed_retry(cpuset_mems_cookie));
+
+		nodemask = &local_node_mask;
+	}
+
+	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