[PATCH v2] userfaultfd: prevent registration of special VMAs

[Mike Rapoport <rppt@kernel.org>]

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

Vova Tokarev says:

  userfaultfd allows registration on shadow stack VMAs.  With userfaultfd
  access, you can register on the shadow stack, discard a page ... and
  inject a page with chosen return addresses via UFFDIO_COPY.

Update vma_can_userfault() to reject VM_SHADOW_STACK.

While on it, also reject VM_SPECIAL so that if a driver would implement
vm_uffd_ops, it wouldn't be possible to register special VMAs with
userfaultfd.

Since VM_SPECIAL includes VM_DONTEXPAND which is set but hugetlb,
exclude hugetlb VMAs from the check for VM_SPECIAL.

Reported-by: vova tokarev <vladimirelitokarev@gmail.com>
Fixes: 54007f818206 ("mm: Introduce VM_SHADOW_STACK for shadow stack memory")
Cc: <stable@vger.kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---

v2 changes: 
* reject all VM_SPECIAL except hugetlb

v1: https://lore.kernel.org/all/20260617194059.2529406-1-rppt@kernel.org

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

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 246af12bf801..c3adedaaf7d5 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2111,7 +2111,10 @@ static bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
 {
 	const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
 
-	if (vma->vm_flags & VM_DROPPABLE)
+	if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK))
+		return false;
+
+	if (!is_vm_hugetlb_page(vma) && (vma->vm_flags & VM_SPECIAL))
 		return false;
 
 	vm_flags &= __VM_UFFD_FLAGS;

base-commit: e3d8707358ea76b78bdec9928937bb9a797f2c8f
-- 
2.53.0