From: Mike Rapoport (Microsoft) <rppt@kernel.org>
Date: Thu, 18 Jun 2026 09:50:17 +0000 (+0300)
Subject: userfaultfd: prevent registration of special VMAs
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=3c58f641e813c3c71039f8fd4d4e2a3aab713288;p=thirdparty%2Flinux.git

userfaultfd: prevent registration of special VMAs

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.

Link: https://lore.kernel.org/20260618095017.2553004-1-rppt@kernel.org
Fixes: 54007f818206 ("mm: Introduce VM_SHADOW_STACK for shadow stack memory")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reported-by: vova tokarev <vladimirelitokarev@gmail.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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;