FAILED: patch "[PATCH] userfaultfd: prevent registration of special VMAs" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 3c58f641e813c3c71039f8fd4d4e2a3aab713288
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072906-estranged-sinless-b8ff@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 3c58f641e813c3c71039f8fd4d4e2a3aab713288 Mon Sep 17 00:00:00 2001
From: "Mike Rapoport (Microsoft)" <[email protected]>
Date: Thu, 18 Jun 2026 12:50:17 +0300
Subject: [PATCH] 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.

index 246af12bf801..c3adedaaf7d5 100644
@@ -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;

From: "Mike Rapoport (Microsoft)" <[email protected]>

[ Upstream commit 3c58f641e813c3c71039f8fd4d4e2a3aab713288 ]

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.

---
 include/linux/userfaultfd_k.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index c0e716aec26aa..796a380664bd8 100644
@@ -214,7 +214,10 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
 {
 	vm_flags &= __VM_UFFD_FLAGS;
 
-	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;
 
 	if ((vm_flags & VM_UFFD_MINOR) &&

2.53.0