[RFC PATCH 8/8] kvm: gmem: Allow restricted userspace mappings

[Patrick Roy <roypat@amazon.co.uk>]

Allow mapping guest_memfd into userspace. Since AS_INACCESSIBLE is set
on the underlying address_space struct, no GUP of guest_memfd will be
possible.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
---
 virt/kvm/guest_memfd.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index dc9b0c2d0b0e..101ec2b248bf 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -319,7 +319,37 @@ static inline struct file *kvm_gmem_get_file(struct kvm_memory_slot *slot)
 	return get_file_active(&slot->gmem.file);
 }
 
+static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
+{
+	struct folio *folio;
+
+	folio = kvm_gmem_get_folio(file_inode(vmf->vma->vm_file), vmf->pgoff, true);
+
+	if (!folio)
+		return VM_FAULT_SIGBUS;
+
+	vmf->page = folio_file_page(folio, vmf->pgoff);
+
+	return VM_FAULT_LOCKED;
+}
+
+static const struct vm_operations_struct kvm_gmem_vm_ops = {
+	.fault = kvm_gmem_fault
+};
+
+static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
+		return -EINVAL;
+
+	vm_flags_set(vma, VM_DONTDUMP);
+	vma->vm_ops = &kvm_gmem_vm_ops;
+
+	return 0;
+}
+
 static struct file_operations kvm_gmem_fops = {
+	.mmap           = kvm_gmem_mmap,
 	.open		= generic_file_open,
 	.release	= kvm_gmem_release,
 	.fallocate	= kvm_gmem_fallocate,
@@ -594,7 +624,6 @@ static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
 		return -EFAULT;
 	}
 
-	gmem = file->private_data;
 	if (xa_load(&gmem->bindings, index) != slot) {
 		WARN_ON_ONCE(xa_load(&gmem->bindings, index));
 		return -EIO;
-- 
2.45.2