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

Patrick Roy
2024-07-09T13:21:24Z

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