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

Fuad Tabba
2024-07-09T14:48:41Z

Hi Patrick,

On Tue, Jul 9, 2024 at 2:21 PM Patrick Roy <roypat@amazon.co.uk> wrote:
>
> 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.

This patch allows mapping guest_memfd() unconditionally. Even if it's
not guppable, there are other reasons why you wouldn't want to allow
this. Maybe a config flag to gate it? e.g.,

https://lore.kernel.org/all/20240222161047.402609-4-tabba@google.com/

>
> 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;

Is this intentional?

Cheers,
/fuad

> if (xa_load(&gmem->bindings, index) != slot) {
> WARN_ON_ONCE(xa_load(&gmem->bindings, index));
> return -EIO;
> --
> 2.45.2
>