Re: [RFC PATCH 14/18] KVM: Add asynchronous userfaults, KVM_READ_USERFAULT

[David Matlack <dmatlack@xxxxxxxxxx>]

On Wed, Jul 10, 2024 at 4:42 PM James Houghton <jthoughton@xxxxxxxxxx> wrote:
>
> +       case KVM_READ_USERFAULT: {
> +               struct kvm_fault fault;
> +               gfn_t gfn;
> +
> +               r = kvm_vm_ioctl_read_userfault(kvm, &gfn);
> +               if (r)
> +                       goto out;
> +
> +               fault.address = gfn;
> +
> +               /* TODO: if this fails, this gfn is lost. */
> +               r = -EFAULT;
> +               if (copy_to_user(&fault, argp, sizeof(fault)))

You could do the copy under the spin_lock() with
copy_to_user_nofault() to avoid losing gfn.