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

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Message-ID: <CA+EHjTynVpsqsudSVRgOBdNSP_XjdgKQkY_LwdqvPkpJAnAYKg@mail.gmail.com>
Date: Tue, 9 Jul 2024 15:48:01 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Patrick Roy <roypat@...zon.co.uk>
Cc: seanjc@...gle.com, pbonzini@...hat.com, akpm@...ux-foundation.org, 
	dwmw@...zon.co.uk, rppt@...nel.org, david@...hat.com, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, willy@...radead.org, graf@...zon.com, derekmn@...zon.com, 
	kalyazin@...zon.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, dmatlack@...gle.com, chao.p.peng@...ux.intel.com, 
	xmarcalx@...zon.co.uk
Subject: Re: [RFC PATCH 8/8] kvm: gmem: Allow restricted userspace mappings

Hi Patrick,

On Tue, Jul 9, 2024 at 2:21 PM Patrick Roy <roypat@...zon.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@...zon.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
>