Re: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Mar 09, 2021 at 12:56:39PM -0700, Alex Williamson wrote:

> And I think this is what we end up with for the current code base:

Yeah, that looks Ok
 
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index 65e7e6b44578..2f247ab18c66 100644
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -1568,19 +1568,24 @@ void vfio_pci_memory_unlock_and_restore(struct vfio_pci_device *vdev, u16 cmd)
>  }
>  
>  /* Caller holds vma_lock */
> -static int __vfio_pci_add_vma(struct vfio_pci_device *vdev,
> -			      struct vm_area_struct *vma)
> +struct vfio_pci_mmap_vma *__vfio_pci_add_vma(struct vfio_pci_device *vdev,
> +					     struct vm_area_struct *vma)
>  {
>  	struct vfio_pci_mmap_vma *mmap_vma;
>  
> +	list_for_each_entry(mmap_vma, &vdev->vma_list, vma_next) {
> +		if (mmap_vma->vma == vma)
> +			return ERR_PTR(-EEXIST);
> +	}
> +
>  	mmap_vma = kmalloc(sizeof(*mmap_vma), GFP_KERNEL);
>  	if (!mmap_vma)
> -		return -ENOMEM;
> +		return ERR_PTR(-ENOMEM);
>  
>  	mmap_vma->vma = vma;
>  	list_add(&mmap_vma->vma_next, &vdev->vma_list);
>  
> -	return 0;
> +	return mmap_vma;
>  }
>  
>  /*
> @@ -1612,30 +1617,39 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>  	struct vfio_pci_device *vdev = vma->vm_private_data;
> -	vm_fault_t ret = VM_FAULT_NOPAGE;
> +	struct vfio_pci_mmap_vma *mmap_vma;
> +	unsigned long vaddr, pfn;
> +	vm_fault_t ret;
>  
>  	mutex_lock(&vdev->vma_lock);
>  	down_read(&vdev->memory_lock);
>  
>  	if (!__vfio_pci_memory_enabled(vdev)) {
>  		ret = VM_FAULT_SIGBUS;
> -		mutex_unlock(&vdev->vma_lock);
>  		goto up_out;
>  	}
>  
> -	if (__vfio_pci_add_vma(vdev, vma)) {
> -		ret = VM_FAULT_OOM;
> -		mutex_unlock(&vdev->vma_lock);
> +	mmap_vma = __vfio_pci_add_vma(vdev, vma);
> +	if (IS_ERR(mmap_vma)) {
> +		/* A concurrent fault might have already inserted the page */
> +		ret = (PTR_ERR(mmap_vma) == -EEXIST) ? VM_FAULT_NOPAGE :
> +						       VM_FAULT_OOM;

I think -EEIXST should not be an error, lets just go down to the
vmf_insert_pfn() and let the MM resolve the race naturally.

I suspect returning VM_FAULT_NOPAGE will be averse to the userspace if
it hits this race??

Also the _prot does look needed at least due to the SME, but possibly
also to ensure NC gets set..

Jason