Re: [PATCH 13/13] vfio/type1: Mark follow_pfn as unsafe

[Jason Gunthorpe]

On Wed, Oct 07, 2020 at 06:44:26PM +0200, Daniel Vetter wrote:
> The code seems to stuff these pfns into iommu pts (or something like
> that, I didn't follow), but there's no mmu_notifier to ensure that
> access is synchronized with pte updates.
> 
> Hence mark these as unsafe. This means that with
> CONFIG_STRICT_FOLLOW_PFN, these will be rejected.
> 
> Real fix is to wire up an mmu_notifier ... somehow. Probably means any
> invalidate is a fatal fault for this vfio device, but then this
> shouldn't ever happen if userspace is reasonable.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-samsung-soc@xxxxxxxxxxxxxxx
> Cc: linux-media@xxxxxxxxxxxxxxx
> Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> ---
>  drivers/vfio/vfio_iommu_type1.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 5fbf0c1f7433..a4d53f3d0a35 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -421,7 +421,7 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
>  {
>  	int ret;
>  
> -	ret = follow_pfn(vma, vaddr, pfn);
> +	ret = unsafe_follow_pfn(vma, vaddr, pfn);
>  	if (ret) {
>  		bool unlocked = false;
>  
> @@ -435,7 +435,7 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
>  		if (ret)
>  			return ret;
>  
> -		ret = follow_pfn(vma, vaddr, pfn);
> +		ret = unsafe_follow_pfn(vma, vaddr, pfn);
>  	}

This is actually being commonly used, so it needs fixing.

When I talked to Alex about this last we had worked out a patch series
that adds a test on vm_ops that the vma came from vfio in the first
place. The VMA's created by VFIO are 'safe' as the PTEs are never changed.

Jason