Re: [PATCH RFC 09/10] vfio/type1: Use follow_pfn for VM_FPNMAP VMAs

[Jason Gunthorpe]

On Fri, Jan 10, 2020 at 07:03:12PM +0000, Joao Martins wrote:
> From: Nikita Leshenko <nikita.leshchenko@xxxxxxxxxx>
> 
> Unconditionally interpreting vm_pgoff as a PFN is incorrect.
> 
> VMAs created by /dev/mem do this, but in general VM_PFNMAP just means
> that the VMA doesn't have an associated struct page and is being managed
> directly by something other than the core mmu.
> 
> Use follow_pfn like KVM does to find the PFN.
> 
> Signed-off-by: Nikita Leshenko <nikita.leshchenko@xxxxxxxxxx>
>  drivers/vfio/vfio_iommu_type1.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 2ada8e6cdb88..1e43581f95ea 100644
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -362,9 +362,9 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>  	vma = find_vma_intersection(mm, vaddr, vaddr + 1);
>  
>  	if (vma && vma->vm_flags & VM_PFNMAP) {
> -		*pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> -		if (is_invalid_reserved_pfn(*pfn))
> -			ret = 0;
> +		ret = follow_pfn(vma, vaddr, pfn);
> +		if (!ret && !is_invalid_reserved_pfn(*pfn))
> +			ret = -EOPNOTSUPP;
>  	}

FWIW this existing code is a huge hack and a security problem.

I'm not sure how you could be successfully using this path on actual
memory without hitting bad bugs?

Fudamentally VFIO can't retain a reference to a page from within a VMA
without some kind of recount/locking/etc to allow the thing that put
the page there to know it is still being used (ie programmed in a
IOMMU) by VFIO.

Otherwise it creates use-after-free style security problems on the
page.

This code needs to be deleted, not extended :(

Jason