Re: [PATCH v2] vfio/type1: check pfn valid before converting to struct page

[Yan Zhao]

On Mon, May 22, 2023 at 01:00:30PM -0600, Alex Williamson wrote:
> On Fri, 19 May 2023 14:58:43 +0800
> Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
> 
> > Check physical PFN is valid before converting the PFN to a struct page
> > pointer to be returned to caller of vfio_pin_pages().
> > 
> > vfio_pin_pages() pins user pages with contiguous IOVA.
> > If the IOVA of a user page to be pinned belongs to vma of vm_flags
> > VM_PFNMAP, pin_user_pages_remote() will return -EFAULT without returning
> > struct page address for this PFN. This is because usually this kind of PFN
> > (e.g. MMIO PFN) has no valid struct page address associated.
> > Upon this error, vaddr_get_pfns() will obtain the physical PFN directly.
> > 
> > While previously vfio_pin_pages() returns to caller PFN arrays directly,
> > after commit
> > 34a255e67615 ("vfio: Replace phys_pfn with pages for vfio_pin_pages()"),
> > PFNs will be converted to "struct page *" unconditionally and therefore
> > the returned "struct page *" array may contain invalid struct page
> > addresses.
> > 
> > Given current in-tree users of vfio_pin_pages() only expect "struct page *
> > returned, check PFN validity and return -EINVAL to let the caller be
> > aware of IOVAs to be pinned containing PFN not able to be returned in
> > "struct page *" array. So that, the caller will not consume the returned
> > pointer (e.g. test PageReserved()) and avoid error like "supervisor read
> > access in kernel mode".
> > 
> > Fixes: 34a255e67615 ("vfio: Replace phys_pfn with pages for vfio_pin_pages()")
> > Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > 
> > ---
> > v2: update commit message to explain background/problem clearly. (Sean)
> > ---
> >  drivers/vfio/vfio_iommu_type1.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index 493c31de0edb..0620dbe5cca0 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -860,6 +860,11 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
> >  		if (ret)
> >  			goto pin_unwind;
> >  
> > +		if (!pfn_valid(phys_pfn)) {
> 
> Why wouldn't we use our is_invalid_reserved_pfn() test here?  Doing
> so would also make it more consistent why we don't need to call
> put_pfn() or rewind accounting for this page.  Thanks,
> 
I actually struggled in choosing is_invalid_reserved_pfn() or
pfn_valid() when writing this patch.

Choosing pfn_valid() is because invalid PFN obviously cannot have
struct page address and it's a bug fix.

While declining reserved pages will have the IOVA range supported by
vfio_pin_pages() even more reduced. So I don't know if there's enough
justification to do so, given that (1) device zone memory usually has
PG_reserved set. (2) vm_normal_page() also contains reserved page.

Thanks
Yan

> 
> > +			ret = -EINVAL;
> > +			goto pin_unwind;
> > +		}
> > +
> >  		ret = vfio_add_to_pfn_list(dma, iova, phys_pfn);
> >  		if (ret) {
> >  			if (put_pfn(phys_pfn, dma->prot) && do_accounting)
> > 
> > base-commit: b3c98052d46948a8d65d2778c7f306ff38366aac
>