Re: [PATCH v2 1/3] vfio/type1: Support faulting PFNMAP vmas

Peter Xu
2020-05-08T14:30:55Z

On Fri, May 08, 2020 at 09:10:13AM -0300, Jason Gunthorpe wrote:
> On Thu, May 07, 2020 at 10:19:39PM -0400, Peter Xu wrote:
> > On Thu, May 07, 2020 at 08:54:21PM -0300, Jason Gunthorpe wrote:
> > > On Thu, May 07, 2020 at 05:24:43PM -0400, Peter Xu wrote:
> > > > On Tue, May 05, 2020 at 03:54:44PM -0600, Alex Williamson wrote:
> > > > > With conversion to follow_pfn(), DMA mapping a PFNMAP range depends on
> > > > > the range being faulted into the vma. Add support to manually provide
> > > > > that, in the same way as done on KVM with hva_to_pfn_remapped().
> > > > >
> > > > > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > > > > drivers/vfio/vfio_iommu_type1.c | 36 +++++++++++++++++++++++++++++++++---
> > > > > 1 file changed, 33 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > > > > index cc1d64765ce7..4a4cb7cd86b2 100644
> > > > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > > > @@ -317,6 +317,32 @@ static int put_pfn(unsigned long pfn, int prot)
> > > > > return 0;
> > > > > }
> > > > >
> > > > > +static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> > > > > + unsigned long vaddr, unsigned long *pfn,
> > > > > + bool write_fault)
> > > > > +{
> > > > > + int ret;
> > > > > +
> > > > > + ret = follow_pfn(vma, vaddr, pfn);
> > > > > + if (ret) {
> > > > > + bool unlocked = false;
> > > > > +
> > > > > + ret = fixup_user_fault(NULL, mm, vaddr,
> > > > > + FAULT_FLAG_REMOTE |
> > > > > + (write_fault ? FAULT_FLAG_WRITE : 0),
> > > > > + &unlocked);
> > > > > + if (unlocked)
> > > > > + return -EAGAIN;
> > > >
> > > > Hi, Alex,
> > > >
> > > > IIUC this retry is not needed too because fixup_user_fault() will guarantee the
> > > > fault-in is done correctly with the valid PTE as long as ret==0, even if
> > > > unlocked==true.
> > >
> > > It is true, and today it is fine, but be careful when reworking this
> > > to use notifiers as unlocked also means things like the vma pointer
> > > are invalidated.
> >
> > Oh right, thanks for noticing that. Then we should probably still keep the
> > retry logic... because otherwise the latter follow_pfn() could be referencing
> > an invalid vma already...
>
> I looked briefly and thought this flow used the vma only once?

ret = follow_pfn(vma, vaddr, pfn);
if (ret) {
bool unlocked = false;

ret = fixup_user_fault(NULL, mm, vaddr,
FAULT_FLAG_REMOTE |
(write_fault ? FAULT_FLAG_WRITE : 0),
&unlocked);
if (unlocked)
return -EAGAIN;

if (ret)
return ret;

ret = follow_pfn(vma, vaddr, pfn); <--------------- [1]
}

So imo the 2nd follow_pfn() [1] could be racy if without the unlocked check.

Thanks,

--
Peter Xu