Re: [PATCH v2 0/3] vfio-pci: Block user access to disabled device MMIO

Peter Xu
2020-05-08T02:32:04Z

On Thu, May 07, 2020 at 04:34:37PM -0600, Alex Williamson wrote:
> On Thu, 7 May 2020 17:59:08 -0400
> Peter Xu <peterx@redhat.com> wrote:
>
> > On Tue, May 05, 2020 at 03:54:36PM -0600, Alex Williamson wrote:
> > > v2:
> > >
> > > Locking in 3/ is substantially changed to avoid the retry scenario
> > > within the fault handler, therefore a caller who does not allow retry
> > > will no longer receive a SIGBUS on contention. IOMMU invalidations
> > > are still not included here, I expect that will be a future follow-on
> > > change as we're not fundamentally changing that issue in this series.
> > > The 'add to vma list only on fault' behavior is also still included
> > > here, per the discussion I think it's still a valid approach and has
> > > some advantages, particularly in a VM scenario where we potentially
> > > defer the mapping until the MMIO BAR is actually DMA mapped into the
> > > VM address space (or the guest driver actually accesses the device
> > > if that DMA mapping is eliminated at some point). Further discussion
> > > and review appreciated. Thanks,
> >
> > Hi, Alex,
> >
> > I have a general question on the series.
> >
> > IIUC this series tries to protect illegal vfio userspace writes to device MMIO
> > regions which may cause platform-level issues. That makes perfect sense to me.
> > However what if the write comes from the devices' side? E.g.:
> >
> > - Device A maps MMIO region X
> >
> > - Device B do VFIO_IOMMU_DMA_MAP on Device A's MMIO region X
> > (so X's MMIO PFNs are mapped in device B's IOMMU page table)
> >
> > - Device A clears PCI_COMMAND_MEMORY (reset, etc.)
> > - this should zap all existing vmas that mapping region X, however device
> > B's IOMMU page table is not aware of this?
> >
> > - Device B writes to MMIO region X of device A even if PCI_COMMAND_MEMORY
> > cleared on device A's PCI_COMMAND register
> >
> > Could this happen?
>
> Yes, this can happen and Jason has brought up variations on this
> scenario that are important to fix as well. I've got some ideas, but
> the access in this series was the current priority. There are also
> issues in the above scenario that if a platform considers a DMA write
> to an invalid IOMMU PTE and triggering an IOMMU fault to have the same
> severity as the write to disabled MMIO space we've prevented, then our
> hands are tied. Thanks,

I see the point now; it makes sense to start with a series like this. Thanks, Alex.

--
Peter Xu