On Wed, Nov 29, 2017 at 06:58:11PM +1100, Alexey Kardashevskiy wrote:
> On 29/11/17 16:17, David Gibson wrote:
> > On Wed, Nov 29, 2017 at 03:38:30PM +1100, Alexey Kardashevskiy wrote:
> >> On 29/11/17 14:57, David Gibson wrote:
> >>> On Wed, Nov 29, 2017 at 02:25:43PM +1100, Alexey Kardashevskiy wrote:
> >>>> On 22/11/17 17:51, David Gibson wrote:
> >>>>> On Tue, Nov 21, 2017 at 10:14:45PM -0700, Alex Williamson wrote:
> >>>>>> On Wed, 22 Nov 2017 15:44:55 +1100
> >>>>>> David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote:
> >>>>>>
> >>>>>>> On Tue, Nov 21, 2017 at 09:28:46PM -0700, Alex Williamson wrote:
> >>>>>>>> On Wed, 22 Nov 2017 15:09:32 +1100
> >>>>>>>> Alexey Kardashevskiy <aik@xxxxxxxxx> wrote:
> >>>>>>>>
> >>>>>>>>> By default VFIO disables mapping of MSIX BAR to the userspace as
> >>>>>>>>> the userspace may program it in a way allowing spurious interrupts;
> >>>>>>>>> instead the userspace uses the VFIO_DEVICE_SET_IRQS ioctl.
> >>>>>>>>>
> >>>>>>>>> This works fine as long as the system page size equals to the MSIX
> >>>>>>>>> alignment requirement which is 4KB. However with a bigger page size
> >>>>>>>>> the existing code prohibits mapping non-MSIX parts of a page with MSIX
> >>>>>>>>> structures so these parts have to be emulated via slow reads/writes on
> >>>>>>>>> a VFIO device fd. If these emulated bits are accessed often, this has
> >>>>>>>>> serious impact on performance.
> >>>>>>>>>
> >>>>>>>>> This adds an ioctl to the vfio-pci device which hides the sparse
> >>>>>>>>> capability and allows the userspace to map a BAR with MSIX structures.
> >>>>>>>>
> >>>>>>>> So the user is in control of telling the kernel whether they're allowed
> >>>>>>>> to mmap the msi-x vector table. That makes absolutely no sense. If
> >>>>>>>> you're trying to figure out how userspace knows whether to implicitly
> >>>>>>>> avoid mmap'ing the msix region, I think there are far better ways in
> >>>>>>>> the existing region info ioctl. We could use a flag, or maybe the
> >>>>>>>> existence of a capability chain pointer, or a new capability. But
> >>>>>>>> absolutely not this. The kernel needs to decide whether it's going to
> >>>>>>>> let the user do this, not the user. Thanks,
> >>>>>>>
> >>>>>>> No, it doesn't. This is actually the approach we discussed in Prague.
> >>>>>>>
> >>>>>>> Remember that intercepting access to the MSI-X table is not a host
> >>>>>>> safety / security issue. It's just that without that we won't wire up
> >>>>>>> the device's MSI-X vectors properly so they won't work.
> >>>>>>>
> >>>>>>> Basically the decision here is between
> >>>>>>>
> >>>>>>> A) Allow MSI-X configuration via standard PCI mechanisms, at the
> >>>>>>> cost of making access slow for any registers sharing a page with
> >>>>>>> the MSI-X table.
> >>>>>>>
> >>>>>>> or
> >>>>>>>
> >>>>>>> B) Make access to BAR registers sharing a page with the MSI-X table
> >>>>>>> fast, at the cost of requiring some alternative mechanism to
> >>>>>>> configure MSI-X vectors.
> >>>>>>>
> >>>>>>> And that is a tradeoff that it is reasonable for userspace to make.
> >>>>>>>
> >>>>>>> In the case of KVM guests, the decision depends entirely on the
> >>>>>>> *guest* platform. Usually we need (A) because the guest expects to be
> >>>>>>> able to poke the MSI-X table in the usual way. However for PAPR
> >>>>>>> guests, there's an alternative mechanism via an RTAS call, which means
> >>>>>>> we can use (B).
> >>>>>>>
> >>>>>>> The host kernel can't make this decision, because it doesn't know the
> >>>>>>> guest platform (well, KVM might, but VFIO doesn't).
> >>>>>>>
> >>>>>>> A userspace VFIO program could also elect for (B) if it does care
> >>>>>>> about performance of access to registers in the same BAR as the MSI-X
> >>>>>>> table, but doesn't need MSI-X for example.
> >>>>>>
> >>>>>> You're asking for an ioctl to allow the kernel to allow the user to
> >>>>>> mmap the page, when instead we could just allow the user to mmap the
> >>>>>> page and whether the user does that and how they make use of it is up
> >>>>>> to them...
> >>>>>
> >>>>> Duh. Sorry. For some reason I was thinking the magic MSI-X
> >>>>> interception was happening in the host kernel rather than in qemu.
> >>>>>
> >>>>>> I understand that there are different virtualization techniques at play
> >>>>>> here, it just doesn't seem relevant. In the case of (A), the user can
> >>>>>> choose not to mmap the page overlapping the vector table even if the
> >>>>>> kernel allows it. The user can also choose to mmap that page, but not
> >>>>>> use the portion overlapping the vector table. QEMU already does this
> >>>>>> by overlaying a MemoryRegion for vector table emulation. We might even
> >>>>>> be able to get away with mmaping that page and emulating the vector
> >>>>>> table elsewhere, which seems like the only option for a 64k page ARM
> >>>>>> system. For (B), clearly it's just a nuisance that we can't currently
> >>>>>> mmap this page, but I still don't see how the user allowing the kernel
> >>>>>> to allow the user to mmap that page makes any sense. I can't even
> >>>>>> describe it without it sounding ridiculous. Thanks,
> >>>>>
> >>>>> Right. Rethinking.. it seems to me we should just completely remove
> >>>>> the logic from the kernel banning mmap()s overlapping the MSI-X
> >>>>> table. All it does is poorly attempt to stop the user shooting
> >>>>> themselves in the foot.
> >>>>>
> >>>>> Then we just need logic in qemu to avoid doing the overlapping memory
> >>>>> region nonsense on a per-machine basis
> >>>>
> >>>>
> >>>> So is there still any plan or we just ditch the feature? I am confused now.
> >>>
> >>> The plan is what I said above. Remove the bogus check logic from the
> >>> kernel, then solve within qemu, by not creating the MSI-X intercept
> >>> region for pseries guests.
> >>
> >>
> >> There were 2 proposals how to do that. Both included platform code to
> >> decide whether to allow mapping or not and some transport to pass that
> >> enablement flag from the plafform code to the VFIO-PCI driver, one was via
> >> an IOMMU group flag, the other via a PCI bus flag. Neither was accepted so
> >> reposting those won't make any progress, what do I miss here? Was there any
> >> agreement on how to do this?
> >
> > As we've discussed the filtering of mmap()able regions in the kernel
> > is not a security concern. All it accomplishes is poorly trying to
> > stop userspace from shooting itself in the foot by directly mapping
> > and accessing the MSI-X table instead of using the VFIO interfaces to
> > set up MSI vectors.
> >
> > We should simply remove - unconditionally - the checks in the kernel
> > which prevent parts of the BARs from being mapped.
>
>
> Just like this? And Alex is ok with that?
Yes, just like this. I was kind of hoping Alex would express an
opinion when I mentioned it above. But at any rate I will advocate
for this approach with him.
At least until someone points out another error in my thinking and I
try again.
>
>
> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index 9ed1ecf..106b825 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -1146,22 +1146,6 @@ static int vfio_pci_mmap(void *device_data, struct
> vm_area_struct *vma)
> if (req_start + req_len > phys_len)
> return -EINVAL;
>
> - if (!vdev->msix_mmap && index == vdev->msix_bar) {
> - /*
> - * Disallow mmaps overlapping the MSI-X table; users don't
> - * get to touch this directly. We could find somewhere
> - * else to map the overlap, but page granularity is only
> - * a recommendation, not a requirement, so the user needs
> - * to know which bits are real. Requiring them to mmap
> - * around the table makes that clear.
> - */
> -
> - /* If neither entirely above nor below, then it overlaps */
> - if (!(req_start >= vdev->msix_offset + vdev->msix_size ||
> - req_start + req_len <= vdev->msix_offset))
> - return -EINVAL;
> - }
>
>
>
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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