Re: [Qemu-devel] [PATCH] vfio/pci-quirks: Only quirk to size of PCI config space

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 2015-11-18 at 10:36 +0100, Laszlo Ersek wrote:
> On 11/18/15 00:41, Alex Williamson wrote:
> > For quirks that support the full PCIe extended config space, limit the
> > quirk to only the size of config space available through vfio.  This
> > allows host systems with broken MMCONFIG regions to still make use of
> > these quirks without generating bad address faults trying to access
> > beyond the end of config space exposed through vfio.  This may expose
> > direct access to parts of extended config space that we'd prefer not
> > to expose, but that's why we have an IOMMU.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > Reported-by: Ronnie Swanink <ronnie@ronnieswanink.nl>
> > ---
> >  hw/vfio/pci-quirks.c |    6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/vfio/pci-quirks.c b/hw/vfio/pci-quirks.c
> > index 30c68a1..e117c41 100644
> > --- a/hw/vfio/pci-quirks.c
> > +++ b/hw/vfio/pci-quirks.c
> > @@ -328,7 +328,7 @@ static void vfio_probe_ati_bar4_quirk(VFIOPCIDevice *vdev, int nr)
> >      window->data_offset = 4;
> >      window->nr_matches = 1;
> >      window->matches[0].match = 0x4000;
> > -    window->matches[0].mask = PCIE_CONFIG_SPACE_SIZE - 1;
> > +    window->matches[0].mask = vdev->config_size - 1;
> >      window->bar = nr;
> >      window->addr_mem = &quirk->mem[0];
> >      window->data_mem = &quirk->mem[1];
> > @@ -674,7 +674,7 @@ static void vfio_probe_nvidia_bar5_quirk(VFIOPCIDevice *vdev, int nr)
> >      window->matches[0].match = 0x1800;
> >      window->matches[0].mask = PCI_CONFIG_SPACE_SIZE - 1;
> >      window->matches[1].match = 0x88000;
> > -    window->matches[1].mask = PCIE_CONFIG_SPACE_SIZE - 1;
> > +    window->matches[1].mask = vdev->config_size - 1;
> >      window->bar = nr;
> >      window->addr_mem = bar5->addr_mem = &quirk->mem[0];
> >      window->data_mem = bar5->data_mem = &quirk->mem[1];
> > @@ -765,7 +765,7 @@ static void vfio_probe_nvidia_bar0_quirk(VFIOPCIDevice *vdev, int nr)
> >      memory_region_init_io(mirror->mem, OBJECT(vdev),
> >                            &vfio_nvidia_mirror_quirk, mirror,
> >                            "vfio-nvidia-bar0-88000-mirror-quirk",
> > -                          PCIE_CONFIG_SPACE_SIZE);
> > +                          vdev->config_size);
> >      memory_region_add_subregion_overlap(&vdev->bars[nr].region.mem,
> >                                          mirror->offset, mirror->mem, 1);
> >  
> > 
> > 
> 
> $ git log -- hw/vfio/pci-quirks.c | grep Reviewed-by
> <nothing>
> 
> Okay, I guess my review won't be deemed immediately unnecessary then. :)

Reviews are very much appreciated.

> (1) Please add to the commit message:
> 
> Link: https://www.redhat.com/archives/vfio-users/2015-November/msg00192.html
> 
> With that reference:
> 
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Done

> (2) Also, one question just to make sure I understand right: the last
> sentence of the commit message means that the left-inclusive,
> right-exclusive config space offset range
> 
>   [vdev->config_size, PCIE_CONFIG_SPACE_SIZE)
> 
> is now directly available to the guest, without QEMU noticing; but,
> we're not worried about that, because the guest can't abuse that freedom
> anyway for doing arbitrary DMA. Is that right?
> 
> If so, then I propose another update to the commit message (replacing
> the last sentence):
> 
>     This may grant the guest direct access to trailing parts of
>     extended config space that we'd prefer not to expose, but that's
>     why we have an IOMMU (the guest can't abuse said config space access
>     for arbitrary DMA).
> 
> Perhaps the above is trivial for you (assuming it is correct at all...);
> it may not be trivial for others.

How's this?:

commit b27c4f5da92a1732a77ddbe98571583a4eac1e14
Author: Alex Williamson <alex.williamson@redhat.com>
Date:   Tue Nov 17 16:37:38 2015 -0700

    vfio/pci-quirks: Only quirk to size of PCI config space
    
    For quirks that support the full PCIe extended config space, limit the
    quirk to only the size of config space available through vfio.  This
    allows host systems with broken MMCONFIG regions to still make use of
    these quirks without generating bad address faults trying to access
    beyond the end of config space exposed through vfio.  This may expose
    direct access to the mirror of extended config space, only trapping
    the sub-range of standard config space, but allowing this makes the
    quirk, and thus the device, functional.  We expect that only device
    specific accesses make use of the mirror, not general extended PCI
    capability accesses, so any virtualization in this space is likely
    unnecessary anyway, and the device is still IOMMU isolated, so it
    should only be able to hurt itself through any bogus configurations
    enabled by this space.
    
    Link: https://www.redhat.com/archives/vfio-users/2015-November/msg00192.html
    Reported-by: Ronnie Swanink <ronnie@ronnieswanink.nl>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>
    Signed-off-by: Alex Williamson <alex.williamson@redhat.com>