Re: [PATCH v4 04/13] PCI: portdrv: Suppress kernel DMA ownership auto-claiming

[Lu Baolu]

Hi Bjorn,

On 12/30/21 5:16 AM, Bjorn Helgaas wrote:
> On Fri, Dec 17, 2021 at 02:36:59PM +0800, Lu Baolu wrote:
> > IOMMU grouping on PCI necessitates that if we lack isolation on a bridge
> > then all of the downstream devices will be part of the same IOMMU group
> > as the bridge. The existing vfio framework allows the portdrv driver to
> > be bound to the bridge while its downstream devices are assigned to user
> > space. The pci_dma_configure() marks the iommu_group as containing only
> > devices with kernel drivers that manage DMA. Avoid this default behavior
> > for the portdrv driver in order for compatibility with the current vfio
> > policy.
>
> A word about the isolation would be useful.  I think you're referring
> to some specific ACS controls, probably P2P Request Redirect?
>
> I guess this is just a wording issue, but I think it's actually the
> *lack* of some ACS controls that forces us to put several devices in
> the same IOMMU group, isn't it?  It's not that we start with "IOMMU
> grouping" and that necessitates something else.
>
> Maybe something like this?
>
>    If a switch lacks ACS P2P Request Redirect (and possibly other
>    controls?), a device below the switch can bypass the IOMMU and DMA
>    directly to other devices below the switch, so all the downstream
>    devices must be in the same IOMMU group as the switch itself.

Yes. That's what it means from the perspective of PCI/PCIe. I will use
this in the next version. Thanks!

> > The commit 5f096b14d421b ("vfio: Whitelist PCI bridges") extended above
> > policy to all kernel drivers of bridge class. This is not always safe.
> > For example, The shpchp_core driver relies on the PCI MMIO access for the
> > controller functionality. With its downstream devices assigned to the
> > userspace, the MMIO might be changed through user initiated P2P accesses
> > without any notification. This might break the kernel driver integrity
> > and lead to some unpredictable consequences.
> >
> > For any bridge driver, in order to avoiding default kernel DMA ownership
> > claiming, we should consider:
> >
> >   1) Does the bridge driver use DMA? Calling pci_set_master() or
> >      a dma_map_* API is a sure indicate the driver is doing DMA
> >
> >   2) If the bridge driver uses MMIO, is it tolerant to hostile
> >      userspace also touching the same MMIO registers via P2P DMA
> >      attacks?
> >
> > Conservatively if the driver maps an MMIO region at all, we can say that
> > it fails the test.
>
> I'm not sure what all this explanation is telling me.  It says
> something done by 5f096b14d421 is not always safe, but this patch
> doesn't fix any of those unsafe things.
>
> If it doesn't explain why we need this patch or how this patch works,
> I don't think we need it in the commit log.
>
> Maybe this is an explanation for why you didn't set
> .suppress_auto_claim_dma_owner for shpc_driver?

You are right. This doesn't explain why this is needed and how it works.
It only explains why we don't do the same thing to other pci port
drivers. I will move this out of the commit message. Perhaps put it
in the cover letter or some patches for vifo.

> Minor typos above:
>    s/in order to avoiding default/before avoiding default/
>    s/relies on the PCI MMIO access/relies on PCI MMIO access/
>    s/For example, The/For example, the/
>    s/is a sure indicate the/is a sure indication the/

Thank you! I will correct these.

> > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Suggested-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > ---
> >   drivers/pci/pcie/portdrv_pci.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/pci/pcie/portdrv_pci.c b/drivers/pci/pcie/portdrv_pci.c
> > index 35eca6277a96..c48a8734f9c4 100644
> > --- a/drivers/pci/pcie/portdrv_pci.c
> > +++ b/drivers/pci/pcie/portdrv_pci.c
> > @@ -202,7 +202,10 @@ static struct pci_driver pcie_portdriver = {
> >   
> >   	.err_handler	= &pcie_portdrv_err_handler,
> >   
> > -	.driver.pm	= PCIE_PORTDRV_PM_OPS,
> > +	.driver		= {
> > +		.pm = PCIE_PORTDRV_PM_OPS,
> > +		.suppress_auto_claim_dma_owner = true,
> > +	},
> >   };
> >   
> >   static int __init dmi_pcie_pme_disable_msi(const struct dmi_system_id *d)
> > --
> > 2.25.1

Best regards,
baolu