Re: [PATCH] iommu/vt-d: Add a fix for devices need extra dtlb flush

[Baolu Lu]

On 11/23/22 7:32 PM, Robin Murphy wrote:
> On 2022-11-23 05:18, Tian, Kevin wrote:
> > > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > > Sent: Wednesday, November 23, 2022 1:04 PM
> > >
> > > On 2022/11/23 9:02, Tian, Kevin wrote:
> > > > > From: Robin Murphy <robin.murphy@xxxxxxx>
> > > > > Sent: Wednesday, November 23, 2022 1:49 AM
> > > > >
> > > > > > +
> > > > > > +/* Impacted QAT device IDs ranging from 0x4940 to 0x4943 */
> > > > > > +#define BUGGY_QAT_DEVID_MASK 0x494c
> > > > > > +static bool dev_needs_extra_dtlb_flush(struct pci_dev *pdev)
> > > > > > +{
> > > > > > +    if (pdev->vendor != PCI_VENDOR_ID_INTEL)
> > > > > > +        return false;
> > > > > > +
> > > > > > +    if ((pdev->device & 0xfffc) != BUGGY_QAT_DEVID_MASK)
> > > > > > +        return false;
> > > > > > +
> > > > > > +    if (risky_device(pdev))
> > > > > > +        return false;
> > > > >
> > > > > Hmm, I'm not sure that that makes much sense to me - what privilege 
> > > > > can
> > > > > the device gain from being told to invalidate things twice? Why 
> > > > > would we
> > > > > want to implicitly *allow* a device to potentially keep using a stale
> > > > > translation if for some bizarre reason firmware has marked it as
> > > > > external, surely that's worse?
> > >
> > >   From the perspective of IOMMU, any quirk is only applicable to trusted
> > > devices. If the IOMMU driver detects that a quirk is being applied to an
> > > untrusted device, it is already buggy or malicious. The IOMMU driver
> > > should let the users know by:
> > >
> > >     pci_info(pdev,
> > >          "Skipping IOMMU quirk for dev [%04X:%04X] on untrusted
> > > PCI link\n",
> > >          pdev->vendor, pdev->device);
> > >     pci_info(pdev, "Please check with your BIOS/Platform vendor about
> > > this\n");
> > >
> > > and stop applying any quirk.
> >
> > A quirk usually relaxes something then you want it only on trusted 
> > devices.
> >
> > but the quirk in this patch is trying to fix a vulnerability. In 
> > concept it's
> > weird to skip it on untrusted devices. This iiuc was the part causing 
> > confusion
> > to Robin.
>
> Right, it's that reasoning in general that seems bogus to me. Clearly 
> any quirk that effectively grants additional privileges, like an 
> identity mapping quirk, should not be applied to untrusted external 
> devices which may be spoofing an affected VID/DID to gain that 
> privilege, but not all quirks imply privilege. If, say, a WiFI 
> controller needs something innocuous like a DMA alias or address width 
> quirk to function correctly, it will still need that regardless of 
> whether it's soldered to a motherboard or to a removable expansion card, 
> and it would do nobody any good to deny correct functionality based on 
> that unnecessary distinction. Yes, I appreciate that in practice many of 
> those kind of quirks will be applied in other layers anyway, but I still 
> think it's wrong to make a sweeping assumption that all IOMMU-level 
> quirks are precious treasure not to be shared with outsiders, rather 
> than assess their impact individually. The detriment in this case is 
> small (just needless code churn), but even that's still not nothing.

Fair enough. I agreed here.

Can we put some comments here so that people can still easily read the
discussion here after a long time?

Best regards,
baolu