On Fri, 13 May 2016 06:50:25 +0000 "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx] > > Sent: Friday, May 13, 2016 1:33 PM > > > > > > > > As argued previously in this thread, there's nothing special about a > > > > DMA write to memory versus a DMA write to a special address that > > > > triggers an MSI vector. If the device is DMA capable, which we assume > > > > all are, it can be fooled into generating those DMA writes regardless > > > > of whether we actively block access to the MSI-X vector table itself. > > > > > > But with DMA remapping above can be blocked. > > > > How? VT-d explicitly ignores DMA writes to 0xFEEx_xxxx, section 3.13: > > > > Write requests without PASID of DWORD length are treated as interrupt > > requests. Interrupt requests are not subjected to DMA remapping[...] > > Instead, remapping hardware can be enabled to subject such interrupt > > requests to interrupt remapping. > > Thanks for catching this! > > > > > > > MSI-X vector table access w/o interrupt remapping is to avoid obvious > > > > collisions if it were to be programmed directly, it doesn't actually > > > > prevent an identical DMA transaction from being generated by other > > > > > > Kernel can enable DMA remapping but disable IRQ remapping. In such > > > case identical DMA transaction can be prevented. > > > > Not according to the VT-d spec as quoted above. If so, how? > > So my argument on this is wrong. sorry. > > > > > > Anyway my point is simple. Let's ignore how Linux kernel implements > > > IRQ remapping on x86 (which may change time to time), and just > > > focus on architectural possibility. Non-x86 platform may implement > > > IRQ remapping completely separate from device side, then checking > > > availability of IRQ remapping is enough to decide whether mmap > > > MSI-X table is safe. x86 with VT-d can be configured to a mode > > > requiring host control of both MSI-X entry and IRQ remapping hardware > > > (without source id check). In such case it's insufficient to make > > > decision simply based on IRQ remapping availability. We need a way > > > to query from IRQ remapping module whether it's actually safe to > > > mmap MSI-X. > > > > We're going in circles here. This patch is attempting to remove > > protection from the MSI-X vector table that is really nothing more than > > security theater already. That "protection" only actually prevents > > casual misuse of the API which is really only a problem when the > > platform offers no form of interrupt isolation, such as VT-d with DMA > > remapping but not interrupt remapping. Disabling source-id checking in > > VT-d should be handled as the equivalent of disabling interrupt > > remapping altogether as far as the IOMMU API is concerned. That's > > a trivial gap that should be fixed. There is no such thing as a secure > > That is the main change I'm asking against original patch, which has: > > +static void pci_check_msi_remapping(struct pci_dev *pdev, > + const struct iommu_ops *ops) > +{ > + struct pci_bus *bus = pdev->bus; > + > + if (ops->capable(IOMMU_CAP_INTR_REMAP) && > + !(bus->bus_flags & PCI_BUS_FLAGS_MSI_REMAP)) > + bus->bus_flags |= PCI_BUS_FLAGS_MSI_REMAP; > +} > + > > Above flag should be cleared when source-id checking is disabled on x86. > Yes, VFIO is part of OS but any assumption we made about other parts > needs to be reflected accurately in the code. I would say this is an independent bug which should be fixed simply as: diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c index e1852e8..60d55c0 100644 --- a/drivers/iommu/intel-iommu.c +++ b/drivers/iommu/intel-iommu.c @@ -4948,7 +4948,7 @@ static bool intel_iommu_capable(enum iommu_cap cap) if (cap == IOMMU_CAP_CACHE_COHERENCY) return domain_update_iommu_snooping(NULL) == 1; if (cap == IOMMU_CAP_INTR_REMAP) - return irq_remapping_enabled == 1; + return irq_remapping_enabled == 1 && !disable_sourceid_checking; return false; } I believe the intent of the IOMMU_CAP_INTR_REMAP flag is simply to indicate interrupt isolation is provided through the IOMMU. Nobody cares about the interrupt remapping support beyond that. If source-id checking is disabled, the remainder of interrupt remapping is irrelevant as far as this capability is concerned imho. Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
