> 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. > MSI-X vector table when untrusted userspace drivers are involved, we > must always assume that a device can generate DMA writes that are > indistinguishable from actual interrupt requests and if the platform > does not provide interrupt isolation we should require the user to > opt-in to an unsafe mode. > > Simply denying direct writes to the vector table or preventing mapping > of the vector table into the user address space does not provide any > tangible form of protection. Many devices make use of window registers > that allow backdoors to arbitrary device registers. Some drivers even > use this as the primary means for configuring MSI-X, which makes them > incompatible with device assignment without device specific quirks to > enable virtualization of these paths. > > If you have an objection to this patch, please show me how preventing > direct CPU access to the MSI-X vector table provides any kind of > security guarantee of the contents of the vector table and also prove > to me that a device cannot spoof a DMA write that is indistinguishable > from one associated with an actual interrupt, regardless of the > contents of the MSI-X vector table. Thanks, > I'm not object to the whole patch series. As replied above, my point is just that current condition of allowing mmap MSI-X in this patch is not accurate, but my argument on security manner is not correct. Thanks for your elaboration to make it clear. Thanks Kevin -- 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
