On 06/06/2019 21:29, Jacob Pan wrote: >>>>>> iommu_unregister_device_fault_handler(&vdev->pdev->dev); >>>>> >>>>> >>>>> But this can fail if there are pending faults which leaves a >>>>> device reference and then the system is broken :( >>>> This series only features unrecoverable errors and for those the >>>> unregistration cannot fail. Now unrecoverable errors were added I >>>> admit this is confusing. We need to sort this out or clean the >>>> dependencies. >>> As Alex pointed out in 4/29, we can make >>> iommu_unregister_device_fault_handler() never fail and clean up all >>> the pending faults in the host IOMMU belong to that device. But the >>> problem is that if a fault, such as PRQ, has already been injected >>> into the guest, the page response may come back after handler is >>> unregistered and registered again. >> >> I'm trying to figure out if that would be harmful in any way. I guess >> it can be a bit nasty if we handle the page response right after >> having injected a new page request that uses the same PRGI. In any >> other case we discard the page response, but here we forward it to >> the endpoint and: >> >> * If the response status is success, endpoint retries the >> translation. The guest probably hasn't had time to handle the new >> page request and translation will fail, which may lead the endpoint >> to give up (two unsuccessful translation requests). Or send a new >> request >> > Good point, there shouldn't be any harm if the page response is a > "fake" success. In fact it could happen in the normal operation when > PRQs to two devices share the same non-leaf translation structure. The > worst case is just a retry. I am not aware of the retry limit, is it in > the PCIe spec? I cannot find it. I don't think so, it's the implementation's choice. In general I don't think devices will have a retry limit, but it doesn't seem like the PCI spec prevents them from implementing one either. It could be useful to stop retrying after a certain number of faults, for preventing livelocks when the OS doesn't fix up the page tables and the device would just repeat the fault indefinitely. > I think we should just document it, similar to having a spurious > interrupt. The PRQ trace event should capture that as well. > >> * otherwise the endpoint won't retry the access, and could also >> disable PRI if the status is failure. >> > That would be true regardless this race condition with handler > registration. So should be fine. We do give an invalid response for the old PRG (because of unregistering), but also for the new one, which has a different address that the guest might be able to page in and would normally return success. >>> We need a way to reject such page response belong >>> to the previous life of the handler. Perhaps a sync call to the >>> guest with your fault queue eventfd? I am not sure. >> >> We could simply expect the device driver not to send any page response >> after unregistering the fault handler. Is there any reason VFIO would >> need to unregister and re-register the fault handler on a live guest? >> > There is no reason for VFIO to unregister and register again, I was > just thinking from security perspective. Someone could write a VFIO app > do this attack. But I agree the damage is within the device, may get > PRI disabled as a result. Yes I think the damage would always be contained within the misbehaving software > So it seems we agree on the following: > - iommu_unregister_device_fault_handler() will never fail > - iommu driver cleans up all pending faults when handler is unregistered > - assume device driver or guest not sending more page response _after_ > handler is unregistered. > - system will tolerate rare spurious response > > Sounds right? Yes, I'll add that to the fault series Thanks, Jean
