Re: [PATCH RFC v2 00/13] IOMMUFD Generic interface

On Tue, Oct 11, 2022 at 04:30:58PM -0400, Steven Sistare wrote: > On 10/11/2022 8:30 AM, Jason Gunthorpe wrote: >> On Mon, Oct 10, 2022 at 04:54:50PM -0400, Steven Sistare wrote: >>>> Do we have a solution to this? >>>> >>>> If not I would like to make a patch removing VFIO_DMA_UNMAP_FLAG_VADDR >>>> >>>> Aside from the approach to use the FD, another idea is to just use >>>> fork. >>>> >>>> qemu would do something like >>>> >>>> .. stop all container ioctl activity .. >>>> fork() >>>> ioctl(CHANGE_MM) // switch all maps to this mm >>>> .. signal parent.. >>>> .. wait parent.. >>>> exit(0) >>>> .. wait child .. >>>> exec() >>>> ioctl(CHANGE_MM) // switch all maps to this mm >>>> ..signal child.. >>>> waitpid(childpid) >>>> >>>> This way the kernel is never left without a page provider for the >>>> maps, the dummy mm_struct belonging to the fork will serve that role >>>> for the gap. >>>> >>>> And the above is only required if we have mdevs, so we could imagine >>>> userspace optimizing it away for, eg vfio-pci only cases. >>>> >>>> It is not as efficient as using a FD backing, but this is super easy >>>> to implement in the kernel. >>> >>> I propose to avoid deadlock for mediated devices as follows. Currently, an >>> mdev calling vfio_pin_pages blocks in vfio_wait while VFIO_DMA_UNMAP_FLAG_VADDR >>> is asserted. >>> >>> * In vfio_wait, I will maintain a list of waiters, each list element >>> consisting of (task, mdev, close_flag=false). >>> >>> * When the vfio device descriptor is closed, vfio_device_fops_release >>> will notify the vfio_iommu driver, which will find the mdev on the waiters >>> list, set elem->close_flag=true, and call wake_up_process for the task. >> >> This alone is not sufficient, the mdev driver can continue to >> establish new mappings until it's close_device function >> returns. Killing only existing mappings is racy. >> >> I think you are focusing on the one issue I pointed at, as I said, I'm >> sure there are more ways than just close to abuse this functionality >> to deadlock the kernel. >> >> I continue to prefer we remove it completely and do something more >> robust. I suggested two options. > > It's not racy. New pin requests also land in vfio_wait if any vaddr's have > been invalidated in any vfio_dma in the iommu. See > vfio_iommu_type1_pin_pages() > if (iommu->vaddr_invalid_count) > vfio_find_dma_valid() > vfio_wait() I mean you can't do a one shot wakeup of only existing waiters, and you can't corrupt the container to wake up waiters for other devices, so I don't see how this can be made to work safely... It also doesn't solve any flow that doesn't trigger file close, like a process thread being stuck on the wait in the kernel. eg because a trapped mmio triggered an access or something. So it doesn't seem like a workable direction to me. > However, I will investigate saving a reference to the file object in > the vfio_dma (for mappings backed by a file) and using that to > translate IOVA's. It is certainly the best flow, but it may be difficult. Eg the memfd work for KVM to do something similar is quite involved. > I think that will be easier to use than fork/CHANGE_MM/exec, and may > even be easier to use than VFIO_DMA_UNMAP_FLAG_VADDR. To be > continued. Yes, certainly easier to use, I suggested CHANGE_MM because the kernel implementation is very easy, I could send you something to test w/ iommufd in a few hours effort probably. Anyhow, I think this conversation has convinced me there is no way to fix VFIO_DMA_UNMAP_FLAG_VADDR. I'll send a patch reverting it due to it being a security bug, basically.