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Thu, 12 Nov 2026 12:25:56 +0000 Message-ID: <97053bb6-c221-4ae0-972c-603b194ec25a@amd.com> Date: Thu, 12 Nov 2026 23:25:46 +1100 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 06/15] iommufd/device: Hold a KVM file reference for the device lifetime To: "Tian, Kevin" , "Aneesh Kumar K.V (Arm)" , "iommu@lists.linux.dev" Cc: Alex Williamson , Bjorn Helgaas , Catalin Marinas , Jacob Pan , Jason Gunthorpe , Joerg Roedel , Jonathan Cameron , Jonathan Hunter , Krishna Reddy , Lukas Wunner , Nicolin Chen , Robin Murphy , Samuel Ortiz , Shameer Kolothum , Steven Price , Suravee Suthikulpanit , Suzuki K Poulose , Thierry Reding , Vasant Hegde , Will Deacon , Xu Yilun , "kvm@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "linux-coco@lists.linux.dev" , "linux-kernel@vger.kernel.org" , "linux-pci@vger.kernel.org" , "linux-tegra@vger.kernel.org" References: <20261104161632.3547423-1-aneesh.kumar@kernel.org> <20261104161632.3547423-7-aneesh.kumar@kernel.org> From: Alexey Kardashevskiy Content-Language: en-US In-Reply-To: Content-Type: text/plain; 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Normally, unbind waits >> for outstanding device references before VFIO drops its KVM reference. >> However, REMOVE_WAIT only waits for 60 seconds. If that timeout expires, >> the device object remains in the iommufd xarray. >> >> For example, an IOMMU_GET_HW_INFO ioctl can hold a device reference >> while >> blocked on a userspace buffer fault: >> >> iommufd_fops_ioctl() >> iommufd_get_hw_info() >> iommufd_get_device() >> iommufd_get_object() >> iommufd_lock_obj() /* takes users and wait_cnt refs */ >> copy_to_user() / clear_user() /* may block before putting refs */ >> >> A concurrent final release of the VFIO device file takes this path: >> >> vfio_device_fops_release() >> vfio_df_unbind_iommufd() >> vfio_df_close() >> vfio_df_device_last_close() >> vfio_df_iommufd_unbind() >> vfio_iommufd_physical_unbind() >> iommufd_device_unbind() >> iommufd_object_destroy_user() >> iommufd_object_remove(..., REMOVE_WAIT) >> iommufd_object_dec_wait() /* timeout: -EBUSY */ >> WARN_ON(ret) >> vfio_device_put_kvm() >> fput(device->kvm) >> >> iommufd_object_destroy_user() warns on -EBUSY but does not propagate it: >> iommufd_device_unbind() returns void. Thus VFIO close effectively ignores >> the unbind failure and proceeds to drop its KVM file reference even >> though the iommufd_device is still alive and its ID remains usable. >> >> Once the remaining KVM file references are released, a later ioctl on >> the still-open iommufd can access the stale pointer: >> >> iommufd_fops_ioctl() >> iommufd_viommu_alloc_ioctl() >> iommufd_get_device() /* finds the surviving device ID */ >> get_file(idev->kvm_file) /* accesses the freed KVM file */ >> >> Take a KVM file reference when binding the device and release it when >> the iommufd_device is actually destroyed. Release it before the igroup >> check in the destructor so that bind failure cleanup is covered too. >> Document the ownership in the bind API and device structure. > > I don't think this needs to be fixed. There is already a WARN_ON() > in iommufd_object_destroy_user() and many bad things could > happen afterwards... imho it is just a right thing to do - hold a reference to the pointer in a structure. > If we really think an ioctl could take more than 60s in sane scenarios, It is quite possible when some TSM operation involves (in)validation of hundreds GBs of MMIO. > it's a more complex issue to be handled in iommufd core. > >> >> Fixes: d8bb80c8f0e5 ("iommufd/device: Associate KVM file pointer with >> iommufd_device") > > btw no fix tag for an issue introduced by an earlier patch in the > same series. +1. should be just squashed into 05/15. Thanks, -- Alexey