Messages in this thread | | | Date | Mon, 5 Sep 2022 10:46:44 +0100 | Subject | Re: [PATCH v4 1/2] iommu/s390: Fix race with release_device ops | From | Robin Murphy <> |
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On 2022-09-02 18:21, Jason Gunthorpe wrote: > On Fri, Sep 02, 2022 at 01:11:09PM -0400, Matthew Rosato wrote: >> On 9/1/22 4:37 PM, Jason Gunthorpe wrote: >>> On Thu, Sep 01, 2022 at 12:14:24PM -0400, Matthew Rosato wrote: >>>> On 9/1/22 6:25 AM, Robin Murphy wrote: >>>>> On 2022-08-31 21:12, Matthew Rosato wrote: >>>>>> With commit fa7e9ecc5e1c ("iommu/s390: Tolerate repeat attach_dev >>>>>> calls") s390-iommu is supposed to handle dynamic switching between IOMMU >>>>>> domains and the DMA API handling. However, this commit does not >>>>>> sufficiently handle the case where the device is released via a call >>>>>> to the release_device op as it may occur at the same time as an opposing >>>>>> attach_dev or detach_dev since the group mutex is not held over >>>>>> release_device. This was observed if the device is deconfigured during a >>>>>> small window during vfio-pci initialization and can result in WARNs and >>>>>> potential kernel panics. >>>>> >>>>> Hmm, the more I think about it, something doesn't sit right about this whole situation... release_device is called via the notifier from device_del() after the device has been removed from its parent bus and largely dismantled; it should definitely not still have a driver bound by that point, so how is VFIO doing things that manage to race at all? >>>>> >>>>> Robin. >>>> >>>> So, I generally have seen the issue manifest as one of the calls >>>> into the iommu core from __vfio_group_unset_container >>>> (e.g. iommu_deatch_group via vfio_type1_iommu) failing with a WARN. >>>> This happens when the vfio group fd is released, which could be >>>> coming e.g. from a userspace ioctl VFIO_GROUP_UNSET_CONTAINER. >>>> AFAICT there's nothing serializing the notion of calling into the >>>> iommu core here against a device that is simultaneously going >>>> through release_device (because we don't enter release_device with >>>> the group mutex held), resulting in unpredictable behavior between >>>> the dueling attach_dev/detach_dev and the release_device for >>>> s390-iommu at least. >>> >>> Oh, this is a vfio bug. >> >> I've been running with your diff applied today on s390 and this >> indeed fixes the issue by preventing the detach-after-release coming >> out of vfio. > > Heh, I'm shocked it worked at all > > I've been trying to understand Robin's latest remarks because maybe I > don't really understand your situation right.
That was really just me thinking out loud to guess at how it must be happening - I wasn't sure whether VFIO is actually intended to allow that or not, so if not then by all means let's look at fixing that, but as I say I think we're only seeing it provoke a problem at the driver level because of 9ac8545199a1, and fixing VFIO doesn't fix that in general. And conversely if we *can* fix that properly at the IOMMU API level then the current VFIO behaviour should become benign again anyway.
> IMHO this is definately a VFIO bug, because in a single-device group > we must not allow the domain to remain attached past remove(). Or more > broadly we shouldn't be holding ownership of a group without also > having a driver attached.
FWIW I was assuming it might be fine for a VFIO user to hold the group open if they expect the device to come back again and re-bind (for example, perhaps over some reconfiguration that requires turning SR-IOV off and on again?)
Cheers, Robin.
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