Re: [PATCH v4 1/2] iommu/s390: Fix race with release_device ops

[Robin Murphy <robin.murphy@arm.com>]

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.