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

Robin Murphy
2022-09-02T10:49:42Z

On 2022-09-01 17:14, 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, of course, I keep forgetting the way VFIO works with groups, so I
guess as long as the group contained a VFIO device and passed the
viability check at some point in the past, the VFIO user can then hold
the fd open and continue to monkey about with it even after the device
itself has unbound from vfio-pci. Plus there's also the obvious case of
a device being hotplugged out of a group that still contains other live
devices, so yes, this seems entirely normal and expected, derp.

From a quick skim through the current mess, I think that 9ac8545199a1
is the culprit here, and has probably broken at least as much as it
fixed - AFAICS it's still possible for release_device to race with a
detach/domain_free and potentially lead to another use-after-free (or
double-detach), it's just less likely than the previous case.

Removing the device from the group *before* calling release_device is
certainly more consistent with the other paths where release_device can
be called with the device group-less, and is also symmetrical with
probe_device being called before the device is first added... It would
almost work if iommu_group_remove_device() captured the current domain
at the point the device is removed from the group list, then passed that
to the release_device callback directly. Except we still can't guarantee
that that domain won't suddenly disappear, so those drivers would need
to synchronise between their domain_free and release_device in some
form, ho hum...

Alternatively, a really sly way around the whole thing would be to find
some pretence for doing an iommu_group_for_each_dev() in your
release_device, but people will probably throw things at me for
suggesting that :)

Robin.

>>> Handle this by tracking when the device is probed/released via
>>> dev_iommu_priv_set/get().  Ensure that once the device is released only
>>> release_device handles the re-init of the device DMA.
>>>
>>> Fixes: fa7e9ecc5e1c ("iommu/s390: Tolerate repeat attach_dev calls")
>>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
>>> ---
>>>   arch/s390/include/asm/pci.h |  1 +
>>>   arch/s390/pci/pci.c         |  1 +
>>>   drivers/iommu/s390-iommu.c  | 39 ++++++++++++++++++++++++++++++++++---
>>>   3 files changed, 38 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
>>> index 7b4cdadbc023..080251e7b275 100644
>>> --- a/arch/s390/include/asm/pci.h
>>> +++ b/arch/s390/include/asm/pci.h
>>> @@ -157,6 +157,7 @@ struct zpci_dev {
>>>       /* DMA stuff */
>>>       unsigned long    *dma_table;
>>>       spinlock_t    dma_table_lock;
>>> +    struct mutex    dma_domain_lock; /* protects s390_domain value */
>>>       int        tlb_refresh;
>>>         spinlock_t    iommu_bitmap_lock;
>>> diff --git a/arch/s390/pci/pci.c b/arch/s390/pci/pci.c
>>> index 73cdc5539384..973edd32ecc9 100644
>>> --- a/arch/s390/pci/pci.c
>>> +++ b/arch/s390/pci/pci.c
>>> @@ -832,6 +832,7 @@ struct zpci_dev *zpci_create_device(u32 fid, u32 fh, enum zpci_state state)
>>>       kref_init(&zdev->kref);
>>>       mutex_init(&zdev->lock);
>>>       mutex_init(&zdev->kzdev_lock);
>>> +    mutex_init(&zdev->dma_domain_lock);
>>>         rc = zpci_init_iommu(zdev);
>>>       if (rc)
>>> diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
>>> index c898bcbbce11..1137d669e849 100644
>>> --- a/drivers/iommu/s390-iommu.c
>>> +++ b/drivers/iommu/s390-iommu.c
>>> @@ -99,6 +99,14 @@ static int s390_iommu_attach_device(struct iommu_domain *domain,
>>>       if (!domain_device)
>>>           return -ENOMEM;
>>>   +    /* Leave now if the device has already been released */
>>> +    mutex_lock(&zdev->dma_domain_lock);
>>> +    if (!dev_iommu_priv_get(dev)) {
>>> +        mutex_unlock(&zdev->dma_domain_lock);
>>> +        kfree(domain_device);
>>> +        return 0;
>>> +    }
>>> +
>>>       if (zdev->dma_table && !zdev->s390_domain) {
>>>           cc = zpci_dma_exit_device(zdev);
>>>           if (cc) {
>>> @@ -132,9 +140,10 @@ static int s390_iommu_attach_device(struct iommu_domain *domain,
>>>           goto out_restore;
>>>       }
>>>       domain_device->zdev = zdev;
>>> -    zdev->s390_domain = s390_domain;
>>>       list_add(&domain_device->list, &s390_domain->devices);
>>>       spin_unlock_irqrestore(&s390_domain->list_lock, flags);
>>> +    zdev->s390_domain = s390_domain;
>>> +    mutex_unlock(&zdev->dma_domain_lock);
>>>         return 0;
>>>   @@ -147,6 +156,7 @@ static int s390_iommu_attach_device(struct iommu_domain *domain,
>>>                      virt_to_phys(zdev->dma_table));
>>>       }
>>>   out_free:
>>> +    mutex_unlock(&zdev->dma_domain_lock);
>>>       kfree(domain_device);
>>>         return rc;
>>> @@ -176,17 +186,22 @@ static void s390_iommu_detach_device(struct iommu_domain *domain,
>>>       }
>>>       spin_unlock_irqrestore(&s390_domain->list_lock, flags);
>>>   -    if (found && (zdev->s390_domain == s390_domain)) {
>>> +    mutex_lock(&zdev->dma_domain_lock);
>>> +    if (found && (zdev->s390_domain == s390_domain) &&
>>> +        dev_iommu_priv_get(dev)) {
>>>           zdev->s390_domain = NULL;
>>>           zpci_unregister_ioat(zdev, 0);
>>>           zpci_dma_init_device(zdev);
>>>       }
>>> +    mutex_unlock(&zdev->dma_domain_lock);
>>>   }
>>>     static struct iommu_device *s390_iommu_probe_device(struct device *dev)
>>>   {
>>>       struct zpci_dev *zdev = to_zpci_dev(dev);
>>>   +    dev_iommu_priv_set(dev, zdev);
>>> +
>>>       return &zdev->iommu_dev;
>>>   }
>>>   @@ -206,10 +221,28 @@ static void s390_iommu_release_device(struct device *dev)
>>>        *
>>>        * So let's call detach_dev from here if it hasn't been called before.
>>>        */
>>> -    if (zdev && zdev->s390_domain) {
>>> +    if (zdev) {
>>> +        /*
>>> +         * Clear priv to block further attaches for this device,
>>> +         * ensure detaches don't init DMA.  Hold the domain lock
>>> +         * to ensure that attach/detach get a consistent view of
>>> +         * whether or not the device is released.
>>> +         */
>>> +        mutex_lock(&zdev->dma_domain_lock);
>>> +        dev_iommu_priv_set(dev, NULL);
>>> +        mutex_unlock(&zdev->dma_domain_lock);
>>> +        /* Make sure this device is removed from the domain list */
>>>           domain = iommu_get_domain_for_dev(dev);
>>>           if (domain)
>>>               s390_iommu_detach_device(domain, dev);
>>> +        /* Now ensure DMA is initialized from here */
>>> +        mutex_lock(&zdev->dma_domain_lock);
>>> +        if (zdev->s390_domain) {
>>> +            zdev->s390_domain = NULL;
>>> +            zpci_unregister_ioat(zdev, 0);
>>> +            zpci_dma_init_device(zdev);
>>> +        }
>>> +        mutex_unlock(&zdev->dma_domain_lock);
>>>       }
>>>   }
>>>
>