Re: [PATCH v4 3/9] s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects

[Anthony Krowiak <akrowiak@linux.ibm.com>]

On 8/12/26 1:28 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:
>
> Pre-existing issues:
> - [Critical] The sysfs file 'status' is exposed to userspace before the device's driver data is fully initialized, leading to a potential NULL pointer dereference in status_show().
> - [High] A race condition in vfio_ap_mdev_probe() allows userspace to bypass the 'one mdev per KVM' isolation restriction.
> - [High] Eventfd contexts for `req_trigger` and `cfg_chg_trigger` are leaked when the mediated device is closed or removed.
> --
>
> --- Patch [3]: [PATCH v4 3/9] s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects ---
> commit b763c5c7bf7a38cb1fc2e2da2ff0df92d3ae360c
> Author: Anthony Krowiak <akrowiak@linux.ibm.com>
>
> s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects
>
> This patch fixes list traversal synchronization for ap_matrix_mdev objects
> by adding guests_lock around mdev_list access. It also removes redundant
> zero-initialization of eventfd triggers in the probe function.
>
>> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
>> index c6bee69cc22f6..f2d662e388bdc 100644
>> --- a/drivers/s390/crypto/vfio_ap_ops.c
>> +++ b/drivers/s390/crypto/vfio_ap_ops.c
>> @@ -800,12 +800,17 @@ static int vfio_ap_mdev_probe(struct mdev_device *mdev)
>>   	ret = vfio_register_emulated_iommu_dev(&matrix_mdev->vdev);
> [Severity: High]
> This isn't a bug introduced by this patch, but does exposing the VFIO
> device to userspace here create a race condition?
>
> vfio_register_emulated_iommu_dev() makes the device visible to userspace
> before it is added to matrix_dev->mdev_list under guests_lock. Could
> userspace open multiple mdevs concurrently, allowing them to bypass the 'one
> mdev per KVM' isolation restriction since vfio_ap_mdev_set_kvm() won't see
> the other devices in the list yet?
>
>>   	if (ret)
>>   		goto err_put_vdev;
>> -	matrix_mdev->req_trigger = NULL;
>> -	matrix_mdev->cfg_chg_trigger = NULL;
> [Severity: High]
> This is a pre-existing issue, but since you are touching the eventfd triggers
> here, are they properly cleaned up when the device is closed?
>
> Userspace can set IRQ triggers via VFIO_DEVICE_SET_IRQS, which calls
> eventfd_ctx_fdget() and takes a reference. If the user closes the device
> without unregistering the IRQs, shouldn't vfio_ap_mdev_close_device() or
> vfio_ap_mdev_remove() call eventfd_ctx_put() for req_trigger and
> cfg_chg_trigger to prevent a memory and file descriptor leak?

The other concerns are fixed in this series, but this concern is not. This
needs to be fixed.

>
>> +
>> +	/*
>> +	 * Take the matrix_dev->guests_lock mutex before adding the matrix_mdev
>> +	 * to the mdev_list. All functions that traverse the list must also hold
>> +	 * this lock to guard against additions to or removals from the list
>> +	 * while it is being traversed.
>> +	 */
>> +	mutex_lock(&matrix_dev->guests_lock);
>>   	dev_set_drvdata(&mdev->dev, matrix_mdev);
>> -	mutex_lock(&matrix_dev->mdevs_lock);
>>   	list_add(&matrix_mdev->node, &matrix_dev->mdev_list);
>> -	mutex_unlock(&matrix_dev->mdevs_lock);
>> +	mutex_unlock(&matrix_dev->guests_lock);
>>   	return 0;
> [ ... ]
>
>> @@ -2316,6 +2323,7 @@ static ssize_t status_show(struct device *dev,
>>   	struct ap_matrix_mdev *matrix_mdev;
>>   	struct ap_device *apdev = to_ap_dev(dev);
>>   
>> +	mutex_lock(&matrix_dev->guests_lock);
>>   	mutex_lock(&matrix_dev->mdevs_lock);
>>   	q = dev_get_drvdata(&apdev->device);
> [Severity: Critical]
> This is a pre-existing issue, but is it possible for dev_get_drvdata() to
> return a NULL pointer here if the status sysfs file is read concurrently
> with device probing?
>
> During vfio_ap_mdev_probe_queue(), sysfs_create_group() creates the status
> attribute before dev_set_drvdata() initializes the driver data. Can a
> concurrent read of the status file fetch a NULL pointer in status_show() and
> pass it to vfio_ap_mdev_for_queue(), causing a crash when it dereferences
> q->apqn?
>
>>   	matrix_mdev = vfio_ap_mdev_for_queue(q);
>> @@ -2343,6 +2351,7 @@ static ssize_t status_show(struct device *dev,
>>   	}
>>   
>>   	mutex_unlock(&matrix_dev->mdevs_lock);
>> +	mutex_unlock(&matrix_dev->guests_lock);
>>   
>>   	return nchars;
>>   }