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

[sashiko-bot@kernel.org]

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?

> +
> +	/*
> +	 * 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;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260812170945.738351-1-akrowiak@linux.ibm.com?part=3