[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>]

In order to traverse or add/remove ap_matrix_mdev objects in the
matrix_dev->mdev_list, the matrix_dev->guests_lock mutex must be held.
There are two functions that access the list without holding the mutex:

vfio_ap_mdev_probe function
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The vfio_ap_mdev_probe function uses the matrix_dev->mdevs_lock
mutex to guard the add of a newly created ap_matrix_mdev object to the
matrix_dev->mdev_list. This mutex does not protect list access; its purpose
is to guard against concurrent access to fields contained in an
ap_matrix_mdev object. This could lead to kernel memory corruption or
use-after-free if another mdev is created or removed concurrently.

The adding of an ap_matrix_mdev object to matrix_dev->mdev_list
is now guarded by the matrix_dev->guests_lock which is the correct
way to protect against concurrent mdev_list access.

Also removed the following two lines of code because the matrix_mdev is
allocated via vfio_alloc_device macro which uses kzalloc, so req_trigger
and cfg_chg_trigger are already zero-initialised when the struct is
allocated before the call to vfio_register_emulated_iommu_dev. This
prevents a window whereby these triggers are set to NULL after
the device is exposed to userspace.

matrix_mdev->req_trigger = NULL;
matrix_mdev->cfg_chg_trigger = NULL;

vfio_ap_mdev_for_queue function
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The status_show function that supports display of the status attribute of
the devices in /sys/bus/ap/devices calls the vfio_ap_mdev_for_queue
function which iterates the matrix_dev->mdev_list to find the object
representing the queue device whose status is to be displayed. In order to
traverse this list, the matrix_dev->guests_lock mutex must be held.

To fix this, the guests_lock mutex is taken prior to taking the
matrix_dev->mdevs_lock mutex in the status_show function. It is taken
there rather than the vfio_ap_mdev_for_queue function - where it is
needed - because it must be taken prior to the mdevs_lock mutex in order to
adhere to the proper locking order and prevent a lockdep splat; also
because the mdevs_lock is needed there to access fields within
the matrix_mdev object in that function.

See the vfio-ap-locking.rst in the linux kernel tree.

Fixes: 2c1ee8983aa3 ("s390/vfio-ap: prepare for dynamic update of guest's APCB on queue probe/remove")
Cc: stable@vger.kernel.org
Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
---
 drivers/s390/crypto/vfio_ap_ops.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index c6bee69cc22f..f2d662e388bd 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);
 	if (ret)
 		goto err_put_vdev;
-	matrix_mdev->req_trigger = NULL;
-	matrix_mdev->cfg_chg_trigger = NULL;
+
+	/*
+	 * 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;
 
 err_put_vdev:
@@ -2297,6 +2302,8 @@ static struct ap_matrix_mdev *vfio_ap_mdev_for_queue(struct vfio_ap_queue *q)
 	unsigned long apid = AP_QID_CARD(q->apqn);
 	unsigned long apqi = AP_QID_QUEUE(q->apqn);
 
+	lockdep_assert_held(&matrix_dev->guests_lock);
+
 	list_for_each_entry(matrix_mdev, &matrix_dev->mdev_list, node) {
 		if (test_bit_inv(apid, matrix_mdev->matrix.apm) &&
 		    test_bit_inv(apqi, matrix_mdev->matrix.aqm))
@@ -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);
 	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;
 }
@@ -2761,6 +2770,12 @@ static void vfio_ap_mdev_cfg_add(unsigned long *apm_add, unsigned long *aqm_add,
 
 	vfio_ap_filter_apid_by_qtype(apm_add, aqm_add);
 
+	/*
+	 * It is safe to traverse this list here because the
+	 * required guard - matrix_dev->guests_lock - is taken in the
+	 * vfio_ap_on_cfg_changed function prior to this function getting
+	 * called.
+	 */
 	list_for_each_entry(matrix_mdev, &matrix_dev->mdev_list, node) {
 		bitmap_and(matrix_mdev->apm_add,
 			   matrix_mdev->matrix.apm, apm_add, AP_DEVICES);
@@ -2820,6 +2835,10 @@ void vfio_ap_on_cfg_changed(struct ap_config_info *cur_cfg_info,
 	if (!cur_cfg_info || !prev_cfg_info)
 		return;
 
+	/*
+	 * Take the guests_lock mutex here to guard access to the
+	 * matrix_dev->mdev_list in the two functions called below.
+	 */
 	mutex_lock(&matrix_dev->guests_lock);
 
 	vfio_ap_mdev_on_cfg_remove(cur_cfg_info, prev_cfg_info);
-- 
2.53.0