In the Linux kernel, the following vulnerability has been resolved: s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mde

In the Linux kernel, the following vulnerability has been resolved:

s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects

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

 1The vfio_ap_mdev_probe function uses the matrix_dev->mdevs_lock
 2mutex to guard the add of a newly created ap_matrix_mdev object to the
 3matrix_dev->mdev_list. This mutex does not protect list access; its purpose
 4is to guard against concurrent access to fields contained in an
 5ap_matrix_mdev object. This could lead to kernel memory corruption or
 6use-after-free if another mdev is created or removed concurrently.
 7 
 8The adding of an ap_matrix_mdev object to matrix_dev->mdev_list
 9is now guarded by the matrix_dev->guests_lock which is the correct
10way to protect against concurrent mdev_list access.
11 
12Also removed the following two lines of code because the matrix_mdev is
13allocated via vfio_alloc_device macro which uses kzalloc, so req_trigger
14and cfg_chg_trigger are already zero-initialised when the struct is
15allocated before the call to vfio_register_emulated_iommu_dev. This
16prevents a window whereby these triggers are set to NULL after
17the device is exposed to userspace.
18 
19matrix_mdev->req_trigger = NULL;
20matrix_mdev->cfg_chg_trigger = NULL;
21 
22vfio_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.