CVE-2026-89956 — s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects

Affected: Linux

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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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 ~~…

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CVE-2026-89956 — s390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects