GHSA-VVFX-87J6-H3VV
Vulnerability from github – Published: 2026-09-16 12:30 – Updated: 2026-09-16 12:30In 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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.
{
"affected": [],
"aliases": [
"CVE-2026-89956"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-09-16T11:17:05Z",
"severity": null
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\ns390/vfio-ap: Fix missing lock required to access list of ap_matrix_mdev objects\n\nIn order to traverse or add/remove ap_matrix_mdev objects in the\nmatrix_dev-\u003emdev_list, the matrix_dev-\u003eguests_lock mutex must be held.\nThere are two functions that access the list without holding the mutex:\n\nvfio_ap_mdev_probe function\n~~~~~~~~~~~~~~~~~~~~~~~~~~~\nThe vfio_ap_mdev_probe function uses the matrix_dev-\u003emdevs_lock\nmutex to guard the add of a newly created ap_matrix_mdev object to the\nmatrix_dev-\u003emdev_list. This mutex does not protect list access; its purpose\nis to guard against concurrent access to fields contained in an\nap_matrix_mdev object. This could lead to kernel memory corruption or\nuse-after-free if another mdev is created or removed concurrently.\n\nThe adding of an ap_matrix_mdev object to matrix_dev-\u003emdev_list\nis now guarded by the matrix_dev-\u003eguests_lock which is the correct\nway to protect against concurrent mdev_list access.\n\nAlso removed the following two lines of code because the matrix_mdev is\nallocated via vfio_alloc_device macro which uses kzalloc, so req_trigger\nand cfg_chg_trigger are already zero-initialised when the struct is\nallocated before the call to vfio_register_emulated_iommu_dev. This\nprevents a window whereby these triggers are set to NULL after\nthe device is exposed to userspace.\n\nmatrix_mdev-\u003ereq_trigger = NULL;\nmatrix_mdev-\u003ecfg_chg_trigger = NULL;\n\nvfio_ap_mdev_for_queue function\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\nThe status_show function that supports display of the status attribute of\nthe devices in /sys/bus/ap/devices calls the vfio_ap_mdev_for_queue\nfunction which iterates the matrix_dev-\u003emdev_list to find the object\nrepresenting the queue device whose status is to be displayed. In order to\ntraverse this list, the matrix_dev-\u003eguests_lock mutex must be held.\n\nTo fix this, the guests_lock mutex is taken prior to taking the\nmatrix_dev-\u003emdevs_lock mutex in the status_show function. It is taken\nthere rather than the vfio_ap_mdev_for_queue function - where it is\nneeded - because it must be taken prior to the mdevs_lock mutex in order to\nadhere to the proper locking order and prevent a lockdep splat; also\nbecause the mdevs_lock is needed there to access fields within\nthe matrix_mdev object in that function.\n\nSee the vfio-ap-locking.rst in the linux kernel tree.",
"id": "GHSA-vvfx-87j6-h3vv",
"modified": "2026-09-16T12:30:43Z",
"published": "2026-09-16T12:30:43Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-89956"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/0fd74477d05743d4cd9b25a3b667daa278d53b93"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/647916988272fe5ecb4bb30cd02b51af9960618a"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/7fa61c29850d05e40ca9ed41bfdf57673023f581"
}
],
"schema_version": "1.4.0",
"severity": []
}
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