CVE-2026-89960

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

s390/vfio-ap: fix stale pqap_hook pointer on error in vfio_ap_mdev_set_kvm()

In vfio_ap_mdev_set_kvm(), kvm->arch.crypto.pqap_hook is set to

&matrix_mdev->pqap_hook before the update locks are acquired and the

mdev list is checked for a conflicting assignment. If another mdev is

already attached to the same KVM instance, the function returns -EPERM

without restoring the hook pointer, leaving kvm->arch.crypto.pqap_hook

pointing at the failing matrix_mdev instead of the mdev that legitimately

owns the KVM.

Since matrix_mdev->kvm is never set on this error path,

vfio_ap_mdev_unset_kvm() will not clean up the hook when matrix_mdev

is later closed. If matrix_mdev is subsequently freed, any PQAP

instruction executed by the guest will dereference the stale pointer

through pqap_hook_rwsem, resulting in a use-after-free.

Since kvm->arch.crypto.pqap_hook is only set in the vfio_ap_mdev_set_kvm()

function and is cleared in the vfio_ap_mdev_unset_kvm() function, a check

for 'kvm->arch.crypto.pqap_hook != NULL' is all that is needed to determine

whether it belongs to another mdev. This will alleviate the need to iterate

the matrix_dev->mdev_list list to see if the kvm object is assigned to

another mdev.This was introduced in v3 to alleviate the need to take the

mdevs_lock while iterating the list; however, this did not prevent a

potential race condition.

The pqap_hook_rwsem(write) is now performed inside

get_update_locks_for_kvm(), which is updated to acquire

pqap_hook_rwsem(write) between kvm->lock and mdevs_lock. This ordering

is consistent with the PQAP intercept path, which acquires pqap_hook_rwsem

in read mode while srcu is held under vcpu->mutex, establishing the

dependency: kvm->lock -> vcpu->mutex -> srcu -> pqap_hook_rwsem(read).

The pqap_hook_rwsem is now released inside the

release_update_locks_for_kvm(), which is updated to release

pqap_hook_rwsem(write) between mdevs_lock and kvm->lock.

Additionally, kvm_put_kvm() in vfio_ap_mdev_unset_kvm() is moved

after release_update_locks_for_kvm(). Previously it was called while

kvm->lock was held; if it were ever the last reference, kvm_destroy_vm()

would run under kvm->lock, which would deadlock.