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

Summary

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.

Timeline

1. Sep 16, 2026, 10:32 AM UTCCVE published
2. Oct 19, 2026, 8:55 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 86956e70761b3292156d668e87126844334dd71b · fixed in eb6cc898501a004c0fa8aab951351d9c959b91cc; From 86956e70761b3292156d668e87126844334dd71b · fixed in 6a180adafc2a8a5fbe815985c71b2201e93ab5e1; From 86956e70761b3292156d668e87126844334dd71b · fixed in fc069d00a0dbef40042fd681554d48dcd5a1d524; From 86956e70761b3292156d668e87126844334dd71b · fixed in 13bfc94eef389bd664ffc67b00184919902c938a; From 86956e70761b3292156d668e87126844334dd71b · fixed in 4400270ec0348d05dc0439d8f0130853ce7f9e20

• Linux Linux

  5.15

• KernelLinux

  From 5.15.0 · fixed in 6.6.157; From 6.7.0 · fixed in 6.12.110; From 6.13.0 · fixed in 6.18.51; From 6.19.0 · fixed in 7.2.5

References 5