[PATCH v2] s390/vfio-ap: fix stale pqap_hook pointer on error in vfio_ap_mdev_set_kvm()

[Anthony Krowiak <akrowiak@linux.ibm.com>]

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 likewise eliminates the need to hold the
matrix_dev->mdevs_lock while iterating the list.

Note that the 'matrix_mdev' pointer retrieved via the 'container_of' macro
in the 'vfio_ap_mdev_open_device()' callback will be available for the
duration of that function because 'matrix_mdev' cannot be freed while
'open_device' (which calls the 'vfio_ap_mdev_open_device()' function) is
running.

Fixes: 86956e70761b3 ("s390/vfio-ap: replace open coded locks for VFIO_GROUP_NOTIFY_SET_KVM notification")
Cc: stable@vger.kernel.org

Signed-off-by: Anthony Krowiak <akrowiak@linux.ibm.com>
---
 drivers/s390/crypto/vfio_ap_ops.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index 44b3a1dcc1b3..b0a038faa93a 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -1821,26 +1821,25 @@ static const struct attribute_group *vfio_ap_mdev_attr_groups[] = {
 static int vfio_ap_mdev_set_kvm(struct ap_matrix_mdev *matrix_mdev,
 				struct kvm *kvm)
 {
-	struct ap_matrix_mdev *m;
-
 	if (kvm->arch.crypto.crycbd) {
+		/*
+		 * Taking the kvm->arch.crypto.pqap_hook_rwsem lock while
+		 * holding the update locks (below) could result in a
+		 * deadlock condition, so let's hold the rwsem here while we
+		 * check and update the pqap_hook.
+		 */
 		down_write(&kvm->arch.crypto.pqap_hook_rwsem);
+		if (kvm->arch.crypto.pqap_hook) {
+			up_write(&kvm->arch.crypto.pqap_hook_rwsem);
+			return -EPERM;
+		}
 		kvm->arch.crypto.pqap_hook = &matrix_mdev->pqap_hook;
 		up_write(&kvm->arch.crypto.pqap_hook_rwsem);
 
 		get_update_locks_for_kvm(kvm);
-
-		list_for_each_entry(m, &matrix_dev->mdev_list, node) {
-			if (m != matrix_mdev && m->kvm == kvm) {
-				release_update_locks_for_kvm(kvm);
-				return -EPERM;
-			}
-		}
-
 		kvm_get_kvm(kvm);
 		matrix_mdev->kvm = kvm;
 		vfio_ap_mdev_update_guest_apcb(matrix_mdev);
-
 		release_update_locks_for_kvm(kvm);
 	}
 
-- 
2.53.0