vhost-scsi: Fix vhost_scsi struct use after free

If vhost_scsi_setup_vq_cmds fails we leave the tpg->vhost_scsi pointer
set. If the device is freed and then the user unmaps the LUN, the call to
vhost_scsi_port_unlink -> vhost_scsi_hotunplug will see the that
tpg->vhost_scsi is still set and try to use it.

This has us clear the vhost_scsi pointer in the failure path. It also
has us take tv_tpg_mutex in this failure path, because tv_tpg_vhost_count
is accessed under this mutex in vhost_scsi_drop_nexus and in the future
we will want to serialize access to tpg->vhost_scsi with that mutex
instead of the vhost_scsi_mutex.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20230321020624.13323-2-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/scsi.c

@@ -1658,7 +1658,10 @@ vhost_scsi_set_endpoint(struct vhost_scsi *vs,
 	for (i = 0; i < VHOST_SCSI_MAX_TARGET; i++) {
 		tpg = vs_tpg[i];
 		if (tpg) {
+			mutex_lock(&tpg->tv_tpg_mutex);
+			tpg->vhost_scsi = NULL;
 			tpg->tv_tpg_vhost_count--;
+			mutex_unlock(&tpg->tv_tpg_mutex);
 			target_undepend_item(&tpg->se_tpg.tpg_group.cg_item);
 		}
 	}