On 2014/9/17 7:56, Cong Wang wrote: > Hi, Tejun > > > We saw some kernel null pointer dereference in > cgroup_pidlist_destroy_work_fn(), more precisely at > __mutex_lock_slowpath(), on 3.14. I can show you the full stack trace > on request. > Yes, please. > Looking at the code, it seems flush_workqueue() doesn't care about new > incoming works, it only processes currently pending ones, if this is > correct, then we could have the following race condition: > > cgroup_pidlist_destroy_all(): > //... > mutex_lock(&cgrp->pidlist_mutex); > list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links) > mod_delayed_work(cgroup_pidlist_destroy_wq, > &l->destroy_dwork, 0); > mutex_unlock(&cgrp->pidlist_mutex); > > // <--- another process calls cgroup_pidlist_start() here > since mutex is released > > flush_workqueue(cgroup_pidlist_destroy_wq); // <--- another > process adds new pidlist and queue work in pararell > BUG_ON(!list_empty(&cgrp->pidlists)); // <--- This check is > passed, list_add() could happen after this > Did you confirm this is what happened when the bug was triggered? I don't think the race condition you described exists. In 3.14 kernel, cgroup_diput() won't be called if there is any thread running cgroup_pidlist_start(). This is guaranteed by vfs. But newer kernels are different. Looks like the bug exists in those kernels. > > Therefore, the newly added pidlist will point to a freed cgroup, and > when it is freed in the delayed work we will crash. > > The attached patch (compile test ONLY) could be a possible fix, since > it will check and hold a refcount on this cgroup in > cgroup_pidlist_start(). But I could very easily miss something here > since there are many cgroup changes after 3.14 and I don't follow > cgroup development. > > What do you think? > -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
