On Thu, Jan 19, 2023 at 5:31 PM Hillf Danton <hdanton@xxxxxxxx> wrote: > > On Thu, 19 Jan 2023 13:01:42 -0800 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > Hi Folks, > > I spent some more time digging into the details and this is what's > > happening. When we call rmdir to delete the cgroup with the pressure > > file being epoll'ed, roughly the following call chain happens in the > > context of the shell process: > > > > do_rmdir > > cgroup_rmdir > > kernfs_drain_open_files > > cgroup_file_release > > cgroup_pressure_release > > psi_trigger_destroy > > > > Later on in the context of our reproducer, the last fput() is called > > causing wait queue removal: > > > > fput > > ep_eventpoll_release > > ep_free > > ep_remove_wait_queue > > remove_wait_queue > > > > By this time psi_trigger_destroy() already destroyed the trigger's > > waitqueue head and we hit UAF. > > I think the conceptual problem here (or maybe that's by design?) is > > that cgroup_file_release() is not really tied to the file's real > > lifetime (when the last fput() is issued). Otherwise fput() would call > > eventpoll_release() before f_op->release() and the order would be fine > > (we would remove the wait queue first in eventpoll_release() and then > > f_op->release() would cause trigger's destruction). > > eventpoll_release > eventpoll_release_file > ep_remove > ep_unregister_pollwait > ep_remove_wait_queue > Yes but fput() calls eventpoll_release() *before* f_op->release(), so waitqueue_head would be removed before trigger destruction. > Different roads run into the same Roma city. You butchered the phrase :) > > > Considering these findings, I think we can use the wake_up_pollfree() > > without contradicting the comment at > > https://elixir.bootlin.com/linux/latest/source/include/linux/wait.h#L253 > > because indeed, cgroup_file_release() and therefore > > psi_trigger_destroy() are not tied to the file's lifetime. > > > > I'm CC'ing Tejun to check if this makes sense to him and > > cgroup_file_release() is working as expected in this case. > > > > Munehisha, if Tejun confirms this is all valid, could you please post > > a patch replacing wake_up_interruptible() with wake_up_pollfree()? We > > don't need to worry about wake_up_all() because we have a limitation > > of one trigger per file descriptor: > > https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L1419, > > so there can be only one waiter. > > Thanks, > > Suren. >
