On Tue 18-12-18 13:24:44, gchen.guomin@xxxxxxxxx wrote: > From: guomin chen <gchen.guomin@xxxxxxxxx> > > When mm->owner is modified by exit_mm, if the new owner directly calls > unuse_mm to exit, it will cause Use-After-Free. Due to the unuse_mm() > directly sets tsk->mm=NULL. > > Under normal circumstances,When do_exit exits, mm->owner will > be updated on exit_mm(). but when the kernel process calls > unuse_mm() and then exits,mm->owner cannot be updated. And it > will point to a task that has been released. > > The current issue flow is as follows: (Process A,B,C use the same mm) > Process C Process A Process B > qemu-system-x86_64: kernel:vhost_net kernel: vhost_net > open /dev/vhost-net > VHOST_SET_OWNER create kthread vhost-%d create kthread vhost-%d > network init use_mm() use_mm() > ... ... > Abnormal exited > ... > do_exit > exit_mm() > update mm->owner to A > exit_files() > close_files() > kthread_should_stop() unuse_mm() > Stop Process A tsk->mm=NULL > do_exit() > can't update owner > A exit completed vhost-%d rcv first package > vhost-%d build rcv buffer for vq > page fault > access mm & mm->owner > NOW,mm->owner still pointer A > kernel UAF > stop Process B > > Although I am having this issue on vhost_net,But it affects all users of > unuse_mm. I am confused. How can we ever assign the owner to a kernel thread. We skip those explicitly. It simply doesn't make any sense to have an owner a kernel thread. -- Michal Hocko SUSE Labs
