On Wed, Mar 20, 2019 at 3:16 PM Jan Kara <jack@xxxxxxx> wrote: > > Hello, > > recently, one of our customers has reported a deadlock with fanotify. The > analysis has shown that a process has put (likely by mistake) FAN_OPEN_PERM > mark on /proc and / filesystem. That resulted in a deadlock like follows: > > process 1: process 2: process 3: > open("/proc/process 2/maps") > - blocks waiting for response to > FAN_OPEN_PERM event > > exec(2) > __do_execve_file() > - grabs current->signal->cred_guard_mutex > do_open_execat() > - blocks waiting for response to > FAN_OPEN_PERM event > > reads fanotify event > generated by process 1 > create_fd() > dentry_open() > proc_maps_open() > blocks on > cred_guard_mutex of process 2. > > Now this is not the only case where you can setup fanotify permissions > events so that your listener deadlocks but I'd argue that this case is > especially nasty and it is unrealistic to expect from userspace that it > would be able to implement fanotify listener in such a way that is > deadlock-free for proc filesystem since the lock dependencies there are > very different. So how about we just forbid placing marks with fanotify > permission events on proc? Any other virtual filesystem we should exclude? > I bet if we forbid placing marks on /proc, some apps would break. I always though that allowing O_PATH in event_f_flags can make sense for some apps. What if instead of forbiding marks of /proc, we would force those marks to use O_PATH for fd creation. Some of the functionality will remain. Apps are less likely to break. Deadlocks will be less likely, although maybe still possible. Note that the new FAN_REPORT_FID listener already excludes marking most virtual filesystems for lack of s_export_op. Thanks, Amir.