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.
