Re: fanotify permission events on virtual filesystem

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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.


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