On Wed 20-03-19 15:46:20, Amir Goldstein wrote:
> On Wed, Mar 20, 2019 at 3:16 PM Jan Kara <jack@xxxxxxx> wrote:
> > 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.
Well, I didn't mean all marks, just the permission ones. I'm not sure there
are apps that place permission events on /proc...
> 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.
Yes, that's another option. But if this is automatic, it is going to be
confusing to potential users - report O_PATH fd if getting normal fd is
dangerous isn't great. And also the deadlocks are there only for permission
events so there's no strong reason to restrict normal ones.
> Note that the new FAN_REPORT_FID listener already excludes
> marking most virtual filesystems for lack of s_export_op.
Yes, I know.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
