Re: malicious filesystems (was Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway)

[Pavel Machek]

Hi!

> > So you want me to handle _malicious_ filesystems now?
> > 
> > That should be easy... :-). You already have nasty deadlocks in FUSE,
> > and you solve them by "root can echo 1 > abort"... so allow me the
> > same possibility.
> > 
> > We can tell fused we are freezing, and if all the requests are not
> > serviced within, say, 30 seconds, we call the filesystem malicious and
> > do echo 1 > abort.
> > 
> > Not ideal, but neither is allowing malicious filesystems in the first
> > place...
> 
> Actually there's also a non-malicious case in which waiting for
> requests to finish won't work: when one fuse filesystem is accessing
> another.
> 
> Since we are blocking new fuse requests, that might block a fuse
> daemon, which in turn makes it impossible to finish the pending
> request.

Hmm, yes, this is ugly. But will it hurt? We'll just wait for number
of active fuse requests to go down to 0 before proceeding. It may
still livelock on system with busy fused-s, but we'll detect that with
timeout, and I believe it is 'don't do that' situation.

> And this is not at all theoretical, I know that encfs is used over
> various other fuse filesystems like sshfs or ntfs-3g.
> 
> Yeah, stacking userspace filesystems could be done entirely in
> userspace, and I'm actually working on that (with fuse-2.7.0 already
> supporting some basic stacking).

Great.

> But the point is, that the "wait for fuse to quiescence" hack would
> not work in todays environment.

Ok, I'll just blame fuse here. 'You have to write to /sys
files for SIGKILL to work' is not funny.
							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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