Re: [PATCH] private mounts

[Miklos Szeredi]

> > > the ability to change the layout underneath, you might trigger bugs in
> > > root programs: Are they really capable of seeing the same filename
> > > twice, or can you throw them into a deep recursion by simulating
> > > infinitely deep directories/circular hardlinks...?
> > Circular or otherwise hardlinked directories are not allowed since it
> > would not only confuse applications but the VFS as well.
> 
> Right, that you can catch. But can you prevent a user fs module from
> creating an infinitely deep directory structure out of thin air? Do you
> limit the maximum path length / depth?

No. 

> (Sending this privately and not to LKML, because I first wanted to check
> the facts ;-)

OK, CC restored.  You shouldn't be afraid to send to LKML.  It's the
ultimate spam list ;)

> > > Certainly a useful tool for hardening applications, but I can see the
> > > point of not wanting to let unwary applications run into a namespace
> > > controlled by a user. Of course, this is sort-of similar to "find
> > > -xdev", but I'm not sure whether it is not indeed new behaviour.
> > 
> > A trivial DoS against any process entering the userspace filesystem is
> > just not to answer the filesystem request.
> > 
> > So it's not just information leak, but also a fine way to _control_
> > certain behavior of applications.
> 
> Yes. I first thought the check was superfluous, because hey, why
> shouldn't root be able to access everything... But then it struck me
> that that might actually be a good idea for all those reasons. root's
> tools don't expect that the namespace they are traversing is
> _completely_ controlled by a user.

Exactly.

Thanks,
Miklos
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