linux-kernel - unionfs unusable on multiuser systems (was Re: [PATCH 01/24] Unionfs: Documentation)

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Date:	Thu, 11 Jan 2007 14:29:56 +0000
From:	Pavel Machek <pavel@...e.cz>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Jan Kara <jack@...e.cz>, Josef Sipek <jsipek@....cs.sunysb.edu>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, hch@...radead.org,
	viro@....linux.org.uk, torvalds@...l.org, mhalcrow@...ibm.com,
	David Quigley <dquigley@....cs.sunysb.edu>,
	Erez Zadok <ezk@...sunysb.edu>
Subject: unionfs unusable on multiuser systems (was Re: [PATCH 01/24] Unionfs: Documentation)

Hi!

> > > That statement is meant to scare people away from modifying the lower fs :)
> > > I tortured unionfs quite a bit, and it can oops but it takes some effort.
> >   But isn't it then potential DOS? If you happen to union two filesystems
> > and an untrusted user has write access to both original filesystem and
> > the union, then you say he'd be able to produce oops? That does not
> > sound very secure to me... And if any secure use of unionfs requires
> > limitting access to the original trees, then I think it's a good reason
> > to implement it in unionfs itself. Just my 2 cents.
> 
> You mean somebody like, say, a perfectly innocent process working on the
> NFS server or some other client that is oblivious to the existence of
> unionfs stacks on your particular machine?
> To me, this has always sounded like a showstopper for using unionfs with
> a remote filesystem.

Actually, it is worse than that. find / (and updatedb) *will* write to
all the filesystems (atime).

Expecting sysadmins to know/prevent this seems like expecting quite a
lot from them. Sounds like a show stopper to me :-(....
							Pavel
-- 
Thanks for all the (sleeping) penguins.
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