Re: [RFC] FUSE permission modell (Was: fuse review bits)

[Daniel Jacobowitz <dan@debian.org>]

On Mon, Apr 11, 2005 at 05:56:09PM +0200, Miklos Szeredi wrote:
> > >   3) No other user should have access to files under the mount, not
> > >      even root[5]
> > 
> > > [5] Obviously root cannot be restricted, but accidental access to
> > > private data is still a good idea.  E.g. root squashing by NFS servers
> > > has a similar affect.
> > 
> > Could you explain a little more?  I don't see the point in denying
> > access to root, but I also can't tell from your explanation whether you
> > do or not.
> 
> Fuse by default does.  This can be disabled by one of two mount
> options: "allow_other" and "allow_root".  The former implies the
> later.  These mount options are only allowed for mounting by root, but
> this can be relaxed with a configuration option.

So the behavior that Cristoph was objecting to here is in fact
configurable?

> > I don't really see the point of this restriction, anyway.  Could you
> > explain why this shouldn't be a matter of policy, and kept out of the
> > kernel?  Have the userspace file servers default to putting restrictive
> > permissions on mounts unless requested otherwise.
> 
> That's an option.  However you can't restrict root that way, and you
> need an extra directory, since permissions on the mountpoint are
> ignored after the mount.

No, you need the userspace daemon to set the permissions on the root
directory of the new mount restrictively.  What am I missing?

> Restricting root is needed, so that a sysadmin won't accidently go
> into a user's private mount (e.g. sshfs to some machine to which the
> sysadmin otherwise has no access).  Root can still gain access by
> doing 'su me', but at least he will have a bad conscience.  This is
> not such a stupid idea as it first sounds IMO, and by default all NFS
> servers exhibit a similar behavior (root squashing).

Root squashing is actually a much less obnoxious restriction.  It means
that local uid 0 doesn't automatically correspond to remote uid 0.

> > >   4) Access should not be further restricted for the owner of the
> > >      mount, even if permission bits, uid or gid would suggest
> > >      otherwise
> > 
> > Similar questions.
> 
> This behavior can be disabled by the "default_permissions" mount
> option (wich is not privileged, since it adds restrictions).  A FUSE
> filesystem mounted by root (and not for private purposes) would
> normally be done with "allow_other,default_permissions".

But why does the kernel need to know anything about this?  Why can't
the userspace library present the permissions appropriately to the
kernel?  I'm going to be pretty confused if I see a mode 666 file that
I can't even read.  So will various programs.

Except for the allow_root bits, I think that having userspace handle
the issue entirely would cover both objections.

> Does this answer your questions?

More or less.

-- 
Daniel Jacobowitz
CodeSourcery, LLC