Re: setting up CDB filters in udev (was Re: [PATCH v2 0/3] block: add queue-private command filter, editable via sysfs)

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

On Fri, 02 Nov 2012 15:49:02 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> Il 31/10/2012 22:22, Tejun Heo ha scritto:
> > Hello, Paolo.
> > 
> > On Thu, Oct 25, 2012 at 02:35:20PM -0400, Paolo Bonzini wrote:
> >>> Disabling filters if opened by root and tranfering via SCM_RIGHTS
> >>> would be the simplest interface-wise (there's no new interface at
> >>> all).  Would that be too dangerous security-wise?
> >>
> >> That would be a change with respect to what we have now.  After
> >> transferring a root-opened (better: CAP_SYS_RAWIO-opened) file
> >> descriptor to an unprivileged process your SG_IO commands get
> >> filtered.  So a ioctl is needed if you want to rely on SCM_RIGHTS.
> > 
> > Yeah, I get that it's a behavior change, but would that be a problem?
> 
> Worse, it's a potential security hole because previously you'd get
> filtering and now you wouldn't.
> 
> Considering that SCM_RIGHTS is usually used to transfer a file
> descriptor from a privileged process to an unprivileged one, I'd be very
> worried of that.

In other contexts you inherit file handles via exec and having a "root
opened so its special" model is bad. Historically it led to things like
the rlogin/rsh hacks on SunOS and friends where a program run by the rsh
daemon got a root opened socket as its stdin/out and could issue ifconfig
ioctls on it at will.

Not a good model. Any removal of filters and passing them to a task
should be explicit. The behaviour really ought to be to permit the
intentional setting of explicit filters then passing them, not touch the
default behaviour.

Alan