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

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Date:	Fri, 2 Nov 2012 15:35:30 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, Ric Wheeler <rwheeler@...hat.com>,
	Petr Matousek <pmatouse@...hat.com>,
	Kay Sievers <kay@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	linux-kernel@...r.kernel.org,
	"James E.J. Bottomley" <James.Bottomley@...senPartnership.com>
Subject: Re: setting up CDB filters in udev (was Re: [PATCH v2 0/3] block:
 add queue-private command filter, editable via sysfs)

On Fri, 02 Nov 2012 15:49:02 +0100
Paolo Bonzini <pbonzini@...hat.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
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