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, 02 Nov 2012 15:49:02 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	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)

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.

>>> I guess I just feel quite reluctant to expose another rather obscure
>>> userland configurable in-kernel filter and at the same time I'm not
>>> sure whether this is flexible enough.  What if a device is shared by
>>> multiple virtual machines which are trusted at different levels?
>>
>> No, you just don't do that.  If a device is passed through to virtual
>> machines, it is between similar virtual machines (for some definition
>> of similar).  The only case where you have this sharing is in practice
>> if either the device is read-only (my patch does give you a basic
>> two-level filtering, with two separate bitmaps for RO and RW) or if you
>> allow persistent reservations (which is as close to full trust as you
>> can get).
> 
> What disturbs me is that it's a completely new interface to userland
> and at the same a very limited one at that.  So, yeah, it's
> bothersome.  I personally would prefer SCM_RIGHTS behavior change +
> hard coded filters per device class.

I think hard-coded filters are bad (I prefer to move policy to
userspace), and SCM_RIGHTS without a ioctl is out of question, really.

> But, I'd really like to hear what other guys are thinking.  Jens?
> Jens? Jens? Jens? Jens? Jens? Jens? Jens? Jens? Jens? Jens? Jens? :P

:P

Paolo

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