Re: some questions

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Fri, Jan 06, 2006 at 03:07:42PM -0500, Thor Lancelot Simon wrote:
> On Fri, Jan 06, 2006 at 05:54:00PM +0100, Manuel Bouyer wrote:
> > On Thu, Jan 05, 2006 at 11:47:55PM -0500, Thor Lancelot Simon wrote:
> > 
> > Or read arbitrary memory. I fully understand that.
> > 
> > > It's not a point to gloss over, because the
> > > security implications are very real -- and the cost of using xbd for
> > > domU disks to mitigate the problem is, really, quite small.
> > 
> > Yes, they're real.?But it's still not exactly the same thing as giving
> > full access to dom0: you have to hack domU's the kernel first, in order
> > to execute code with kernel privileges.
> 
> I don't understand what you mean.  Given a malicious application
> running in dom0 (a "privileged" domain), it must "hack dom0's kernel
> first" in order to write arbitrary memory.
> 
> If you allow domU kernels to access PCI DMA engines, a malicious
> application running in such a domU (what people erroneously, once
> it has DMA access, call an "unprivileged" domain) must "hack domU's
> kernel first".
> 
> In both cases, a bug in the OS kernel running in the domain the
> malicious application is running in allows arbitrary memory to be
> written, including that belonging to other domains.
> 
> In other words, just as I originally said, if you grant an
> "unprivileged" domain access to a PCI device that does DMA, you no
> longer have a domain that is actually unprivileged at all -- a bug in
> its kernel has precisely the same consequences as a bug in the domain
> 0 kernel; the "privileged/unprivileged" distinction disappears.

Yes, of course, I understand that. However a bug in a domU application
gives you root in that domU; you then have to find a kernel bug to
gets to dom0. It's one more step than direct root in domain0.

> 
> This is why I strongly recommend against granting domains in which
> one intends to run any network-facing applications (or to which one
> intends to grant access to potentially untrustworthy users) access
> to any PCI device.  Use xbd instead: it is simple, enforces proper
> access controls, and imposes only a small performance penalty.

Sure. There are cases where the risk is acceptable though. It really depends
on the application.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--