linux-kernel - Re: [PATCH V3 02/11] PCI: Lock down BAR access when module security is enabled

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Date:	Wed, 4 Sep 2013 19:01:54 +0000
From:	Matthew Garrett <matthew.garrett@...ula.com>
To:	David Woodhouse <dwmw2@...radead.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH V3 02/11] PCI: Lock down BAR access when module security
 is enabled

On Wed, 2013-09-04 at 19:58 +0100, David Woodhouse wrote:
> On Wed, 2013-09-04 at 17:04 +0000, Matthew Garrett wrote:

> > How does virt passthrough work in this case? The current situation
> > appears to be that qemu just passes the BARs through to the guest, and
> > it's the guest that sets things up. We'd need to be able to ensure that
> > there's no way the guest driver can cause DMA into the host kernel.
> 
> We set up the IOMMU page tables so that the virtual bus addresses seen
> by the PCI device are 1:1 mapped to the guest "physical" address space.
> 
> That is, what the PCI device sees as a "physical" address is equivalent
> to what the guest sees as a "physical" address space. It can access
> memory which belongs to that guest, and nothing else. So that should be
> fine.

But presumably the guest's view of RAM is what gets written to the BARs?
I guess if we know it's constrained then there's no need to restrict the
addresses that can be set - we know that they'll be unable to overlap
the host RAM.

-- 
Matthew Garrett <matthew.garrett@...ula.com>