Re: [coreboot] DMA protection? [AMD-Vi]

[Timothy Pearson]

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On 11/20/2016 05:56 PM, ron minnich wrote:
> man. Most of these BME things revolve around intel. Not surprising, but
> not good.
>
> Does anyone care about the realtek 8168? And why on earth does it need
> BME? Can it just be initialized but not have DMA enabled?
>
> I wonder if we should scan for anything with BME set, at each stage
> transition, and print a warning for each one found?

A quick check through the source seems to indicate that the generic
pci_set_resource function will enable bus mastering on any PCI bridges.
 From that point on, if I'm not mistaken, any malicious device that
exposed a bridge interface could enable mastering for any logical
devices behind the bridge and attack the host.  Am I missing something?

Thanks!

- -- 
Timothy Pearson
Raptor Engineering
[phone] (direct line)
[phone] (switchboard)
https://www.raptorengineering.com
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DMA protection? [AMD-Vi]

[ron minnich]

On Mon, Nov 21, 2016 at 7:53 AM Timothy Pearson <
[e-mail]> wrote:
> A quick check through the source seems to indicate that the generic
> pci_set_resource function will enable bus mastering on any PCI bridges.
>  From that point on, if I'm not mistaken, any malicious device that
> exposed a bridge interface could enable mastering for any logical
> devices behind the bridge and attack the host.  Am I missing something?

probably not. Back in the early days this kind of thing was not an issue.

We've always had to adjust for limits in the kernels we support. We have a
PCI subsystem mainly because, in 2000, linux could not handle an
unconfigured PCI bus -- it interpreted a "0 bar" as meaning "device
disabled by BIOS" -- really!

I suspect the BME enable on bridges was done because Linux or other guests
didn't know how to configure bridges correctly. But Linux and other kernels
are a lot better now than they were; I wonder if we should stop enabling
BME on bridges.

In any event, however, if we make this change it should be done in small
steps, and I think a good first small step is to start with things that
*look* obvious, like the aforementioned NIC. I am going to submit a CL
today to remove BME from that and see how much upset it causes :-)

ron