Re: Hiding physical memory from OS and VT-d/IOMMU

[Andrew Fish <afish@apple.com>]

> On Mar 3, 2017, at 8:32 AM, Tomas Pilar (tpilar) <tpilar@solarflare.com> wrote:
> 
> Hi,
> 
> 
> I am trying to implement a message-box communication protocol between
> PCIe devices in the same host without the assistance of the OS. For
> irreducible reasons, I can't use PCIe endpoint-to-endpoint communication
> so I thought I could create a DMA based message-box protocol where in
> UEFI driver probe (during DXE) I blot out some physical memory (by
> leaking a page of memory allocated as EfiRuntimeServicesData) that the
> devices will then use to communicate even when the OS loads.
> 
> 
> This runs into a problem when VT-d/IOMMU is involved because it still
> stops the device from DMA into that page, even though the OS shouldn't
> touch the page as it's been allocated using EfiRuntimeServicesData.
> 
> 
> So my query is: Can I achieve this by allocating the box as a different
> memory type (such as EfiUnusableMemory or EfiReservedMemoryType) and if
> not, what would be a better way of doing this?
> 

No VT-d is designed to stop attacks like yours. By default VT-d blocks all DMA, and only allows DMA when it is properly requested by an OS driver that is trusted. 

You need an OS driver to map the DMA region for use by the PCI devices prior to using it on a system with VT-d enabled. If there was a way around this that would be a security bug in the IOMMU that an attacker could exploit.

Thanks,

Andrew Fish

> 
> Cheers,
> 
> Tom
> 
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