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

From: "Tomas Pilar (tpilar)" <tpilar@solarflare.com>
To: Andrew Fish <afish@apple.com>
Cc: "edk2-devel@lists.01.org" <edk2-devel@lists.01.org>
Subject: Re: Hiding physical memory from OS and VT-d/IOMMU
Date: Fri, 3 Mar 2017 18:03:15 +0000	[thread overview]
Message-ID: <f9d160f5-5768-fa38-9fe8-f189951d1346@solarflare.com> (raw)
In-Reply-To: <8239214C-DAB8-4CED-A683-90DB1AA0DE47@apple.com>

Yeah, I was worried about this.

Thanks,

Tom

On 03/03/17 17:06, Andrew Fish wrote:
>> 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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