linux-kernel - Re: [PATCH 1/9] iommu/amd: Introduce helper functions for managing IOMMU memory

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Date: Tue, 14 May 2024 01:59:33 +0700
From: "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev, joro@...tes.org,
 thomas.lendacky@....com, vasant.hegde@....com, michael.roth@....com,
 jon.grimm@....com, rientjes@...gle.com
Subject: Re: [PATCH 1/9] iommu/amd: Introduce helper functions for managing
 IOMMU memory

Jason

On 5/1/2024 11:17 PM, Jason Gunthorpe wrote:
> On Tue, Apr 30, 2024 at 03:24:22PM +0000, Suravee Suthikulpanit wrote:
>> Depending on the modes of operation, certain AMD IOMMU data structures are
>> allocated with constraints. For example:
>>
>>   * Some buffers must be 4K-aligned when running in SNP-enabled host
>>
>>   * To support AMD IOMMU emulation in an SEV guest, some data structures
>>     cannot be encrypted so that the VMM can access the memory successfully.
> 
> Uh, this seems like a really bad idea. The VM's integrity strongly
> depends on the correct function of the HW. If the IOMMU datastructures
> are not protected then the whole thing is not secure.
> 
> For instance allowing hostile VMs to manipulate the DTE, or interfere
> with the command queue, destroys any possibility to have secure DMA.

Currently, we have already set the area used for guest SWIOTLB region as 
shared memory to support DMA in SEV guest. Here, we are setting 
additional guest IOMMU data structures as shared:

* Device Table
* Command Buffer
* Completion-Wait Semaphore Buffer
* Per-device Interrupt Remapping Table

, which are necessary for QEMU interrupt remapping emulation. Therefore,
we are not making the VM any less secure from device perspective.

> Is this some precursor to implementing a secure iommu where the data
> structures will remain encrypted? 

Yes, the is precursor to secure vIOMMU support in the guest.

> What is even the point of putting a non-secure viommu into a SEV guest anyhow?

This is needed to provide interrupt remapping support for vcpu with 
x2APIC ID (> 255) in the guest, which is already available w/ 
QEMU-emulated AMD vIOMMU.

Thanks,
Suravee