Re: [PATCH v3 4/4] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver

Sathyanarayanan Kuppuswamy
2022-04-20T23:45:57Z

Hi,

On 4/20/22 4:18 PM, Kai Huang wrote:
> On Fri, 2022-04-15 at 15:01 -0700, Kuppuswamy Sathyanarayanan wrote:
>> TDX guest supports encrypted disk as root or secondary drives.
>> Decryption keys required to access such drives are usually maintained
>> by 3rd party key servers. Attestation is required by 3rd party key
>> servers to get the key for an encrypted disk volume, or possibly other
>> encrypted services. Attestation is used to prove to the key server that
>> the TD guest is running in a valid TD and the kernel and virtual BIOS
>> and other environment are secure.
>>
>> During the boot process various components before the kernel accumulate
>> hashes in the TDX module, which can then combined into a report. This
>> would typically include a hash of the bios, bios configuration, boot
>> loader, command line, kernel, initrd. After checking the hashes the
>> key server will securely release the keys.
>>
>> The actual details of the attestation protocol depend on the particular
>> key server configuration, but some parts are common and need to
>> communicate with the TDX module.
>
> As we discussed "key provisioning from key server to support disk decryption" is
> only one use case of attestation, so I don't think you need 3 paragraphs to talk
> about details of this use case here. The attestation flow is documented in GHCI

Not everyone understands the attestation context and usage. So I have
attempted to explain one of the use-case in detail.

> so it's clear. The attestation flow (and what this patch does) does not have
> any direct relation to the "disk decryption" details above. I think you can
> discard above entirely or using one or two simple sentences to explain.

Ok. I will try to summarize the details in few lines

>
> Also, as you agreed you will remove the 8K shared memory assumption:
>
> https://lore.kernel.org/lkml/20220415220109.282834-5-sathyanarayanan.kuppuswamy@linux.intel.com/T/

Yes. I have already removed this restriction. This will be part of my
next submission.

>
> and if you agree with my approach (again, I recommend) to split the driver to
> two parts (reorganize your patches essentially):

Yes. I have moved the GetQuote support to a new patch (but without new
config).

>
> https://lore.kernel.org/lkml/20220415220109.282834-5-sathyanarayanan.kuppuswamy@linux.intel.com/T/#m9e3c5115df0be0b53d41f987e1eda1715255d1d8
>
> I'll review again once you finish updating the driver.

Thanks.

>
> Btw some minor comments below.
>
>
> [...]
>
>> --- /dev/null
>> +++ b/drivers/platform/x86/intel/tdx/intel_tdx_attest.c
>
> Perhaps attest.c is enough, no matter where the file will reside.

Noted. I will change it.

>
>> @@ -0,0 +1,302 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * intel_tdx_attest.c - TDX guest attestation interface driver.
>> + *
>> + * Implements user interface to trigger attestation process and
>> + * read the TD Quote result.
>> + *
>> + * Copyright (C) 2021-2022 Intel Corporation
>
> For upstream I guess just need 2022.
>
> [...]

Noted. I will change it.

>
>> +struct attest_dev {
>> + /* Mutex to serialize attestation requests */
>> + struct mutex lock;
>
> I think we need a comment to explain why the driver doesn't support multiple
> GetQuote requests in parallel. In fact the updated GHCI spec doesn't explicitly
> say GetQuote cannot be done in parallel. It has a new "GET_QUOTE_IN_FLIGHT"
> flag introduced, which can be used to determine which Quote is finished I think.
>
> I am fine with only supporting GetQuote in serialized way, but perhaps we need
> to call out the reason somewhere.

If we want to support multiple GetQuote requests in parallel, then we
need some way to uniquely identify the GetQuote requests. So that when
we get completion notification, we can understand which request is
completed. This part is not mentioned/discussed in ABI spec. So we want
to serialize the requests for now.

I will include the details about it in the commit log.

>
> [...]
>
>> +
>> + /* Allocate DMA buffer to get TDQUOTE data from the VMM */
>> + adev->tdquote_buf = dma_alloc_coherent(dev, GET_QUOTE_MAX_SIZE,
>> + &adev->handle,
>> + GFP_KERNEL | __GFP_ZERO);
>> + if (!adev->tdquote_buf) {
>> + ret = -ENOMEM;
>> + goto failed;
>> + }
>
> The buffer needs to be shared. Guest must have called MapGPA to convert the
> buffer to shared. Is this guaranteed by calling dma_alloc_coherent(), since
> seems I didn't see any MapGPA in the driver? Anyway this deserves a comment I
> think.

Yes. It is taken care by dma_alloc_*() API. DMA memory is marked by
default as shared in TDX guest. I will add comment about it.

>
>

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer