linux-kernel - Re: [PATCH v2] virt: tdx-guest: Handle GetQuote request error code

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Date: Thu, 22 Feb 2024 22:18:13 -0800
From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: Dan Williams <dan.j.williams@...el.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, x86@...nel.org
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
 Xiaoyao Li <xiaoyao.li@...el.com>, linux-kernel@...r.kernel.org,
 linux-coco@...ts.linux.dev
Subject: Re: [PATCH v2] virt: tdx-guest: Handle GetQuote request error code


On 2/22/24 9:48 PM, Dan Williams wrote:
> Kuppuswamy Sathyanarayanan wrote:
>> During the TDX guest attestation process, TSM ConfigFS ABI is used by
>> the user attestation agent to get the signed VM measurement data (a.k.a
>> Quote), which can be used by a remote verifier to validate the
>> trustworthiness of the guest. When a user requests for the Quote data
>> via the ConfigFS ABI, the TDX Quote generation handler
>> (tdx_report_new()) forwards the request to VMM (or QE) via a hypercall,
>> and then shares the output with the user.
>>
>> Currently, when handling the Quote generation request, tdx_report_new()
>> handler only checks whether the VMM successfully processed the request
>> and if it is true it returns success and shares the output to the user
>> without actually validating the output data. Since the VMM can return
>> error even after processing the Quote request, always returning success
>> for the processed requests is incorrect and will create confusion to
>> the user. Although for the failed request, output buffer length will
>> be zero and can also be used by the user to identify the failure case,
>> it will be more clear to return error for all failed cases.
> This is a lot of text. More is not necessarily better.
>
> ---
> The tdx-guest driver marshals requests via hypercall to have a quoting
> enclave sign attestation evidence about the current state of the TD.
> There are 2 possible failures, a transport failure (failure to
> communicate with the quoting agent) and payload failure (a failed
> quote). The driver only checks the former, update it to consider the
> latter payload errors as well.
> ---

Looks better. I will use it in next version.

>
>
>> Validate the Quote data output status and return error code for all
>> failed cases.
>>
>> Fixes: f4738f56d1dc ("virt: tdx-guest: Add Quote generation support using TSM_REPORTS")
>> Reported-by: Xiaoyao Li <xiaoyao.li@...el.com>
>> Closes: https://lore.kernel.org/linux-coco/6bdf569c-684a-4459-af7c-4430691804eb@linux.intel.com/T/#u
>> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
>> ---
>>
>> Changes since v1:
>>  * Updated the commit log (Kirill)
>>
>>  drivers/virt/coco/tdx-guest/tdx-guest.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/drivers/virt/coco/tdx-guest/tdx-guest.c b/drivers/virt/coco/tdx-guest/tdx-guest.c
>> index 1253bf76b570..61368318fa39 100644
>> --- a/drivers/virt/coco/tdx-guest/tdx-guest.c
>> +++ b/drivers/virt/coco/tdx-guest/tdx-guest.c
>> @@ -228,6 +228,12 @@ static int tdx_report_new(struct tsm_report *report, void *data)
>>  		goto done;
>>  	}
>>  
>> +	if (quote_buf->status != GET_QUOTE_SUCCESS) {
>> +		pr_err("GetQuote request failed, ret %llx\n", quote_buf->status);
> Do you really want to spam the log on every error? I would expect
> pr_err() for events that are fatal to driver operation that might
> indicate conditions where maybe the TD should give up on the host.
>
> Yes, there are other pr_err() in this function and I am kicking myself
> for not scrutinizing those more closely. It is likely enough to
> distinguish transport errors vs payload / quote errors with ENXIO and
> EIO.
>
> Otherwise if there is an exceedingly good reason to keep this driver
> chirping into the kernel log then these likely also want to be
> rate-limited. If they are "just in case" debug messages, then move them
> to pr_debug().

Ok. Makes sense. I will convert it into a pr_debug.

I will submit a separate patch to fix other pr_err usage in this driver.
Expect Quote timeout, the rest of the failure does not affect the
usage of the driver.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer