linux-kernel - Re: [PATCH v3 10/14] configfs-tsm: Allow the privlevel

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Date: Tue, 16 Apr 2024 11:17:38 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Dan Williams <dan.j.williams@...el.com>, linux-kernel@...r.kernel.org,
 x86@...nel.org, linux-coco@...ts.linux.dev, svsm-devel@...onut-svsm.dev
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Michael Roth <michael.roth@....com>,
 Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v3 10/14] configfs-tsm: Allow the privlevel_floor
 attribute to be updated

On 4/16/24 10:57, Dan Williams wrote:
> Tom Lendacky wrote:
>> On 4/15/24 23:55, Dan Williams wrote:
>>> Tom Lendacky wrote:
>>>> With the introduction of an SVSM, Linux will be running at a non-zero
>>>> VMPL. Any request for an attestation report at a higher priviledge VMPL
>>>> than what Linux is currently running will result in an error. Allow for
>>>> the privlevel_floor attribute to be updated dynamically so that the
>>>> attribute may be set dynamically.
>>>>
>>>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
>>>> ---
>>>>    drivers/virt/coco/sev-guest/sev-guest.c | 5 ++++-
>>>>    include/linux/tsm.h                     | 2 +-
>>>>    2 files changed, 5 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
>>>> index 1ff897913bf4..bba6531cb606 100644
>>>> --- a/drivers/virt/coco/sev-guest/sev-guest.c
>>>> +++ b/drivers/virt/coco/sev-guest/sev-guest.c
>>>> @@ -885,7 +885,7 @@ static int sev_report_new(struct tsm_report *report, void *data)
>>>>    	return 0;
>>>>    }
>>>>    
>>>> -static const struct tsm_ops sev_tsm_ops = {
>>>> +static struct tsm_ops sev_tsm_ops = {
>>>>    	.name = KBUILD_MODNAME,
>>>>    	.report_new = sev_report_new,
>>>>    };
>>>> @@ -972,6 +972,9 @@ static int __init sev_guest_probe(struct platform_device *pdev)
>>>>    	snp_dev->input.resp_gpa = __pa(snp_dev->response);
>>>>    	snp_dev->input.data_gpa = __pa(snp_dev->certs_data);
>>>>    
>>>> +	/* Set the privlevel_floor attribute based on the current VMPL */
>>>> +	sev_tsm_ops.privlevel_floor = snp_get_vmpl();
>>>
>>> Why is this not vmpck_id?
>>
>> Good catch, this probably should be pulled out separately and submitted
>> as a Fixes: against the current support. If you think it's important
>> enough, I can do that and put this at the beginning of the series. Or I
>> can just modify this to use the vmpck_id value. Any preference?
> 
> I dunno, you tell me. What breaks if privlevel_floor is mismatched vs
> vmpl and/or vmpck_id? If it warrants a "Fixes:" it should probably be
> broken out.
> 
> However, I *guess* it is just adding some sanity checking precision to
> userspace requests and makes some input validation not catch errors when
> userspace tries to generate reports from the wrong level, right? I.e.
> privlevel_floor may be lower than expected, but userspace should not be
> depending on that since the report generation will fail.

Yeah, it just results in a different type of error. If the VMPL 
specified by the user is numerically lower than the vmpck_id, then the 
request will fail with a specific return code value. With the change to 
privlevel_floor to use vmpck_id, then you would just get the error that 
much sooner when trying to set a value that is lower than the floor.

Since I don't think the vmpck_id module parameter is a common case 
today, let's just leave that change in this patch.

Thanks,
Tom