linux-kernel - Re: [PATCH v4 09/15] x86/sev: Provide guest VMPL level to userspace

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Message-ID: <20240527135101.GBZlSPxQ6iBSlMDefY@fat_crate.local>
Date: Mon, 27 May 2024 15:51:01 +0200
From: Borislav Petkov <bp@...en8.de>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-coco@...ts.linux.dev, svsm-devel@...onut-svsm.dev,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Michael Roth <michael.roth@....com>,
	Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v4 09/15] x86/sev: Provide guest VMPL level to userspace

On Wed, Apr 24, 2024 at 10:58:05AM -0500, Tom Lendacky wrote:
> Requesting an attestation report from userspace involves providing the
> VMPL level for the report. Currently any value from 0-3 is valid because
> Linux enforces running at VMPL0.
> 
> When an SVSM is present, though, Linux will not be running at VMPL0 and
> only VMPL values starting at the VMPL level Linux is running at to 3 are
> valid. In order to allow userspace to determine the minimum VMPL value
> that can be supplied to an attestation report, create a sysfs entry that
> can be used to retrieve the current VMPL level of Linux.

So what is the use case here: you create the attestation report *on* the
running guest and as part of that, the script which does that should do

cat /sys/.../sev/vmpl

?

But then sev-guest does some VMPL including into some report:

struct snp_report_req {
        /* user data that should be included in the report */
        __u8 user_data[SNP_REPORT_USER_DATA_SIZE];

        /* The vmpl level to be included in the report */
        __u32 vmpl;

Why do you need this and can't use sev-guest?

> +static int __init sev_sysfs_init(void)
> +{
> +	struct kobject *sev_kobj;
> +	int ret;
> +
> +	if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
> +		return -ENODEV;
> +
> +	sev_kobj = kobject_create_and_add("sev", kernel_kobj);

In the main hierarchy?!

This is a x86 CPU thing, so if anything, it should be under
/sys/devices/system/cpu/

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette