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.
Signed-off-by: Tom Lendacky <[email protected]>
---
arch/x86/kernel/sev.c | 37 +++++++++++++++++++++++++++++++++++++
1 file changed, 37 insertions(+)
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 121a9bad86c9..9844c772099c 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -2479,3 +2479,40 @@ void __init snp_remap_svsm_ca(void)
/* Update the CAA to a proper kernel address */
boot_svsm_caa = &boot_svsm_ca_page;
}
+
+static ssize_t vmpl_show(struct kobject *kobj,
+ struct kobj_attribute *attr, char *buf)
+{
+ return sysfs_emit(buf, "%d\n", vmpl);
+}
+
+static struct kobj_attribute vmpl_attr = __ATTR_RO(vmpl);
+
+static struct attribute *vmpl_attrs[] = {
+ &vmpl_attr.attr,
+ NULL
+};
+
+static struct attribute_group sev_attr_group = {
+ .attrs = vmpl_attrs,
+};
+
+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);
+ if (!sev_kobj)
+ return -ENOMEM;
+
+ ret = sysfs_create_group(sev_kobj, &sev_attr_group);
+ if (ret)
+ kobject_put(sev_kobj);
+
+ return ret;
+}
+arch_initcall(sev_sysfs_init);
--
2.42.0
On Fri, Jan 26, 2024 at 2:19 PM Tom Lendacky <[email protected]> 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.
Is this not the intended meaning of privlevel_floor in
/sys/kernel/config/tsm/report/$report0/privlevel_floor?
>
> Signed-off-by: Tom Lendacky <[email protected]>
> ---
> arch/x86/kernel/sev.c | 37 +++++++++++++++++++++++++++++++++++++
> 1 file changed, 37 insertions(+)
>
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 121a9bad86c9..9844c772099c 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2479,3 +2479,40 @@ void __init snp_remap_svsm_ca(void)
> /* Update the CAA to a proper kernel address */
> boot_svsm_caa = &boot_svsm_ca_page;
> }
> +
> +static ssize_t vmpl_show(struct kobject *kobj,
> + struct kobj_attribute *attr, char *buf)
> +{
> + return sysfs_emit(buf, "%d\n", vmpl);
> +}
> +
> +static struct kobj_attribute vmpl_attr = __ATTR_RO(vmpl);
> +
> +static struct attribute *vmpl_attrs[] = {
> + &vmpl_attr.attr,
> + NULL
> +};
> +
> +static struct attribute_group sev_attr_group = {
> + .attrs = vmpl_attrs,
> +};
> +
> +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);
> + if (!sev_kobj)
> + return -ENOMEM;
> +
> + ret = sysfs_create_group(sev_kobj, &sev_attr_group);
> + if (ret)
> + kobject_put(sev_kobj);
> +
> + return ret;
> +}
> +arch_initcall(sev_sysfs_init);
> --
> 2.42.0
>
>
--
-Dionna Glaze, PhD (she/her)
On 1/26/24 19:06, Dionna Amalie Glaze wrote:
> On Fri, Jan 26, 2024 at 2:19 PM Tom Lendacky <[email protected]> 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.
>
> Is this not the intended meaning of privlevel_floor in
> /sys/kernel/config/tsm/report/$report0/privlevel_floor?
Hmmm... possibly. But that would make someone using the ioctl() (which is
still available) have to use the config-tsm support to get the value. If
the overall consensus is not to have a sysfs entry, I'll remove it, but it
could be useful beyond just attestation.
Your comment does make me realize that I did miss changing privlevel_floor
for the TSM support. I need to set privlevel_floor to the current VMPL level.
Thanks,
Tom
>