Re: [PATCH v2 13/14] x86/sev: Hide SVSM attestation entries if not running under an SVSM

["Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@intel.com>]

On 3/8/24 10:35 AM, Tom Lendacky wrote:
> Config-fs provides support to hide individual attribute entries. Using
> this support, base the display of the SVSM related entries on the presence
> of an SVSM.
>
> Cc: Joel Becker <jlbec@evilplan.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/coco/core.c        |  4 ++++
>  drivers/virt/coco/tsm.c     | 13 +++++++++----
>  include/linux/cc_platform.h |  8 ++++++++
>  3 files changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> index d07be9d05cd0..efa0f648f754 100644
> --- a/arch/x86/coco/core.c
> +++ b/arch/x86/coco/core.c
> @@ -12,6 +12,7 @@
>  
>  #include <asm/coco.h>
>  #include <asm/processor.h>
> +#include <asm/sev.h>
>  
>  enum cc_vendor cc_vendor __ro_after_init = CC_VENDOR_NONE;
>  u64 cc_mask __ro_after_init;
> @@ -78,6 +79,9 @@ static bool noinstr amd_cc_platform_has(enum cc_attr attr)
>  	case CC_ATTR_GUEST_STATE_ENCRYPT:
>  		return sev_status & MSR_AMD64_SEV_ES_ENABLED;
>  
> +	case CC_ATTR_GUEST_SVSM_PRESENT:
> +		return snp_get_vmpl();
> +
>  	/*
>  	 * With SEV, the rep string I/O instructions need to be unrolled
>  	 * but SEV-ES supports them through the #VC handler.
> diff --git a/drivers/virt/coco/tsm.c b/drivers/virt/coco/tsm.c
> index 07b4c95ce704..2efa6e578477 100644
> --- a/drivers/virt/coco/tsm.c
> +++ b/drivers/virt/coco/tsm.c
> @@ -64,6 +64,11 @@ static struct tsm_report_state *to_state(struct tsm_report *report)
>  	return container_of(report, struct tsm_report_state, report);
>  }
>  
> +static umode_t svsm_visibility(const struct config_item *item, const struct configfs_attribute *attr)
> +{
> +	return cc_platform_has(CC_ATTR_GUEST_SVSM_PRESENT) ? attr->ca_mode : 0;
> +}
> +

Instead of directly checking for CC flags here, I am wondering if it would make
sense to add a callback to vendor drivers and let the callback decide whether
the attribute is valid or not? We can't add a CC flag for every ConfigFS attribute,
right? For example, privlevel is not used by TDX. If there is a callback, then
TDX driver can make this attribute invalid for it.

>  static int try_advance_write_generation(struct tsm_report *report)
>  {
>  	struct tsm_report_state *state = to_state(report);
> @@ -139,7 +144,7 @@ static ssize_t tsm_report_svsm_store(struct config_item *cfg,
>  
>  	return len;
>  }
> -CONFIGFS_ATTR_WO(tsm_report_, svsm);
> +CONFIGFS_ATTR_VISIBLE_WO(tsm_report_, svsm, svsm_visibility);
>  
>  static ssize_t tsm_report_service_guid_store(struct config_item *cfg,
>  					     const char *buf, size_t len)
> @@ -168,7 +173,7 @@ static ssize_t tsm_report_service_guid_store(struct config_item *cfg,
>  
>  	return len;
>  }
> -CONFIGFS_ATTR_WO(tsm_report_, service_guid);
> +CONFIGFS_ATTR_VISIBLE_WO(tsm_report_, service_guid, svsm_visibility);
>  
>  static ssize_t tsm_report_service_manifest_version_store(struct config_item *cfg,
>  							 const char *buf, size_t len)
> @@ -189,7 +194,7 @@ static ssize_t tsm_report_service_manifest_version_store(struct config_item *cfg
>  
>  	return len;
>  }
> -CONFIGFS_ATTR_WO(tsm_report_, service_manifest_version);
> +CONFIGFS_ATTR_VISIBLE_WO(tsm_report_, service_manifest_version, svsm_visibility);
>  
>  static ssize_t tsm_report_inblob_write(struct config_item *cfg,
>  				       const void *buf, size_t count)
> @@ -336,7 +341,7 @@ static ssize_t tsm_report_manifestblob_read(struct config_item *cfg, void *buf,
>  
>  	return tsm_report_read(report, buf, count, TSM_MANIFEST);
>  }
> -CONFIGFS_BIN_ATTR_RO(tsm_report_, manifestblob, NULL, TSM_OUTBLOB_MAX);
> +CONFIGFS_BIN_ATTR_VISIBLE_RO(tsm_report_, manifestblob, NULL, TSM_OUTBLOB_MAX, svsm_visibility);
>  
>  #define TSM_DEFAULT_ATTRS() \
>  	&tsm_report_attr_generation, \
> diff --git a/include/linux/cc_platform.h b/include/linux/cc_platform.h
> index cb0d6cd1c12f..f1b4266c1484 100644
> --- a/include/linux/cc_platform.h
> +++ b/include/linux/cc_platform.h
> @@ -90,6 +90,14 @@ enum cc_attr {
>  	 * Examples include TDX Guest.
>  	 */
>  	CC_ATTR_HOTPLUG_DISABLED,
> +
> +	/**
> +	 * @CC_ATTR_GUEST_SVSM_PRESENT: Guest is running under an SVSM
> +	 *
> +	 * The platform/OS is running as a guest/virtual machine and is
> +	 * running under a Secure VM Service Module (SVSM).
> +	 */
> +	CC_ATTR_GUEST_SVSM_PRESENT,
>  };
>  
>  #ifdef CONFIG_ARCH_HAS_CC_PLATFORM

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer