Re: [PATCH v7 7/7] KVM: SEV: Add SEV-SNP CipherTextHiding support

[Kim Phillips <kim.phillips@amd.com>]

Hi Ashish,

For patches 1 through 6 in this series:

Reviewed-by: Kim Phillips <kim.phillips@amd.com>

For this 7/7 patch, consider making the simplification changes I've supplied
in the diff at the bottom of this email: it cuts the number of lines for
check_and_enable_sev_snp_ciphertext_hiding() in half.

Thanks,

Kim

On 7/21/25 9:14 AM, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@amd.com>
>
> Ciphertext hiding prevents host accesses from reading the ciphertext of
> SNP guest private memory. Instead of reading ciphertext, the host reads
> will see constant default values (0xff).
>
> The SEV ASID space is split into SEV and SEV-ES/SEV-SNP ASID ranges.
> Enabling ciphertext hiding further splits the SEV-ES/SEV-SNP ASID space
> into separate ASID ranges for SEV-ES and SEV-SNP guests.
>
> Add new module parameter to the KVM module to enable ciphertext hiding
> support and a user configurable system-wide maximum SNP ASID value. If
> the module parameter value is "max" then the complete SEV-ES/SEV-SNP
> ASID space is allocated to SEV-SNP guests.
>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> ---
>   .../admin-guide/kernel-parameters.txt         | 18 ++++++
>   arch/x86/kvm/svm/sev.c                        | 60 ++++++++++++++++++-
>   2 files changed, 77 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index eb2fab9bd0dc..379350d7ae19 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2942,6 +2942,24 @@
>   			(enabled). Disable by KVM if hardware lacks support
>   			for NPT.
>   
> +	kvm-amd.ciphertext_hiding_asids=
> +			[KVM,AMD] Ciphertext hiding prevents host accesses from reading
> +			the ciphertext of SNP guest private memory. Instead of reading
> +			ciphertext, the host will see constant default values (0xff).
> +			The SEV ASID space is split into SEV and joint SEV-ES and SEV-SNP
> +			ASID space. Ciphertext hiding further partitions the joint
> +			SEV-ES/SEV-SNP ASID space into separate SEV-ES and SEV-SNP ASID
> +			ranges with the SEV-SNP ASID range starting at 1. For SEV-ES/
> +			SEV-SNP guests the maximum ASID available is MIN_SEV_ASID - 1
> +			where MIN_SEV_ASID value is discovered by CPUID Fn8000_001F[EDX].
> +
> +			Format: { <unsigned int> | "max" }
> +			A non-zero value enables SEV-SNP ciphertext hiding feature and sets
> +			the ASID range available for SEV-SNP guests.
> +			A Value of "max" assigns all ASIDs available in the joint SEV-ES
> +			and SEV-SNP ASID range to SNP guests, effectively disabling
> +			SEV-ES.
> +
>   	kvm-arm.mode=
>   			[KVM,ARM,EARLY] Select one of KVM/arm64's modes of
>   			operation.
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index b5f4e69ff579..7ac0f0f25e68 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -59,6 +59,11 @@ static bool sev_es_debug_swap_enabled = true;
>   module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0444);
>   static u64 sev_supported_vmsa_features;
>   
> +static char ciphertext_hiding_asids[16];
> +module_param_string(ciphertext_hiding_asids, ciphertext_hiding_asids,
> +		    sizeof(ciphertext_hiding_asids), 0444);
> +MODULE_PARM_DESC(ciphertext_hiding_asids, "  Enable ciphertext hiding for SEV-SNP guests and specify the number of ASIDs to use ('max' to utilize all available SEV-SNP ASIDs");
> +
>   #define AP_RESET_HOLD_NONE		0
>   #define AP_RESET_HOLD_NAE_EVENT		1
>   #define AP_RESET_HOLD_MSR_PROTO		2
> @@ -201,6 +206,9 @@ static int sev_asid_new(struct kvm_sev_info *sev, unsigned long vm_type)
>   	/*
>   	 * The min ASID can end up larger than the max if basic SEV support is
>   	 * effectively disabled by disallowing use of ASIDs for SEV guests.
> +	 * Similarly for SEV-ES guests the min ASID can end up larger than the
> +	 * max when ciphertext hiding is enabled, effectively disabling SEV-ES
> +	 * support.
>   	 */
>   	if (min_asid > max_asid)
>   		return -ENOTTY;
> @@ -2269,6 +2277,7 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn_start, kvm_pfn_t pf
>   				ret = -EFAULT;
>   				goto err;
>   			}
> +
>   			kunmap_local(vaddr);
>   		}
>   
> @@ -2959,6 +2968,46 @@ static bool is_sev_snp_initialized(void)
>   	return initialized;
>   }
>   
> +static bool check_and_enable_sev_snp_ciphertext_hiding(void)
> +{
> +	unsigned int ciphertext_hiding_asid_nr = 0;
> +
> +	if (!ciphertext_hiding_asids[0])
> +		return false;
> +
> +	if (!sev_is_snp_ciphertext_hiding_supported()) {
> +		pr_warn("Module parameter ciphertext_hiding_asids specified but ciphertext hiding not supported\n");
> +		return false;
> +	}
> +
> +	if (isdigit(ciphertext_hiding_asids[0])) {
> +		if (kstrtoint(ciphertext_hiding_asids, 10, &ciphertext_hiding_asid_nr))
> +			goto invalid_parameter;
> +
> +		/* Do sanity check on user-defined ciphertext_hiding_asids */
> +		if (ciphertext_hiding_asid_nr >= min_sev_asid) {
> +			pr_warn("Module parameter ciphertext_hiding_asids (%u) exceeds or equals minimum SEV ASID (%u)\n",
> +				ciphertext_hiding_asid_nr, min_sev_asid);
> +			return false;
> +		}
> +	} else if (!strcmp(ciphertext_hiding_asids, "max")) {
> +		ciphertext_hiding_asid_nr = min_sev_asid - 1;
> +	}
> +
> +	if (ciphertext_hiding_asid_nr) {
> +		max_snp_asid = ciphertext_hiding_asid_nr;
> +		min_sev_es_asid = max_snp_asid + 1;
> +		pr_info("SEV-SNP ciphertext hiding enabled\n");
> +
> +		return true;
> +	}
> +
> +invalid_parameter:
> +	pr_warn("Module parameter ciphertext_hiding_asids (%s) invalid\n",
> +		ciphertext_hiding_asids);
> +	return false;
> +}
> +
>   void __init sev_hardware_setup(void)
>   {
>   	unsigned int eax, ebx, ecx, edx, sev_asid_count, sev_es_asid_count;
> @@ -3068,6 +3117,13 @@ void __init sev_hardware_setup(void)
>   out:
>   	if (sev_enabled) {
>   		init_args.probe = true;
> +		/*
> +		 * The ciphertext hiding feature partitions the joint SEV-ES/SEV-SNP
> +		 * ASID range into separate SEV-ES and SEV-SNP ASID ranges with
> +		 * the SEV-SNP ASID starting at 1.
> +		 */
> +		if (check_and_enable_sev_snp_ciphertext_hiding())
> +			init_args.max_snp_asid = max_snp_asid;
>   		if (sev_platform_init(&init_args))
>   			sev_supported = sev_es_supported = sev_snp_supported = false;
>   		else if (sev_snp_supported)
> @@ -3082,7 +3138,9 @@ void __init sev_hardware_setup(void)
>   			min_sev_asid, max_sev_asid);
>   	if (boot_cpu_has(X86_FEATURE_SEV_ES))
>   		pr_info("SEV-ES %s (ASIDs %u - %u)\n",
> -			str_enabled_disabled(sev_es_supported),
> +			sev_es_supported ? min_sev_es_asid <= max_sev_es_asid ? "enabled" :
> +										"unusable" :
> +										"disabled",
>   			min_sev_es_asid, max_sev_es_asid);
>   	if (boot_cpu_has(X86_FEATURE_SEV_SNP))
>   		pr_info("SEV-SNP %s (ASIDs %u - %u)\n",


diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 7ac0f0f25e68..bd0947360e18 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -59,7 +59,7 @@ static bool sev_es_debug_swap_enabled = true;
  module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0444);
  static u64 sev_supported_vmsa_features;

-static char ciphertext_hiding_asids[16];
+static char ciphertext_hiding_asids[10];
  module_param_string(ciphertext_hiding_asids, ciphertext_hiding_asids,
              sizeof(ciphertext_hiding_asids), 0444);
  MODULE_PARM_DESC(ciphertext_hiding_asids, "  Enable ciphertext hiding 
for SEV-SNP guests and specify the number of ASIDs to use ('max' to 
utilize all available SEV-SNP ASIDs");
@@ -2970,42 +2970,22 @@ static bool is_sev_snp_initialized(void)

  static bool check_and_enable_sev_snp_ciphertext_hiding(void)
  {
-    unsigned int ciphertext_hiding_asid_nr = 0;
-
-    if (!ciphertext_hiding_asids[0])
-        return false;
-
-    if (!sev_is_snp_ciphertext_hiding_supported()) {
-        pr_warn("Module parameter ciphertext_hiding_asids specified but 
ciphertext hiding not supported\n");
-        return false;
-    }
-
-    if (isdigit(ciphertext_hiding_asids[0])) {
-        if (kstrtoint(ciphertext_hiding_asids, 10, 
&ciphertext_hiding_asid_nr))
-            goto invalid_parameter;
-
-        /* Do sanity check on user-defined ciphertext_hiding_asids */
-        if (ciphertext_hiding_asid_nr >= min_sev_asid) {
-            pr_warn("Module parameter ciphertext_hiding_asids (%u) 
exceeds or equals minimum SEV ASID (%u)\n",
-                ciphertext_hiding_asid_nr, min_sev_asid);
-            return false;
-        }
-    } else if (!strcmp(ciphertext_hiding_asids, "max")) {
-        ciphertext_hiding_asid_nr = min_sev_asid - 1;
+    if (!strcmp(ciphertext_hiding_asids, "max")) {
+        max_snp_asid = min_sev_asid - 1;
+        return true;
      }

-    if (ciphertext_hiding_asid_nr) {
-        max_snp_asid = ciphertext_hiding_asid_nr;
-        min_sev_es_asid = max_snp_asid + 1;
-        pr_info("SEV-SNP ciphertext hiding enabled\n");
-
-        return true;
+    /* Do sanity check on user-defined ciphertext_hiding_asids */
+    if (kstrtoint(ciphertext_hiding_asids, 
sizeof(ciphertext_hiding_asids), &max_snp_asid) ||
+        max_snp_asid >= min_sev_asid ||
+        !sev_is_snp_ciphertext_hiding_supported()) {
+        pr_warn("ciphertext_hiding not supported, or invalid 
ciphertext_hiding_asids \"%s\", or !(0 < %u < minimum SEV ASID %u)\n",
+            ciphertext_hiding_asids, max_snp_asid, min_sev_asid);
+        max_snp_asid = min_sev_asid - 1;
+        return false;
      }

-invalid_parameter:
-    pr_warn("Module parameter ciphertext_hiding_asids (%s) invalid\n",
-        ciphertext_hiding_asids);
-    return false;
+    return true;
  }

  void __init sev_hardware_setup(void)
@@ -3122,8 +3102,11 @@ void __init sev_hardware_setup(void)
           * ASID range into separate SEV-ES and SEV-SNP ASID ranges with
           * the SEV-SNP ASID starting at 1.
           */
-        if (check_and_enable_sev_snp_ciphertext_hiding())
+        if (check_and_enable_sev_snp_ciphertext_hiding()) {
+            pr_info("SEV-SNP ciphertext hiding enabled\n");
              init_args.max_snp_asid = max_snp_asid;
+            min_sev_es_asid = max_snp_asid + 1;
+        }
          if (sev_platform_init(&init_args))
              sev_supported = sev_es_supported = sev_snp_supported = false;
          else if (sev_snp_supported)