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Message-ID: <c39af745-35b4-6201-be05-80b19636381b@amd.com>
Date: Tue, 15 Jul 2025 11:44:47 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Ashish Kalra <Ashish.Kalra@....com>, corbet@....net, seanjc@...gle.com,
pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
john.allen@....com, herbert@...dor.apana.org.au, davem@...emloft.net,
akpm@...ux-foundation.org, rostedt@...dmis.org, paulmck@...nel.org
Cc: nikunj@....com, Neeraj.Upadhyay@....com, aik@....com, ardb@...nel.org,
michael.roth@....com, arnd@...db.de, linux-doc@...r.kernel.org,
linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org
Subject: Re: [PATCH v6 7/7] KVM: SEV: Add SEV-SNP CipherTextHiding support
On 7/14/25 17:41, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@....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@...gle.com>
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
Just a minor comment below in case you need to submit another version.
> ---
> .../admin-guide/kernel-parameters.txt | 18 ++++++
> arch/x86/kvm/svm/sev.c | 61 ++++++++++++++++++-
> 2 files changed, 78 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 6305257de698..de086bfd7e27 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 77f7c103134e..b795ec988025 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;
> @@ -2258,6 +2266,7 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn_start, kvm_pfn_t pf
> ret = -EFAULT;
> goto err;
> }
> +
> kunmap_local(vaddr);
> }
>
> @@ -2938,10 +2947,48 @@ 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 (!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;
> struct sev_platform_init_args init_args = {0};
> + bool snp_ciphertext_hiding_enabled = false;
> bool sev_snp_supported = false;
> bool sev_es_supported = false;
> bool sev_supported = false;
> @@ -3039,6 +3086,14 @@ void __init sev_hardware_setup(void)
> min_sev_es_asid = min_snp_asid = 1;
> max_sev_es_asid = max_snp_asid = min_sev_asid - 1;
>
> + /*
> + * 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 (ciphertext_hiding_asids[0])
> + snp_ciphertext_hiding_enabled = check_and_enable_sev_snp_ciphertext_hiding();
This check could be in check_and_enable_sev_snp_ciphertext_hiding().
That would eliminate having to initialize snp_ciphertext_hiding_enabled
to false and keep all the logic related to the parameter in a single
function.
Thanks,
Tom
> +
> sev_es_asid_count = min_sev_asid - 1;
> WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV_ES, sev_es_asid_count));
> sev_es_supported = true;
> @@ -3047,6 +3102,8 @@ void __init sev_hardware_setup(void)
> out:
> if (sev_enabled) {
> init_args.probe = true;
> + if (snp_ciphertext_hiding_enabled)
> + 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)
> @@ -3061,7 +3118,9 @@ 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",
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