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Message-ID: <9132edc0-1bc2-440a-ac90-64ed13d3c30c@amd.com>
Date: Fri, 25 Jul 2025 12:58:24 -0500
From: Kim Phillips <kim.phillips@....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>, <thomas.lendacky@....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 v7 7/7] KVM: SEV: Add SEV-SNP CipherTextHiding support
Hi Ashish,
For patches 1 through 6 in this series:
Reviewed-by: Kim Phillips <kim.phillips@....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@....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>
> Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
> Signed-off-by: Ashish Kalra <ashish.kalra@....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)
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