Commit 6c7c6205 authored by Ashish Kalra's avatar Ashish Kalra Committed by Sean Christopherson
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KVM: SEV: Add SEV-SNP CipherTextHiding support 



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 a new off-by-default kvm-amd module parameter to enable ciphertext
hiding and allow the admin to configure the SEV-ES and SEV-SNP ASID
ranges. Simply cap the maximum SEV-SNP ASID as appropriate, i.e. don't
reject loading KVM or disable ciphertest hiding for a too-big value, as
KVM's general approach for module params is to sanitize inputs based on
hardware/kernel support, not burn the world down. This also allows the
admin to use -1u to assign all SEV-ES/SNP ASIDs to SNP without needing
dedicated handling in KVM.

Suggested-by: default avatarSean Christopherson <seanjc@google.com>
Signed-off-by: default avatarAshish Kalra <ashish.kalra@amd.com>
Co-developed-by: default avatarSean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/95abc49edfde36d4fb791570ea2a4be6ad95fd0d.1755721927.git.ashish.kalra@amd.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent d7fc7d98

Documentation/admin-guide/kernel-parameters.txt

+21 −0
Original line number Diff line number Diff line
@@ -2957,6 +2957,27 @@ 
			(enabled). Disable by KVM if hardware lacks support

			for NPT.



	kvm-amd.ciphertext_hiding_asids=

			[KVM,AMD] Ciphertext hiding prevents disallowed accesses

			to SNP private memory from reading ciphertext.  Instead,

			reads will see constant default values (0xff).



			If ciphertext hiding is enabled, the joint SEV-ES and

			SEV-SNP ASID space is partitioned into separate SEV-ES

			and SEV-SNP ASID ranges, with the SEV-SNP range being

			[1..max_snp_asid] and the SEV-ES range being

			(max_snp_asid..min_sev_asid), where min_sev_asid is

			enumerated by CPUID.0x.8000_001F[EDX].



			A non-zero value enables SEV-SNP ciphertext hiding and

			adjusts the ASID ranges for SEV-ES and SEV-SNP guests.

			KVM caps the number of SEV-SNP ASIDs at the maximum

			possible value, e.g. specifying -1u will assign all

			joint SEV-ES and SEV-SNP ASIDs to SEV-SNP.  Note,

			assigning all joint ASIDs to SEV-SNP, i.e. configuring

			max_snp_asid == min_sev_asid-1, will effectively make

			SEV-ES unusable.



	kvm-arm.mode=

			[KVM,ARM,EARLY] Select one of KVM/arm64's modes of

			operation.

arch/x86/kvm/svm/sev.c

+31 −1
Original line number Diff line number Diff line
@@ -59,6 +59,9 @@ 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 unsigned int nr_ciphertext_hiding_asids;

module_param_named(ciphertext_hiding_asids, nr_ciphertext_hiding_asids, uint, 0444);



#define AP_RESET_HOLD_NONE		0

#define AP_RESET_HOLD_NAE_EVENT		1

#define AP_RESET_HOLD_MSR_PROTO		2
@@ -201,6 +204,9 @@ 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;
@@ -3068,10 +3074,32 @@ void __init sev_hardware_setup(void) 
out:

	if (sev_enabled) {

		init_args.probe = true;



		if (sev_is_snp_ciphertext_hiding_supported())

			init_args.max_snp_asid = min(nr_ciphertext_hiding_asids,

						     min_sev_asid - 1);



		if (sev_platform_init(&init_args))

			sev_supported = sev_es_supported = sev_snp_supported = false;

		else if (sev_snp_supported)

			sev_snp_supported = is_sev_snp_initialized();



		if (sev_snp_supported)

			nr_ciphertext_hiding_asids = init_args.max_snp_asid;



		/*

		 * If ciphertext hiding is enabled, the joint SEV-ES/SEV-SNP

		 * ASID range is partitioned into separate SEV-ES and SEV-SNP

		 * ASID ranges, with the SEV-SNP range being [1..max_snp_asid]

		 * and the SEV-ES range being (max_snp_asid..max_sev_es_asid].

		 * Note, SEV-ES may effectively be disabled if all ASIDs from

		 * the joint range are assigned to SEV-SNP.

		 */

		if (nr_ciphertext_hiding_asids) {

			max_snp_asid = nr_ciphertext_hiding_asids;

			min_sev_es_asid = max_snp_asid + 1;

			pr_info("SEV-SNP ciphertext hiding enabled\n");

		}

	}



	if (boot_cpu_has(X86_FEATURE_SEV))
@@ -3082,7 +3110,9 @@ 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",