Merge tag 'kvm-x86-ciphertext-6.18' of https://github.com/kvm-x86/linux into HEAD

KVM SEV-SNP CipherText Hiding support for 6.18

Add support for SEV-SNP's CipherText Hiding, an opt-in feature that prevents
unauthorized CPU accesses from reading the ciphertext of SNP guest private
memory, e.g. to attempt an offline attack.  Instead of ciphertext, the CPU
will always read back all FFs when CipherText Hiding is enabled.

Add new module parameter to the KVM module to enable CipherText Hiding and
control the number of ASIDs that can be used for VMs with CipherText Hiding,
which is in effect the number of SNP VMs.  When CipherText Hiding is enabled,
the shared SEV-ES/SEV-SNP ASID space is split into separate ranges for SEV-ES
and SEV-SNP guests, i.e. ASIDs that can be used for CipherText Hiding cannot
be used to run SEV-ES guests.

Author: bonzini

Documentation/admin-guide/kernel-parameters.txt

@@ -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

@@ -58,6 +58,9 @@ 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
@@ -84,6 +87,10 @@ static DECLARE_RWSEM(sev_deactivate_lock);
 static DEFINE_MUTEX(sev_bitmap_lock);
 unsigned int max_sev_asid;
 static unsigned int min_sev_asid;
+static unsigned int max_sev_es_asid;
+static unsigned int min_sev_es_asid;
+static unsigned int max_snp_asid;
+static unsigned int min_snp_asid;
 static unsigned long sev_me_mask;
 static unsigned int nr_asids;
 static unsigned long *sev_asid_bitmap;
@@ -180,20 +187,34 @@ static void sev_misc_cg_uncharge(struct kvm_sev_info *sev)
 	misc_cg_uncharge(type, sev->misc_cg, 1);
 }
 
-static int sev_asid_new(struct kvm_sev_info *sev)
+static int sev_asid_new(struct kvm_sev_info *sev, unsigned long vm_type)
 {
 	/*
 	 * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
 	 * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
-	 * Note: min ASID can end up larger than the max if basic SEV support is
-	 * effectively disabled by disallowing use of ASIDs for SEV guests.
 	 */
-	unsigned int min_asid = sev->es_active ? 1 : min_sev_asid;
-	unsigned int max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
-	unsigned int asid;
+	unsigned int min_asid, max_asid, asid;
 	bool retry = true;
 	int ret;
 
+	if (vm_type == KVM_X86_SNP_VM) {
+		min_asid = min_snp_asid;
+		max_asid = max_snp_asid;
+	} else if (sev->es_active) {
+		min_asid = min_sev_es_asid;
+		max_asid = max_sev_es_asid;
+	} else {
+		min_asid = min_sev_asid;
+		max_asid = max_sev_asid;
+	}
+
+	/*
+	 * 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;
 
@@ -454,7 +475,7 @@ static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
 	if (snp_active)
 		sev->vmsa_features |= SVM_SEV_FEAT_SNP_ACTIVE;
 
-	ret = sev_asid_new(sev);
+	ret = sev_asid_new(sev, vm_type);
 	if (ret)
 		goto e_no_asid;
 
@@ -3058,6 +3079,9 @@ void __init sev_hardware_setup(void)
 	if (min_sev_asid == 1)
 		goto out;
 
+	min_sev_es_asid = min_snp_asid = 1;
+	max_sev_es_asid = max_snp_asid = min_sev_asid - 1;
+
 	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;
@@ -3066,10 +3090,32 @@ 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))
@@ -3080,12 +3126,14 @@ 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),
-			min_sev_asid > 1 ? 1 : 0, min_sev_asid - 1);
+			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",
 			str_enabled_disabled(sev_snp_supported),
-			min_sev_asid > 1 ? 1 : 0, min_sev_asid - 1);
+			min_snp_asid, max_snp_asid);
 
 	sev_enabled = sev_supported;
 	sev_es_enabled = sev_es_supported;