KVM: SVM: Call SEV Guest Decommission if ASID binding fails

Send SEV_CMD_DECOMMISSION command to PSP firmware if ASID binding
fails. If a failure happens after  a successful LAUNCH_START command,
a decommission command should be executed. Otherwise, guest context
will be unfreed inside the AMD SP. After the firmware will not have
memory to allocate more SEV guest context, LAUNCH_START command will
begin to fail with SEV_RET_RESOURCE_LIMIT error.

The existing code calls decommission inside sev_unbind_asid, but it is
not called if a failure happens before guest activation succeeds. If
sev_bind_asid fails, decommission is never called. PSP firmware has a
limit for the number of guests. If sev_asid_binding fails many times,
PSP firmware will not have resources to create another guest context.

Cc: stable@vger.kernel.org
Fixes: 59414c98 ("KVM: SVM: Add support for KVM_SEV_LAUNCH_START command")
Reported-by: Peter Gonda <pgonda@google.com>
Signed-off-by: Alper Gun <alpergun@google.com>
Reviewed-by: Marc Orr <marcorr@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210610174604.2554090-1-alpergun@google.com>

arch/x86/kvm/svm/sev.c

@@ -199,9 +199,19 @@ static void sev_asid_free(struct kvm_sev_info *sev)
 	sev->misc_cg = NULL;
 }
 
-static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
+static void sev_decommission(unsigned int handle)
 {
 	struct sev_data_decommission decommission;
+
+	if (!handle)
+		return;
+
+	decommission.handle = handle;
+	sev_guest_decommission(&decommission, NULL);
+}
+
+static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
+{
 	struct sev_data_deactivate deactivate;
 
 	if (!handle)
@@ -214,9 +224,7 @@ static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
 	sev_guest_deactivate(&deactivate, NULL);
 	up_read(&sev_deactivate_lock);
 
-	/* decommission handle */
-	decommission.handle = handle;
-	sev_guest_decommission(&decommission, NULL);
+	sev_decommission(handle);
 }
 
 static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
@@ -341,8 +349,10 @@ static int sev_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
 
 	/* Bind ASID to this guest */
 	ret = sev_bind_asid(kvm, start.handle, error);
-	if (ret)
+	if (ret) {
+		sev_decommission(start.handle);
 		goto e_free_session;
+	}
 
 	/* return handle to userspace */
 	params.handle = start.handle;