[PATCH 04/21] KVM: SEV: Disallow LAUNCH_FINISH if vCPUs are actively being created

[Sean Christopherson <seanjc@google.com>]

Reject LAUNCH_FINISH for SEV-ES and SNP VMs if KVM is actively creating
one or more vCPUs, as KVM needs to process and encrypt each vCPU's VMSA.
Letting userspace create vCPUs while LAUNCH_FINISH is in-progress is
"fine", at least in the current code base, as kvm_for_each_vcpu() operates
on online_vcpus, LAUNCH_FINISH (all SEV+ sub-ioctls) holds kvm->mutex, and
fully onlining a vCPU in kvm_vm_ioctl_create_vcpu() is done under
kvm->mutex.  I.e. there's no difference between an in-progress vCPU and a
vCPU that is created entirely after LAUNCH_FINISH.

However, given that concurrent LAUNCH_FINISH and vCPU creation can't
possibly work (for any reasonable definition of "work"), since userspace
can't guarantee whether a particular vCPU will be encrypted or not,
disallow the combination as a hardening measure, to reduce the probability
of introducing bugs in the future, and to avoid having to reason about the
safety of future changes related to LAUNCH_FINISH.

Cc: Jethro Beekman <jethro@fortanix.com>
Closes: https://lore.kernel.org/all/b31f7c6e-2807-4662-bcdd-eea2c1e132fa@fortanix.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c   | 10 ++++++++--
 include/linux/kvm_host.h |  7 +++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 7da040baba1c..5de36bbc4c53 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1030,6 +1030,9 @@ static int sev_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	if (!sev_es_guest(kvm))
 		return -ENOTTY;
 
+	if (kvm_is_vcpu_creation_in_progress(kvm))
+		return -EBUSY;
+
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		ret = mutex_lock_killable(&vcpu->mutex);
 		if (ret)
@@ -2050,8 +2053,8 @@ static int sev_check_source_vcpus(struct kvm *dst, struct kvm *src)
 	struct kvm_vcpu *src_vcpu;
 	unsigned long i;
 
-	if (src->created_vcpus != atomic_read(&src->online_vcpus) ||
-	    dst->created_vcpus != atomic_read(&dst->online_vcpus))
+	if (kvm_is_vcpu_creation_in_progress(src) ||
+	    kvm_is_vcpu_creation_in_progress(dst))
 		return -EBUSY;
 
 	if (!sev_es_guest(src))
@@ -2450,6 +2453,9 @@ static int snp_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	unsigned long i;
 	int ret;
 
+	if (kvm_is_vcpu_creation_in_progress(kvm))
+		return -EBUSY;
+
 	data.gctx_paddr = __psp_pa(sev->snp_context);
 	data.page_type = SNP_PAGE_TYPE_VMSA;
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 34759a262b28..3c7f8557f7af 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1029,6 +1029,13 @@ static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
 	return NULL;
 }
 
+static inline bool kvm_is_vcpu_creation_in_progress(struct kvm *kvm)
+{
+	lockdep_assert_held(&kvm->lock);
+
+	return kvm->created_vcpus != atomic_read(&kvm->online_vcpus);
+}
+
 void kvm_destroy_vcpus(struct kvm *kvm);
 
 int kvm_trylock_all_vcpus(struct kvm *kvm);
-- 
2.53.0.473.g4a7958ca14-goog