Lock all vCPUs when synchronizing and encrypting VMSAs for SNP guests, as
allowing userspace to manipulate and/or run a vCPU while its state is being
synchronized would at best corrupt vCPU state, and at worst crash the host
kernel.

Opportunistically assert that vcpu->mutex is held when synchronizing its
VMSA (the SEV-ES path already locks vCPUs).

Fixes: ad27ce155566 ("KVM: SEV: Add KVM_SEV_SNP_LAUNCH_FINISH command")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 5de36bbc4c53..c10c71608208 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -882,6 +882,8 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
 	u8 *d;
 	int i;
 
+	lockdep_assert_held(&vcpu->mutex);
+
 	if (vcpu->arch.guest_state_protected)
 		return -EINVAL;
 
@@ -2456,6 +2458,10 @@ static int snp_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	if (kvm_is_vcpu_creation_in_progress(kvm))
 		return -EBUSY;
 
+	ret = kvm_lock_all_vcpus(kvm);
+	if (ret)
+		return ret;
+
 	data.gctx_paddr = __psp_pa(sev->snp_context);
 	data.page_type = SNP_PAGE_TYPE_VMSA;
 
@@ -2465,12 +2471,12 @@ static int snp_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
 
 		ret = sev_es_sync_vmsa(svm);
 		if (ret)
-			return ret;
+			goto err;
 
 		/* Transition the VMSA page to a firmware state. */
 		ret = rmp_make_private(pfn, INITIAL_VMSA_GPA, PG_LEVEL_4K, sev->asid, true);
 		if (ret)
-			return ret;
+			goto err;
 
 		/* Issue the SNP command to encrypt the VMSA */
 		data.address = __sme_pa(svm->sev_es.vmsa);
@@ -2479,7 +2485,7 @@ static int snp_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		if (ret) {
 			snp_page_reclaim(kvm, pfn);
 
-			return ret;
+			goto err;
 		}
 
 		svm->vcpu.arch.guest_state_protected = true;
@@ -2494,6 +2500,10 @@ static int snp_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	}
 
 	return 0;
+
+err:
+	kvm_unlock_all_vcpus(kvm);
+	return ret;
 }
 
 static int snp_launch_finish(struct kvm *kvm, struct kvm_sev_cmd *argp)
-- 
2.53.0.473.g4a7958ca14-goog