[PATCH v2 2/6] KVM: x86: Reject clearing EFER.SVME via KVM_SYNC_X86_SREGS if L2 is active

[Sean Christopherson <seanjc@google.com>]

Disallow clearing EFER.SVME via KVM_SYNC_X86_SREGS if L2 is active so that
svm_set_efer() can treat yanking the vCPU out of L2 during KVM_RUN as a KVM
bug.  Ideally, kvm_is_valid_sregs() itself would reject attempts to clear
SVME while L2 is active, but the risk of breaking userspace, e.g. across
save/restore, is too high.  On the other hand, yanking the vCPU out of L2
during save/restore is already "broken" as doing so would queue a triple
fault for L1.

Link: https://lore.kernel.org/all/sashiko-outbox-182127@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/regs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/regs.c b/arch/x86/kvm/regs.c
index 3621f1a47d37..e53496fdeb82 100644
--- a/arch/x86/kvm/regs.c
+++ b/arch/x86/kvm/regs.c
@@ -738,6 +738,10 @@ int kvm_run_sync_regs_from_user(struct kvm_vcpu *vcpu)
 	if (vcpu->run->kvm_dirty_regs & KVM_SYNC_X86_SREGS) {
 		struct kvm_sregs sregs = vcpu->run->s.regs.sregs;
 
+		if (is_guest_mode(vcpu) &&
+		    (vcpu->arch.efer & EFER_SVME) && !(sregs.efer & EFER_SVME))
+			return -EINVAL;
+
 		if (__set_sregs(vcpu, &sregs))
 			return -EINVAL;
 
-- 
2.56.0.307.g5fe7bd680c-goog