KVM: SVM: Triple fault L1 on unintercepted EFER.SVME clear by L2

KVM tracks when EFER.SVME is set and cleared to initialize and tear down
nested state. However, it doesn't differentiate if EFER.SVME is getting
toggled in L1 or L2+. If L2 clears EFER.SVME, and L1 does not intercept
the EFER write, KVM exits guest mode and tears down nested state while
L2 is running, executing L1 without injecting a proper #VMEXIT.

According to the APM:

    The effect of turning off EFER.SVME while a guest is running is
    undefined; therefore, the VMM should always prevent guests from
    writing EFER.

Since the behavior is architecturally undefined, KVM gets to choose what
to do. Inject a triple fault into L1 as a more graceful option that
running L1 with corrupted state.

Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
base-commit: 95deaec3557dced322e2540bfa426e60e5373d46
Link: https://patch.msgid.link/20260209195142.2554532-2-yosry.ahmed@linux.dev
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 2c511f86b79d95282b7a178f7f257348174fb2c8..4bf0f5d7167faa3a98b1f8dc3590b971d33d6f44 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -217,6 +217,19 @@ int svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
 
        if ((old_efer & EFER_SVME) != (efer & EFER_SVME)) {
                if (!(efer & EFER_SVME)) {
+                       /*
+                        * Architecturally, clearing EFER.SVME while a guest is
+                        * running yields undefined behavior, i.e. KVM can do
+                        * literally anything.  Force the vCPU back into L1 as
+                        * that is the safest option for KVM, but synthesize a
+                        * triple fault (for L1!) so that KVM at least doesn't
+                        * run random L2 code in the context of L1.  Do so if
+                        * and only if the vCPU is actively running, e.g. to
+                        * avoid positives if userspace is stuffing state.
+                        */
+                       if (is_guest_mode(vcpu) && vcpu->wants_to_run)
+                               kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
+
                        svm_leave_nested(vcpu);
                        /* #GP intercept is still needed for vmware backdoor */
                        if (!enable_vmware_backdoor)