[PATCH] KVM: nVMX: Reject nested VM-entry in wait-for-SIPI with a pending event injection

[Amirmohammad Eftekhar <amirmohammad.eftekhar@cispa.de>]

nested_check_guest_non_reg_state() only verifies that
guest_activity_state is one of ACTIVE, HLT or WAIT_SIPI; it performs
none of the activity-state cross-checks that VM entry requires.  Per
Intel's SDM, "Checks on Guest Non-Register State", when the valid bit of
the VM-entry interruption-information field is set the interruption to
be delivered "must not be one that would normally be blocked while a
logical processor is in the activity state", and for the wait-for-SIPI
state "No interruptions are allowed".  The same section also requires
that the activity state indicate the active state when the
interruptibility-state field blocks by STI or MOV SS.

KVM accepts a vmcs12 that violates both rules, and hardware does not
catch the miss: prepare_vmcs02() never writes GUEST_ACTIVITY_STATE, so
vmcs02 always carries active, and wait-for-SIPI is emulated in software
only after a successful entry (the mp_state = INIT_RECEIVED arm).  The
injection, in contrast, is copied into vmcs02.  The physical VMLAUNCH
therefore sees {active, valid injection} -- a legal pair -- and
succeeds.  Because the offending field is stripped before hardware sees
vmcs02, the missed-exit WARN in __nested_vmx_vmexit() can never fire for
this class either.

From L1's point of view a VMLAUNCH that bare metal rejects with a
VM-entry failure (invalid guest state) instead succeeds, with L2 left
parked in wait-for-SIPI holding a stale VM_ENTRY_INTR_INFO in vmcs02.

Add the two activity-state cross-checks that KVM strips before hardware
can enforce them.  The HLT state additionally restricts which
interruption types may be injected (external interrupt / NMI / #DB / #MC
/ pending-MTF only); that matrix is left for a follow-up as it needs the
per-type table, whereas the two checks added here are unconditional and
cover the fields KVM elides.

Signed-off-by: Amirmohammad Eftekhar <amirmohammad.eftekhar@cispa.de>
---
 arch/x86/kvm/vmx/nested.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 9b0bfa2f8..66c9d1ffe 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3311,6 +3311,24 @@ static int nested_check_guest_non_reg_state(struct vmcs12 *vmcs12)
 	       vmcs12->guest_activity_state != GUEST_ACTIVITY_WAIT_SIPI))
 		return -EINVAL;
 
+	/*
+	 * The activity state must be active if the interruptibility state
+	 * blocks by STI or MOV SS.
+	 */
+	if (CC(vmcs12->guest_activity_state != GUEST_ACTIVITY_ACTIVE &&
+	       (vmcs12->guest_interruptibility_info &
+		(GUEST_INTR_STATE_STI | GUEST_INTR_STATE_MOV_SS))))
+		return -EINVAL;
+
+	/*
+	 * An injected event must not be one that would be blocked in the
+	 * guest's activity state; no interruptions are allowed in the
+	 * wait-for-SIPI state.
+	 */
+	if (vmcs12->guest_activity_state == GUEST_ACTIVITY_WAIT_SIPI &&
+	    CC(vmcs12->vm_entry_intr_info_field & INTR_INFO_VALID_MASK))
+		return -EINVAL;
+
 	return 0;
 }
 
-- 
2.54.0 (Apple Git-157)