KVM: nVMX: Prioritize TSS T-flag #DBs over Monitor Trap Flag

Service TSS T-flag #DBs prior to pending MTFs, as such #DBs are higher
priority than MTF.  KVM itself doesn't emulate TSS #DBs, and any such
exceptions injected from L1 will be handled by hardware (or morphed to
a fault-like exception if injection fails), but theoretically userspace
could pend a TSS T-flag #DB in conjunction with a pending MTF.

Note, there's no known use case this fixes, it's purely to be technically
correct with respect to Intel's SDM.

Cc: Oliver Upton <oupton@google.com>
Cc: Peter Shier <pshier@google.com>
Fixes: 5ef8acbdd687 ("KVM: nVMX: Emulate MTF when performing instruction emulation")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20220830231614.3580124-8-seanjc@google.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 9e25e759b486bf4c0dccb32aefb888c9e13db375..60e6a261c723fd02bc61f81562df124b04188033 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3985,15 +3985,17 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
        }
 
        /*
-        * Process any exceptions that are not debug traps before MTF.
+        * Process exceptions that are higher priority than Monitor Trap Flag:
+        * fault-like exceptions, TSS T flag #DB (not emulated by KVM, but
+        * could theoretically come in from userspace), and ICEBP (INT1).
         *
         * Note that only a pending nested run can block a pending exception.
         * Otherwise an injected NMI/interrupt should either be
         * lost or delivered to the nested hypervisor in the IDT_VECTORING_INFO,
         * while delivering the pending exception.
         */
-
-       if (vcpu->arch.exception.pending && !vmx_get_pending_dbg_trap(vcpu)) {
+       if (vcpu->arch.exception.pending &&
+           !(vmx_get_pending_dbg_trap(vcpu) & ~DR6_BT)) {
                if (vmx->nested.nested_run_pending)
                        return -EBUSY;
                if (!nested_vmx_check_exception(vcpu, &exit_qual))