KVM: VMX: return correct segment limit and flags for CS/SS registers in real mode

VMX without unrestricted mode cannot virtualize real mode, so if
emulate_invalid_guest_state=0 kvm uses vm86 mode to approximate
it. Sometimes, when guest moves from protected mode to real mode, it
leaves segment descriptors in a state not suitable for use by vm86 mode
virtualization, so we keep shadow copy of segment descriptors for internal
use and load fake register to VMCS for guest entry to succeed. Till
now we kept shadow for all segments except SS and CS (for SS and CS we
returned parameters directly from VMCS), but since commit a5625189
emulator enforces segment limits in real mode. This causes #GP during move
from protected mode to real mode when emulator fetches first instruction
after moving to real mode since it uses incorrect CS base and limit to
linearize the %rip. Fix by keeping shadow for SS and CS too.
Reviewed-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

arch/x86/kvm/vmx.c

@@ -2856,6 +2856,8 @@ static void enter_rmode(struct kvm_vcpu *vcpu)
 	vmx_get_segment(vcpu, &vmx->rmode.segs[VCPU_SREG_DS], VCPU_SREG_DS);
 	vmx_get_segment(vcpu, &vmx->rmode.segs[VCPU_SREG_FS], VCPU_SREG_FS);
 	vmx_get_segment(vcpu, &vmx->rmode.segs[VCPU_SREG_GS], VCPU_SREG_GS);
+	vmx_get_segment(vcpu, &vmx->rmode.segs[VCPU_SREG_SS], VCPU_SREG_SS);
+	vmx_get_segment(vcpu, &vmx->rmode.segs[VCPU_SREG_CS], VCPU_SREG_CS);
 
 	vmx->emulation_required = 1;
 	vmx->rmode.vm86_active = 1;
@@ -3171,10 +3173,7 @@ static void vmx_get_segment(struct kvm_vcpu *vcpu,
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	u32 ar;
 
-	if (vmx->rmode.vm86_active
-	    && (seg == VCPU_SREG_TR || seg == VCPU_SREG_ES
-		|| seg == VCPU_SREG_DS || seg == VCPU_SREG_FS
-		|| seg == VCPU_SREG_GS)) {
+	if (vmx->rmode.vm86_active && seg != VCPU_SREG_LDTR) {
 		*var = vmx->rmode.segs[seg];
 		if (seg == VCPU_SREG_TR
 		    || var->selector == vmx_read_guest_seg_selector(vmx, seg))