static int em_sysexit(struct x86_emulate_ctxt *ctxt)
{
	const struct x86_emulate_ops *ops = ctxt->ops;
	struct desc_struct cs, ss;
	u64 msr_data, rcx, rdx;
	int usermode;
	u16 cs_sel = 0, ss_sel = 0;

	/* inject #GP if in real mode or Virtual 8086 mode */
	if (ctxt->mode == X86EMUL_MODE_REAL ||
	    ctxt->mode == X86EMUL_MODE_VM86)
		return emulate_gp(ctxt, 0);

	setup_syscalls_segments(ctxt, &cs, &ss);

	if ((ctxt->rex_prefix & 0x8) != 0x0)
		usermode = X86EMUL_MODE_PROT64;
	else
		usermode = X86EMUL_MODE_PROT32;

	rcx = reg_read(ctxt, VCPU_REGS_RCX);
	rdx = reg_read(ctxt, VCPU_REGS_RDX);

	cs.dpl = 3;
	ss.dpl = 3;
	ops->get_msr(ctxt, MSR_IA32_SYSENTER_CS, &msr_data);
	switch (usermode) {
	case X86EMUL_MODE_PROT32:
		cs_sel = (u16)(msr_data + 16);
		if ((msr_data & 0xfffc) == 0x0)
			return emulate_gp(ctxt, 0);
		ss_sel = (u16)(msr_data + 24);
		rcx = (u32)rcx;
		rdx = (u32)rdx;
		break;
	case X86EMUL_MODE_PROT64:
		cs_sel = (u16)(msr_data + 32);
		if (msr_data == 0x0)
			return emulate_gp(ctxt, 0);
		ss_sel = cs_sel + 8;
		cs.d = 0;
		cs.l = 1;
		if (is_noncanonical_address(rcx) ||
		    is_noncanonical_address(rdx))
			return emulate_gp(ctxt, 0);
		break;
	}
	cs_sel |= SELECTOR_RPL_MASK;
	ss_sel |= SELECTOR_RPL_MASK;

	ops->set_segment(ctxt, cs_sel, &cs, 0, VCPU_SREG_CS);
	ops->set_segment(ctxt, ss_sel, &ss, 0, VCPU_SREG_SS);

	ctxt->_eip = rdx;
	*reg_write(ctxt, VCPU_REGS_RSP) = rcx;

	return X86EMUL_CONTINUE;
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.