On Tue, Jun 23, 2020 at 04:53:44PM +0200, Peter Zijlstra wrote:

+noinstr void idtentry_validate_ist(struct pt_regs *regs)
+{
+	if ((regs->sp & ~(EXCEPTION_STKSZ-1)) ==
+	    (_RET_IP_ & ~(EXCEPTION_STKSZ-1)))
+		die("IST stack recursion", regs, 0);
+}

Yes, this is a start, it doesn't cover the case where the NMI stack is
in-between, so I think you need to walk down regs->sp too. The dumpstack
code already has some logic for this.


	Joerg