From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F0760C433E0 for ; Tue, 23 Jun 2020 15:39:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BC86A20723 for ; Tue, 23 Jun 2020 15:39:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732979AbgFWPi7 (ORCPT ); Tue, 23 Jun 2020 11:38:59 -0400 Received: from mx2.suse.de ([195.135.220.15]:45762 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732521AbgFWPi7 (ORCPT ); Tue, 23 Jun 2020 11:38:59 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 0A2E6AF3D; Tue, 23 Jun 2020 15:38:57 +0000 (UTC) Date: Tue, 23 Jun 2020 17:38:55 +0200 From: Joerg Roedel To: Peter Zijlstra Cc: Andy Lutomirski , Joerg Roedel , Dave Hansen , Tom Lendacky , Mike Stunes , Dan Williams , Dave Hansen , "H. Peter Anvin" , Juergen Gross , Jiri Slaby , Kees Cook , kvm list , LKML , Thomas Hellstrom , Linux Virtualization , X86 ML , Sean Christopherson , Andrew Cooper Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace) Message-ID: <20200623153855.GM14101@suse.de> References: <20200623110706.GB4817@hirez.programming.kicks-ass.net> <20200623113007.GH31822@suse.de> <20200623114818.GD4817@hirez.programming.kicks-ass.net> <20200623120433.GB14101@suse.de> <20200623125201.GG4817@hirez.programming.kicks-ass.net> <20200623134003.GD14101@suse.de> <20200623135916.GI4817@hirez.programming.kicks-ass.net> <20200623145344.GA117543@hirez.programming.kicks-ass.net> <20200623145914.GF14101@suse.de> <20200623152326.GL4817@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200623152326.GL4817@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 23, 2020 at 05:23:26PM +0200, Peter Zijlstra wrote: > On Tue, Jun 23, 2020 at 04:59:14PM +0200, Joerg Roedel wrote: > > 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. > > That shouldn't be possible with the current code, I think. Not with the current code, but possibly with SNP #VC exceptions: -> First #VC -> NMI before VC handler switched off its IST stack (now on NMI IST stack) -> Second SNP #VC exception before the NMI handler did the #VC stack check (because HV messed around with some pages touched there). In the second #VC you use the same IST stack as in the first #VC, but the the NMI-stack in-between. > Reliability of that depends on the unwinder, I wouldn't want the guess > uwinder to OOPS me by accident. It doesn't use the full unwinder, it just assumes that there is a pt_regs struct at the top of every kernel stack and walks through them until SP points to a user-space stack. As long as the assumption that there is a pt_regs struct on top of every stack holds, this should be safe. The assumption might be wrong when an exception happens during SYSCALL/SYSENTER entry, when the return frame is not written by hardware. Joerg