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 Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 09D2FC433F5 for ; Fri, 3 Dec 2021 15:31:01 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.237772.412376 (Exim 4.92) (envelope-from ) id 1mtAWI-0006QD-HQ; Fri, 03 Dec 2021 15:30:42 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 237772.412376; Fri, 03 Dec 2021 15:30:42 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1mtAWI-0006Q6-ES; Fri, 03 Dec 2021 15:30:42 +0000 Received: by outflank-mailman (input) for mailman id 237772; Fri, 03 Dec 2021 15:30:40 +0000 Received: from se1-gles-flk1-in.inumbo.com ([94.247.172.50] helo=se1-gles-flk1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1mtAWG-0006Q0-SN for xen-devel@lists.xenproject.org; Fri, 03 Dec 2021 15:30:40 +0000 Received: from ppsw-43.csi.cam.ac.uk (ppsw-43.csi.cam.ac.uk [131.111.8.143]) by se1-gles-flk1.inumbo.com (Halon) with ESMTPS id f6a7e812-544d-11ec-b1df-f38ee3fbfdf7; Fri, 03 Dec 2021 16:30:37 +0100 (CET) Received: from hades.srcf.societies.cam.ac.uk ([131.111.179.67]:40562) by ppsw-43.csi.cam.ac.uk (ppsw.cam.ac.uk [131.111.8.139]:25) with esmtps (TLS1.2:ECDHE-RSA-AES256-GCM-SHA384:256) id 1mtAWC-000bGB-oS (Exim 4.95) (return-path ); Fri, 03 Dec 2021 15:30:36 +0000 Received: from [192.168.1.10] (host-92-12-61-86.as13285.net [92.12.61.86]) (Authenticated sender: amc96) by hades.srcf.societies.cam.ac.uk (Postfix) with ESMTPSA id 692C81FC17; Fri, 3 Dec 2021 15:30:36 +0000 (GMT) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: f6a7e812-544d-11ec-b1df-f38ee3fbfdf7 X-Cam-AntiVirus: no malware found X-Cam-ScannerInfo: https://help.uis.cam.ac.uk/email-scanner-virus Message-ID: Date: Fri, 3 Dec 2021 15:30:36 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.2 Content-Language: en-GB To: Jan Beulich , Andrew Cooper Cc: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Wei Liu , Xen-devel References: <20211126123446.32324-1-andrew.cooper3@citrix.com> <20211126123446.32324-63-andrew.cooper3@citrix.com> From: Andrew Cooper Subject: Re: [PATCH 62/65] x86/entry: Make IDT entrypoints CET-IBT compatible In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 03/12/2021 13:32, Jan Beulich wrote: > On 26.11.2021 13:34, Andrew Cooper wrote: >> Each IDT vector needs to land on an endbr64 instruction. This is especially >> important for the #CP handler, which will escalate to #DF if the endbr64 is >> missing. > One question here: How does this work? Honestly, I'm not sure. > I don't recall there being any "CET > shadow" along the lines of "STI shadow" and "SS shadow", yet there's > clearly an insn boundary here that gets "skipped" if the 2nd #CP gets > converted to #DF. And fetching of the first handler insn also isn't part > of exception delivery (and could cause other exceptions first, like #PF). I can't make my observations of real hardware behaviour match the description in the spec. Given what a mess it all is, I wouldn't be surprised if the exception delivery microcode has a special case to escalate this to #DF. If it didn't escalate to #DF, then you'd end up with an infinite stream of #CP's, which will most likely cause a stack overflow because #CP needs to be not-IST for shadow stack reasons. ~Andrew