Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965163AbeALS4b (ORCPT + 1 other); Fri, 12 Jan 2018 13:56:31 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:41684 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965031AbeALS4a (ORCPT ); Fri, 12 Jan 2018 13:56:30 -0500 Message-ID: <1515783378.22302.482.camel@infradead.org> Subject: Re: [PATCH] x86/retpoline: Fill RSB on context switch for affected CPUs From: David Woodhouse To: Andrew Cooper , Andi Kleen Cc: Paul Turner , LKML , Linus Torvalds , Greg Kroah-Hartman , Tim Chen , Dave Hansen , tglx@linutronix.de, Kees Cook , Rik van Riel , Peter Zijlstra , Andy Lutomirski , Jiri Kosina , gnomes@lxorguk.ukuu.org.uk, x86@kernel.org, thomas.lendacky@amd.com, Josh Poimboeuf In-Reply-To: References: <1515779365-9032-1-git-send-email-dwmw@amazon.co.uk> Face: 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 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-5kxrU0wDQ3Uqog8kEsE6" Date: Fri, 12 Jan 2018 18:56:18 +0000 Mime-Version: 1.0 X-Mailer: Evolution 3.18.5.2-0ubuntu3.2 X-SRS-Rewrite: SMTP reverse-path rewritten from by twosheds.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: --=-5kxrU0wDQ3Uqog8kEsE6 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2018-01-12 at 18:05 +0000, Andrew Cooper wrote: >=20 > If you unconditionally fill the RSB on every entry to supervisor mode, > then there are never guest-controlled RSB values to be found. >=20 > With that property (and IBRS to protect Skylake+), you shouldn't need > RSB filling anywhere in the middle. Yes, that's right. We have a choice =E2=80=94 we can do it on kernel entry (in the interrupt a= nd syscall and NMI paths), and that's nice and easy and really safe because we know there's *never* a bad RSB entry lurking while we're in the kernel. The alternative, which is what we seem to be learning towards now in the latest tables from Dave (https://goo.gl/pXbvBE=C2=A0and https://goo.gl/Grbuhf), is to do it on context switch when we might be switching from a shallow call stack to a deeper one. Which has much better performance characteristics for processes which make non- sleeping syscalls. The caveat with the latter approach is that we do depend on the fact that context switches are the only imbalance in the kernel. But that's OK =E2=80=94 we don't have a longjmp or anything else like that. Especially that goes into a *deeper* call stack. 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