From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure Date: Tue, 30 Jan 2018 11:35:51 +0000 Message-ID: <1517312151.18619.109.camel@infradead.org> References: <1516476182-5153-6-git-send-email-karahmed@amazon.de> <20180129201404.GA1588@localhost.localdomain> <1517257022.18619.30.camel@infradead.org> <20180129204256.GV25150@localhost.localdomain> <31415b7f-9c76-c102-86cd-6bf4e23e3aee@linux.intel.com> <1517259759.18619.38.camel@infradead.org> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-ssZ5qZ48uBqCIDF0cVVC" Cc: Arjan van de Ven , Eduardo Habkost , KarimAllah Ahmed , Linux Kernel Mailing List , Andi Kleen , Andrea Arcangeli , Andy Lutomirski , Ashok Raj , Asit Mallick , Borislav Petkov , Dan Williams , Dave Hansen , Greg Kroah-Hartman , "H . Peter Anvin" , Ingo Molnar , Janakarajan Natarajan , Joerg Roedel , Jun Nakajima , Laura Abbott , Masami To: Linus Torvalds Return-path: Received: from twosheds.infradead.org ([90.155.92.209]:47796 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751262AbeA3LgI (ORCPT ); Tue, 30 Jan 2018 06:36:08 -0500 In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: --=-ssZ5qZ48uBqCIDF0cVVC Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 2018-01-29 at 16:23 -0800, Linus Torvalds wrote: >=20 > Note on the unhappiness with some of the patches involved: what I do > *not* want to see is the "on every kernel entry" kind of garbage. >=20 > So my unhappiness with the intel microcode patches is two-fold: >=20 > =C2=A0(a) the interface is nasty and wrong, and I absolutely detest how I= ntel did it. >=20 > =C2=A0(b) the write to random MSR's on every kernel entry/exit is wrong >=20 > but that doesn't mean that I will necessarily end up NAK'ing every > single IBRS/IBPB patch. >=20 > My concern with (a) is that unlike meltdown, the intel work-around > isn't forward-looking, and doesn't have a "we fixed it" bit. Instead, > it has a "we have a nasty workaround that may or may not be horribly > expensive" bit, and isn't all that well-defined. The lack of a "we fixed it" bit is certainly problematic. But as an interim hack for the upcoming hardware, IBRS_ALL isn't so badly defined. Sure, the reassurances about performance all got ripped out before the document saw the light of day =E2=80=94 quelle surprise? =E2= =80=94 but my understanding is that it *will* be fast. It is expected to be fast enough that we can ALTERNATIVE away the retpolines, set it once and leave it set. The reason it isn't just a "we fixed it" bit is because we'll still need the IBPB on context/vCPU switches. I suspect they managed to tag BTB entries with VMX mode and ring, but *not* the full VMID/PCID tagging (and associated automatic flushing) that they'd need to truly say "we fixed it". I seriously hope they're working on a complete fix for the subsequent generation, and just neglected to mention it in their public documentation that far in advance. > My dislike of (b) comes from "we have retpoline and various wondrous > RSB filling crud already, we're smarter than that". So it's not that I > refuse any IBRS/IBPB use, I refuse the stupid and _mindless_ kind of > use. Well... for Skylake we probably need something like Ingo's cunning plan to abuse function tracing to count call depth. I won't be utterly shocked if, by the time we have all that pulled together, it ends up being fairly much as fugly as the IBRS version =E2=80=94 for less complete protection. But we'll see. :) It may also be that some of the last remaining holes can be declared just too unlikely for us to jump through fugly hoops for. In fact that *has* to be our answer for the SMI issue if we're not using IBRS on Skylake, so now it's just a question of degree =E2=80=94 how many of the *other* theoretical holes are we happy to do the same thing for? That's a genuine question, not a rhetorical device arguing for IBRS. I just haven't seen a clear analysis, other than some hand-waving, of how feasible some of those attack vectors really are. 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