From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-6.0 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 837297D2EF for ; Mon, 11 Mar 2019 13:13:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726958AbfCKNNq (ORCPT ); Mon, 11 Mar 2019 09:13:46 -0400 Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:42637 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725943AbfCKNNp (ORCPT ); Mon, 11 Mar 2019 09:13:45 -0400 Received: by atrey.karlin.mff.cuni.cz (Postfix, from userid 512) id 3728F805DA; Mon, 11 Mar 2019 14:13:35 +0100 (CET) Date: Mon, 11 Mar 2019 14:13:41 +0100 From: Pavel Machek To: Thomas Gleixner Cc: corbet@lwn.net, LKML , Linus Torvalds , x86@kernel.org, Peter Zijlstra , Jiri Kosina , Josh Poimboeuf , Dave Hansen , Andy Lutomirski , Greg KH , Konrad Rzeszutek Wilk , David Woodhouse , Tom Lendacky , Paolo Bonzini , Joerg Roedel , Tony Luck , Salvatore Bonaccorso , linux-doc@vger.kernel.org Subject: Re: [patch] Fix up l1ft documentation was Re: Taking a break - time to look back Message-ID: <20190311131341.GA28223@amd> References: <20190102235152.GA24163@amd> <20190311102109.GA14118@amd> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="lrZ03NoBR/3+SXJZ" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org --lrZ03NoBR/3+SXJZ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon 2019-03-11 14:05:07, Thomas Gleixner wrote: > On Mon, 11 Mar 2019, Pavel Machek wrote: > > On Thu 2019-01-03 00:51:52, Pavel Machek wrote: > > > Hi! > > >=20 > > > > The next round of speculation-related issues including the scary L1= TF > > > > hardware bug was a way more "pleasant" experience to work on. While= for > > > > obvious reasons the mitigation development happened behind closed d= oors in > > > > a smaller group of people, we were at least able to collaborate in = a way > > > > which is somehow close to what we are used to. > > >=20 > > > Ok, I guess L1TF was a lot of fun, and there was not time for a good > > > documentation. > > >=20 > > > There's admin guide that is written as an advertisment, and >=20 > What's advertisement there? "No problem here, no performance issues, nothing to be seen unless you are running VM." > > > unfortunately is slightly "inaccurate" at places (to the point of > > > lying). >=20 > Huch? Care to tell what's a lie instead of making bold statements? Take a care to look at the patch I submitted? Lie: # A system with an up to date kernel is protected against attacks from # malicious user space applications. 3GB system running 32bit kernel is not protected. Same is true for for really big 64bit systems. If I do what dmesg suggests, this becomes untrue: # The Linux kernel contains a mitigation for this attack vector, PTE # inversion, which is permanently enabled and has no performance # impact. Limiting memory to 2GB _is_ going to have severe perfomance impact. Pavel commit 9664b4dabdb132433a6843aefe05814953f1342f Author: Pavel Date: Thu Jan 3 00:48:40 2019 +0100 Ok, I guess L1TF was a lot of fun, and there was not time for a good documentation. =20 There's admin guide that is written as an advertisment, and unfortunately is slightly "inaccurate" at places (to the point of lying). =20 Plus, I believe it should go to x86/ directory, as this is really Intel issue, and not anything ARM (or RISC-V) people need to know. =20 Signed-off-by: Pavel Machek diff --git a/Documentation/admin-guide/l1tf.rst b/Documentation/admin-guide= /l1tf.rst index 9af9773..05c5422 100644 --- a/Documentation/admin-guide/l1tf.rst +++ b/Documentation/admin-guide/l1tf.rst @@ -1,10 +1,11 @@ L1TF - L1 Terminal Fault =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =20 -L1 Terminal Fault is a hardware vulnerability which allows unprivileged -speculative access to data which is available in the Level 1 Data Cache -when the page table entry controlling the virtual address, which is used -for the access, has the Present bit cleared or other reserved bits set. +L1 Terminal Fault is a hardware vulnerability on most recent Intel x86 +CPUs which allows unprivileged speculative access to data which is +available in the Level 1 Data Cache when the page table entry +controlling the virtual address, which is used for the access, has the +Present bit cleared or other reserved bits set. =20 Affected processors ------------------- @@ -76,12 +77,14 @@ Attack scenarios deterministic and more practical. =20 The Linux kernel contains a mitigation for this attack vector, PTE - inversion, which is permanently enabled and has no performance - impact. The kernel ensures that the address bits of PTEs, which are not - marked present, never point to cacheable physical memory space. - - A system with an up to date kernel is protected against attacks from - malicious user space applications. + inversion, which is permanently enabled and has no measurable + performance impact in most configurations. The kernel ensures that + the address bits of PTEs, which are not marked present, never point + to cacheable physical memory space. On x86-32, this physical memory + needs to be limited to 2GiB to make mitigation effective. + + Mitigation is present in kernels v4.19 and newer, and in + recent -stable kernels. =20 2. Malicious guest in a virtual machine ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ --=20 (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo= g.html --lrZ03NoBR/3+SXJZ Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iEYEARECAAYFAlyGXwUACgkQMOfwapXb+vJFoQCfWZDKue2kMKNscc4j/gsZ8GnD VcIAniqQ8g91FoiTBoojnhz44ehgnvDr =xJ1/ -----END PGP SIGNATURE----- --lrZ03NoBR/3+SXJZ--