Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753425AbeADTTa (ORCPT + 1 other); Thu, 4 Jan 2018 14:19:30 -0500 Received: from twosheds.infradead.org ([90.155.92.209]:45658 "EHLO twosheds.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750990AbeADTT3 (ORCPT ); Thu, 4 Jan 2018 14:19:29 -0500 Message-ID: <1515093549.29312.11.camel@infradead.org> Subject: Re: [PATCH 0/7] IBRS patch series From: David Woodhouse To: Linus Torvalds , Tim Chen Cc: Thomas Gleixner , Andy Lutomirski , Greg KH , Dave Hansen , Andrea Arcangeli , Andi Kleen , Arjan Van De Ven , Linux Kernel Mailing List In-Reply-To: References: Face: iVBORw0KGgoAAAANSUhEUgAAADAAAAAwBAMAAAClLOS0AAAAG1BMVEUHBwcUFBQpKSlGRkZhYWF9fX2Xl5eysrLMzMxFF+rXAAACaElEQVQ4y21UQXbbIBQE9wJALmAg6ToWON22FrhZthHgbvssUPathC7QWMful2JHSmtWwGg+zPxBCE0DU4QoJQgRgsg4w2gJjBNE8PjFBZgnQMBs+uZ1NQNQjZO3BV4AGDFC0f+l4DBG0VUAM4yv7SO8IgRdHXQ+A78HKL5OAeCfNQV5cHX8DsBUyIJKtYbt98BKaGNCKjfgFVkqYVLbkHKsRsbSCSa0T6npIqLrpRBgQKHUpQmgs9eEKaiUcooE8WWfCGVnBiUcn1uF2XhbfmN9apKnmMP2K4kizKkQWxuaVNOpU2cACIyxO1Po8ETHcXEDMVnozcejkAYA9iaD4pU0ZvNQ8VurNnTuFAYVtuIPUZW25PjDIjQAlGyffIiRQxoWAZBmJ0LTdW2Nyc0iP3DqRhxizvGJkBWZmyFVyZkddWzmBoIBVMpCCJ1CFzl98xav4VJKSSD45KbUT75ixikTphDSRh8+Uz7JLgUTAgAFwzqzjxc/nDY7WUApqY0OMdTwCKZSXplSKkgIRCHElCp8ZnhnKqXuwcNbk1L0VXE+I9alUXoHlLHl3mv7/dWQlJwtjREC7mu9L/U2jQyMUuO2EDS4q9Kl2ddm232bxIE5pjJuVwiljNn/Cfv25/T0cu5cZbwHGVq7h/zp0B4n3S99V/utD+Uo8BiGx9xCsOAV5z7/tjo4Z4z1Lvb90KZ7eFOoOeXOukqF2seo234YYuaQPpRP+cVZU5adT1Edun5Iz3z8fTz3+eSDh0Ip1c7zx1MaijGzTd/3MbRuBHz8cvcVgCMBRpOHvgu59WDhoat+nIZm+LWm9C/aaaGq5DCP9QAAAABJRU5ErkJggg== Content-Type: multipart/signed; micalg="sha-256"; protocol="application/x-pkcs7-signature"; boundary="=-9BFViQXqn0LIS7VngFj5" Date: Thu, 04 Jan 2018 19:19:09 +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: --=-9BFViQXqn0LIS7VngFj5 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2018-01-04 at 11:00 -0800, Linus Torvalds wrote: > On Thu, Jan 4, 2018 at 9:56 AM, Tim Chen > wrote: > >=20 > >=20 > > Speculation on Skylake and later requires these patches ("dynamic > > IBRS") > > be used instead of retpoline[1]. > Can somebody explain this part? >=20 > I was assuming that retpoline would work around this issue on all > uarchs. >=20 > This seems to say "retpoline does nothing on Skylake+" On Skylake the target for a 'ret' instruction may also come from the BTB. So if you ever let the RSB (which remembers where the 'call's came from get empty, you end up vulnerable. Other than the obvious call stack of more than 16 calls in depth, there's also a big list of other things which can empty the RSB, including an SMI. Which basically makes retpoline on Skylake+ *very* hard to use reliably. 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