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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F0A06C49ED7 for ; Fri, 20 Sep 2019 14:33:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D0F86206C2 for ; Fri, 20 Sep 2019 14:33:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391343AbfITOdE (ORCPT ); Fri, 20 Sep 2019 10:33:04 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:36426 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388260AbfITOZJ (ORCPT ); Fri, 20 Sep 2019 10:25:09 -0400 Received: from [192.168.4.242] (helo=deadeye) by shadbolt.decadent.org.uk with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.89) (envelope-from ) id 1iBJqN-0004y6-4X; Fri, 20 Sep 2019 15:25:07 +0100 Received: from ben by deadeye with local (Exim 4.92.1) (envelope-from ) id 1iBJqG-0007wv-LN; Fri, 20 Sep 2019 15:25:00 +0100 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, Denis Kirjanov , "Borislav Petkov" , "Ingo Molnar" , "Thomas Gleixner" , "Peter Zijlstra" , "Andy Lutomirski" , "Linus Torvalds" , "Frederic Weisbecker" , "Greg Kroah-Hartman" , "Jon Masters" Date: Fri, 20 Sep 2019 15:23:35 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) X-Patchwork-Hint: ignore Subject: [PATCH 3.16 101/132] x86/speculation/mds: Improve CPU buffer clear documentation In-Reply-To: X-SA-Exim-Connect-IP: 192.168.4.242 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.16.74-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Andy Lutomirski commit 9d8d0294e78a164d407133dea05caf4b84247d6a upstream. On x86_64, all returns to usermode go through prepare_exit_to_usermode(), with the sole exception of do_nmi(). This even includes machine checks -- this was added several years ago to support MCE recovery. Update the documentation. Signed-off-by: Andy Lutomirski Cc: Borislav Petkov Cc: Frederic Weisbecker Cc: Greg Kroah-Hartman Cc: Jon Masters Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user") Link: http://lkml.kernel.org/r/999fa9e126ba6a48e9d214d2f18dbde5c62ac55c.1557865329.git.luto@kernel.org Signed-off-by: Ingo Molnar Signed-off-by: Ben Hutchings --- Documentation/x86/mds.rst | 39 +++++++-------------------------------- 1 file changed, 7 insertions(+), 32 deletions(-) --- a/Documentation/x86/mds.rst +++ b/Documentation/x86/mds.rst @@ -142,38 +142,13 @@ Mitigation points mds_user_clear. The mitigation is invoked in prepare_exit_to_usermode() which covers - most of the kernel to user space transitions. There are a few exceptions - which are not invoking prepare_exit_to_usermode() on return to user - space. These exceptions use the paranoid exit code. - - - Non Maskable Interrupt (NMI): - - Access to sensible data like keys, credentials in the NMI context is - mostly theoretical: The CPU can do prefetching or execute a - misspeculated code path and thereby fetching data which might end up - leaking through a buffer. - - But for mounting other attacks the kernel stack address of the task is - already valuable information. So in full mitigation mode, the NMI is - mitigated on the return from do_nmi() to provide almost complete - coverage. - - - Machine Check Exception (#MC): - - Another corner case is a #MC which hits between the CPU buffer clear - invocation and the actual return to user. As this still is in kernel - space it takes the paranoid exit path which does not clear the CPU - buffers. So the #MC handler repopulates the buffers to some - extent. Machine checks are not reliably controllable and the window is - extremly small so mitigation would just tick a checkbox that this - theoretical corner case is covered. To keep the amount of special - cases small, ignore #MC. - - - Debug Exception (#DB): - - This takes the paranoid exit path only when the INT1 breakpoint is in - kernel space. #DB on a user space address takes the regular exit path, - so no extra mitigation required. + all but one of the kernel to user space transitions. The exception + is when we return from a Non Maskable Interrupt (NMI), which is + handled directly in do_nmi(). + + (The reason that NMI is special is that prepare_exit_to_usermode() can + enable IRQs. In NMI context, NMIs are blocked, and we don't want to + enable IRQs with NMIs blocked.) 2. C-State transition