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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 564ABC04AAF for ; Thu, 16 May 2019 16:00:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 33F2720833 for ; Thu, 16 May 2019 16:00:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727932AbfEPQAp (ORCPT ); Thu, 16 May 2019 12:00:45 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:43010 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727074AbfEPP6r (ORCPT ); Thu, 16 May 2019 11:58:47 -0400 Received: from [167.98.27.226] (helo=deadeye) by shadbolt.decadent.org.uk with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.89) (envelope-from ) id 1hRImK-0006zm-AQ; Thu, 16 May 2019 16:58:44 +0100 Received: from ben by deadeye with local (Exim 4.92) (envelope-from ) id 1hRImE-0001S9-QF; Thu, 16 May 2019 16:58:38 +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 , "Jon Masters" , "Thomas Gleixner" , "Borislav Petkov" Date: Thu, 16 May 2019 16:55:33 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) X-Patchwork-Hint: ignore Subject: [PATCH 3.16 72/86] x86/speculation/mds: Add mitigation mode VMWERV In-Reply-To: X-SA-Exim-Connect-IP: 167.98.27.226 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 3.16.68-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Thomas Gleixner commit 22dd8365088b6403630b82423cf906491859b65e upstream. In virtualized environments it can happen that the host has the microcode update which utilizes the VERW instruction to clear CPU buffers, but the hypervisor is not yet updated to expose the X86_FEATURE_MD_CLEAR CPUID bit to guests. Introduce an internal mitigation mode VMWERV which enables the invocation of the CPU buffer clearing even if X86_FEATURE_MD_CLEAR is not set. If the system has no updated microcode this results in a pointless execution of the VERW instruction wasting a few CPU cycles. If the microcode is updated, but not exposed to a guest then the CPU buffers will be cleared. That said: Virtual Machines Will Eventually Receive Vaccine Signed-off-by: Thomas Gleixner Reviewed-by: Borislav Petkov Reviewed-by: Jon Masters Tested-by: Jon Masters Signed-off-by: Ben Hutchings --- --- a/Documentation/x86/mds.rst +++ b/Documentation/x86/mds.rst @@ -93,11 +93,38 @@ The kernel provides a function to invoke The mitigation is invoked on kernel/userspace, hypervisor/guest and C-state (idle) transitions. +As a special quirk to address virtualization scenarios where the host has +the microcode updated, but the hypervisor does not (yet) expose the +MD_CLEAR CPUID bit to guests, the kernel issues the VERW instruction in the +hope that it might actually clear the buffers. The state is reflected +accordingly. + According to current knowledge additional mitigations inside the kernel itself are not required because the necessary gadgets to expose the leaked data cannot be controlled in a way which allows exploitation from malicious user space or VM guests. +Kernel internal mitigation modes +-------------------------------- + + ======= ============================================================ + off Mitigation is disabled. Either the CPU is not affected or + mds=off is supplied on the kernel command line + + full Mitigation is eanbled. CPU is affected and MD_CLEAR is + advertised in CPUID. + + vmwerv Mitigation is enabled. CPU is affected and MD_CLEAR is not + advertised in CPUID. That is mainly for virtualization + scenarios where the host has the updated microcode but the + hypervisor does not expose MD_CLEAR in CPUID. It's a best + effort approach without guarantee. + ======= ============================================================ + +If the CPU is affected and mds=off is not supplied on the kernel command +line then the kernel selects the appropriate mitigation mode depending on +the availability of the MD_CLEAR CPUID bit. + Mitigation points ----------------- --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -957,6 +957,7 @@ void df_debug(struct pt_regs *regs, long enum mds_mitigations { MDS_MITIGATION_OFF, MDS_MITIGATION_FULL, + MDS_MITIGATION_VMWERV, }; #endif /* _ASM_X86_PROCESSOR_H */ --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -279,7 +279,8 @@ static enum mds_mitigations mds_mitigati static const char * const mds_strings[] = { [MDS_MITIGATION_OFF] = "Vulnerable", - [MDS_MITIGATION_FULL] = "Mitigation: Clear CPU buffers" + [MDS_MITIGATION_FULL] = "Mitigation: Clear CPU buffers", + [MDS_MITIGATION_VMWERV] = "Vulnerable: Clear CPU buffers attempted, no microcode", }; static void __init mds_select_mitigation(void) @@ -290,10 +291,9 @@ static void __init mds_select_mitigation } if (mds_mitigation == MDS_MITIGATION_FULL) { - if (boot_cpu_has(X86_FEATURE_MD_CLEAR)) - static_branch_enable(&mds_user_clear); - else - mds_mitigation = MDS_MITIGATION_OFF; + if (!boot_cpu_has(X86_FEATURE_MD_CLEAR)) + mds_mitigation = MDS_MITIGATION_VMWERV; + static_branch_enable(&mds_user_clear); } pr_info("%s\n", mds_strings[mds_mitigation]); } @@ -753,8 +753,14 @@ void arch_smt_update(void) break; } - if (mds_mitigation == MDS_MITIGATION_FULL) + switch (mds_mitigation) { + case MDS_MITIGATION_FULL: + case MDS_MITIGATION_VMWERV: update_mds_branch_idle(); + break; + case MDS_MITIGATION_OFF: + break; + } mutex_unlock(&spec_ctrl_mutex); }