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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,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 A2D5BC433DF for ; Tue, 19 May 2020 21:13:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 76E8A20758 for ; Tue, 19 May 2020 21:13:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728039AbgESVNi (ORCPT ); Tue, 19 May 2020 17:13:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35576 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727905AbgESVNc (ORCPT ); Tue, 19 May 2020 17:13:32 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D30FFC08C5C2; Tue, 19 May 2020 14:13:31 -0700 (PDT) Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jb9YA-0001zE-GS; Tue, 19 May 2020 23:13:22 +0200 Received: from nanos.tec.linutronix.de (localhost [IPv6:::1]) by nanos.tec.linutronix.de (Postfix) with ESMTP id D60D6100D00; Tue, 19 May 2020 23:13:21 +0200 (CEST) Message-Id: <20200519211224.753841434@linutronix.de> User-Agent: quilt/0.65 Date: Tue, 19 May 2020 22:31:34 +0200 From: Thomas Gleixner To: LKML Cc: x86@kernel.org, Paolo Bonzini , kvm@vger.kernel.org, Juergen Gross , Tom Lendacky , Alexandre Chartre , Peter Zijlstra Subject: [patch 6/7] x86/kvm/svm: Use uninstrumented wrmsrl() to restore GS References: <20200519203128.773151484@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-transfer-encoding: 8-bit X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On guest exit MSR_GS_BASE contains whatever the guest wrote to it and the first action after returning from the ASM code is to set it to the host kernel value. This uses wrmsrl() which is interesting at least. wrmsrl() is either using native_write_msr() or the paravirt variant. The XEN_PV code is uninteresting as nested SVM in a XEN_PV guest does not work. But native_write_msr() can be placed out of line by the compiler especially when paravirtualization is enabled in the kernel configuration. The function is marked notrace, but still can be probed if CONFIG_KPROBE_EVENTS_ON_NOTRACE is enabled. That would be a fatal problem as kprobe events use per-CPU variables which are GS based and would be accessed with the guest GS. Depending on the GS value this would either explode in colorful ways or lead to completely undebugable data corruption. Aside of that native_write_msr() contains a tracepoint which objtool complains about as it is invoked from the noinstr section. As this cannot run inside a XEN_PV guest there is no point in using wrmsrl(). Use native_wrmsrl() instead which is just a plain native WRMSR without tracing or anything else attached. Signed-off-by: Thomas Gleixner Acked-by: Juergen Gross Cc: Paolo Bonzini Cc: Tom Lendacky diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 35bd95dd64dd..663333d34c84 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3353,7 +3353,7 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs); #ifdef CONFIG_X86_64 - wrmsrl(MSR_GS_BASE, svm->host.gs_base); + native_wrmsrl(MSR_GS_BASE, svm->host.gs_base); #else loadsegment(fs, svm->host.fs); #ifndef CONFIG_X86_32_LAZY_GS