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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 C98CAC43A1D for ; Thu, 12 Jul 2018 09:31:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8D1F620C0C for ; Thu, 12 Jul 2018 09:31:48 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8D1F620C0C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726580AbeGLJk3 (ORCPT ); Thu, 12 Jul 2018 05:40:29 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:50664 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726040AbeGLJk3 (ORCPT ); Thu, 12 Jul 2018 05:40:29 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id EF3C1401EF26; Thu, 12 Jul 2018 09:31:43 +0000 (UTC) Received: from vitty.brq.redhat.com.redhat.com (unknown [10.43.2.155]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 82DE7111AF0E; Thu, 12 Jul 2018 09:31:42 +0000 (UTC) From: Vitaly Kuznetsov To: Wanpeng Li Cc: kvm , Paolo Bonzini , Radim Krcmar , "the arch\/x86 maintainers" , Andy Lutomirski , ldv@altlinux.org, yamato@redhat.com, LKML Subject: Re: [PATCH] x86/kvm/vmx: don't read current->thread.{fs,gs}base of legacy tasks References: <20180711173718.8850-1-vkuznets@redhat.com> Date: Thu, 12 Jul 2018 11:31:41 +0200 In-Reply-To: (Wanpeng Li's message of "Thu, 12 Jul 2018 09:39:09 +0800") Message-ID: <877em0ztoi.fsf@vitty.brq.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Thu, 12 Jul 2018 09:31:44 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Thu, 12 Jul 2018 09:31:44 +0000 (UTC) for IP:'10.11.54.3' DOMAIN:'int-mx03.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'vkuznets@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Wanpeng Li writes: > On Thu, 12 Jul 2018 at 08:07, Vitaly Kuznetsov wrote: >> >> When we switched from doing rdmsr() to reading FS/GS base values from >> current->thread we completely forgot about legacy 32-bit userspaces which >> we still support in KVM (why?). task->thread.{fsbase,gsbase} are only >> synced for 64-bit processes, calling save_fsgs_for_kvm() and using >> its result from current is illegal for legacy processes. >> >> There's no ARCH_SET_FS/GS prctls for legacy applications. Base MSRs are, >> however, not always equal to zero. Intel's manual says (3.4.4 Segment >> Loading Instructions in IA-32e Mode): >> >> "In order to set up compatibility mode for an application, segment-load >> instructions (MOV to Sreg, POP Sreg) work normally in 64-bit mode. An >> entry is read from the system descriptor table (GDT or LDT) and is loaded >> in the hidden portion of the segment register. >> ... >> The hidden descriptor register fields for FS.base and GS.base are >> physically mapped to MSRs in order to load all address bits supported by >> a 64-bit implementation. >> " >> >> The issue was found by strace test suite where 32-bit ioctl_kvm_run test >> started segfaulting. > > Test suite: MSR switch > PASS: VM entry MSR load > PASS: VM exit MSR store > PASS: VM exit MSR load > FAIL: VM entry MSR load: try to load FS_BASE > SUMMARY: 4 tests, 1 unexpected failures > > kvm-unit-tests fails w/ and w/o the patch, maybe it is another issue, > i didn't dig further, you can have a look if you are interested in. :) The patch only changes the behavior for legacy userspaces and I can reproduce the failure on native x86_64, it is something different. I'm, however, interested so stay tuned :-) -- Vitaly