From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752506AbdLLPIo (ORCPT ); Tue, 12 Dec 2017 10:08:44 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51474 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751566AbdLLPIm (ORCPT ); Tue, 12 Dec 2017 10:08:42 -0500 Subject: Re: [PATCH] KVM: X86: Fix host dr6 miss restore To: Wanpeng Li , David Hildenbrand Cc: "linux-kernel@vger.kernel.org" , kvm , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Wanpeng Li , Dmitry Vyukov References: <1512724322-11678-1-git-send-email-wanpeng.li@hotmail.com> <036e6e3d-547c-e7b3-8188-b6a1bf29fb72@redhat.com> From: Paolo Bonzini Message-ID: <9e3b4365-5606-2c97-d231-ec2a7603dc3e@redhat.com> Date: Tue, 12 Dec 2017 16:08:25 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Tue, 12 Dec 2017 15:08:42 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/12/2017 01:44, Wanpeng Li wrote: > 2017-12-08 20:39 GMT+08:00 David Hildenbrand : >> On 08.12.2017 10:12, Wanpeng Li wrote: >>> From: Wanpeng Li >>> >>> Reported by syzkaller: >>> >>> WARNING: CPU: 0 PID: 12927 at arch/x86/kernel/traps.c:780 do_debug+0x222/0x250 >>> CPU: 0 PID: 12927 Comm: syz-executor Tainted: G OE 4.15.0-rc2+ #16 >>> RIP: 0010:do_debug+0x222/0x250 >>> Call Trace: >>> <#DB> >>> debug+0x3e/0x70 >>> RIP: 0010:copy_user_enhanced_fast_string+0x10/0x20 >>> >>> _copy_from_user+0x5b/0x90 >>> SyS_timer_create+0x33/0x80 >>> entry_SYSCALL_64_fastpath+0x23/0x9a >>> >>> The syzkaller will mmap a buffer which is also the struct sigevent parameter of >>> timer_create(), it will also call perf_event_open() to set a BP for the buffer, >>> so when the implementation of timer_create() in kernel tries to get the struct >>> sigevent parameter by copy_from_user(), rep movsb triggers the BP. The syzkaller >>> testcase also sets the debug registers for the guest, however, the kvm just >>> restores host debug registers when we have active breakpoints. I can observe >>> the dr6 single step bit is set and !hw_breakpoint_active() sporadically by print >>> when running the testcase heavy multithreading. The do_debug() which is triggered >>> by rep movsb will splash when (dr6 & DR_STEP && !user_mode(regs)). >>> >>> This patch fixes it by restoring host dr6 unconditionally before preempt/irq >>> enable. >>> >>> Reported-by: Dmitry Vyukov >>> Cc: Paolo Bonzini >>> Cc: Radim Krčmář >>> Cc: David Hildenbrand >>> Cc: Dmitry Vyukov >>> Signed-off-by: Wanpeng Li >>> --- >>> arch/x86/kvm/x86.c | 2 ++ >>> 1 file changed, 2 insertions(+) >>> >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> index 0c5d55c..a6370fd 100644 >>> --- a/arch/x86/kvm/x86.c >>> +++ b/arch/x86/kvm/x86.c >>> @@ -7065,6 +7065,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) >>> */ >>> if (hw_breakpoint_active()) >>> hw_breakpoint_restore(); >>> + else >>> + set_debugreg(current->thread.debugreg6, 6); >>> >>> vcpu->arch.last_guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()); >>> >>> >> >> If you haven't seen it, I analyzed this in >> https://lkml.org/lkml/2017/11/7/638 but nobody would respond for now to >> my suggestion/question. > > I think it's fine to restore dr6 before preempt/irq enable. If no breakpoint is active, you should be able to restore it only in sched_out. Paolo