Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751379AbdLJAot (ORCPT ); Sat, 9 Dec 2017 19:44:49 -0500 Received: from mail-ot0-f193.google.com ([74.125.82.193]:44854 "EHLO mail-ot0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751190AbdLJAoq (ORCPT ); Sat, 9 Dec 2017 19:44:46 -0500 X-Google-Smtp-Source: AGs4zMasdJpnRXInGbL3WxePOsfB+UyEFwI7lB6i/O+fS8uxWEGNZujSqTjlLf3qETUE7G8yBTz/Ih0eRvyn05MztNU= MIME-Version: 1.0 In-Reply-To: <036e6e3d-547c-e7b3-8188-b6a1bf29fb72@redhat.com> References: <1512724322-11678-1-git-send-email-wanpeng.li@hotmail.com> <036e6e3d-547c-e7b3-8188-b6a1bf29fb72@redhat.com> From: Wanpeng Li Date: Sun, 10 Dec 2017 08:44:45 +0800 Message-ID: Subject: Re: [PATCH] KVM: X86: Fix host dr6 miss restore To: David Hildenbrand Cc: "linux-kernel@vger.kernel.org" , kvm , Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Wanpeng Li , Dmitry Vyukov Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by nfs id vBA0ithN000344 Content-Length: 2505 Lines: 64 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. Regards, Wanpeng Li