Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752607AbdLMJnA (ORCPT ); Wed, 13 Dec 2017 04:43:00 -0500 Received: from mx1.redhat.com ([209.132.183.28]:55564 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752523AbdLMJmn (ORCPT ); Wed, 13 Dec 2017 04:42:43 -0500 Subject: Re: [PATCH v2] KVM: X86: Fix host dr6 miss restore To: David Hildenbrand , Wanpeng Li , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Wanpeng Li , Dmitry Vyukov References: <1513134613-7900-1-git-send-email-wanpeng.li@hotmail.com> <6e7feb72-6b65-5dc8-6d4a-e93772dbfed8@redhat.com> From: Paolo Bonzini Message-ID: <9fbc72fa-567b-5672-648f-4721b15e58b0@redhat.com> Date: Wed, 13 Dec 2017 10:42:38 +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: <6e7feb72-6b65-5dc8-6d4a-e93772dbfed8@redhat.com> 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]); Wed, 13 Dec 2017 09:42:43 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2772 Lines: 75 On 13/12/2017 10:18, David Hildenbrand wrote: > On 13.12.2017 04:10, 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 in sched_out if no breakpoint is active. >> >> Reported-by: Dmitry Vyukov >> Cc: Paolo Bonzini >> Cc: Radim Krčmář >> Cc: David Hildenbrand >> Cc: Dmitry Vyukov >> Reviewed-by: David Hildenbrand >> Signed-off-by: Wanpeng Li >> --- >> v1 -> v2: >> * move to sched_out path >> >> 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 1c5c7a3..76886c4 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -2964,6 +2964,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) >> pagefault_enable(); >> kvm_x86_ops->vcpu_put(vcpu); >> vcpu->arch.last_host_tsc = rdtsc(); > > Can you add a comment like > > /* With active breakpoints we already restored all debugregs in > vcpu_enter_guest(), however without active breakpoints we have to > restore debugreg 6 before scheduled out. > */ Actually, we should make it unconditionally zero, not reset it to current->thread.debugreg6. That's because the invariant at exit from do_debug is DR6 = 0. /* * do_debug expects dr6 to be cleared after it runs, but here * we might have a stale dr6 from the guest. */ set_debugreg(0, 6); I'll push the patch to kvm/queue. Thanks, Paolo