From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757604AbdELHpD (ORCPT ); Fri, 12 May 2017 03:45:03 -0400 Received: from mail-wr0-f195.google.com ([209.85.128.195]:35956 "EHLO mail-wr0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756285AbdELHpA (ORCPT ); Fri, 12 May 2017 03:45:00 -0400 Subject: Re: [PATCH v3] KVM: x86: Fix potential preemption when get the current kvmclock timestamp To: Wanpeng Li , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Wanpeng Li , stable References: <1494551525-4100-1-git-send-email-wanpeng.li@hotmail.com> From: Paolo Bonzini Message-ID: Date: Fri, 12 May 2017 09:44:54 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.0 MIME-Version: 1.0 In-Reply-To: <1494551525-4100-1-git-send-email-wanpeng.li@hotmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/05/2017 03:12, Wanpeng Li wrote: > From: Wanpeng Li > > BUG: using __this_cpu_read() in preemptible [00000000] code: qemu-system-x86/2809 > caller is __this_cpu_preempt_check+0x13/0x20 > CPU: 2 PID: 2809 Comm: qemu-system-x86 Not tainted 4.11.0+ #13 > Call Trace: > dump_stack+0x99/0xce > check_preemption_disabled+0xf5/0x100 > __this_cpu_preempt_check+0x13/0x20 > get_kvmclock_ns+0x6f/0x110 [kvm] > get_time_ref_counter+0x5d/0x80 [kvm] > kvm_hv_process_stimers+0x2a1/0x8a0 [kvm] > ? kvm_hv_process_stimers+0x2a1/0x8a0 [kvm] > ? kvm_arch_vcpu_ioctl_run+0xac9/0x1ce0 [kvm] > kvm_arch_vcpu_ioctl_run+0x5bf/0x1ce0 [kvm] > kvm_vcpu_ioctl+0x384/0x7b0 [kvm] > ? kvm_vcpu_ioctl+0x384/0x7b0 [kvm] > ? __fget+0xf3/0x210 > do_vfs_ioctl+0xa4/0x700 > ? __fget+0x114/0x210 > SyS_ioctl+0x79/0x90 > entry_SYSCALL_64_fastpath+0x23/0xc2 > RIP: 0033:0x7f9d164ed357 > ? __this_cpu_preempt_check+0x13/0x20 > > This can be reproduced by run kvm-unit-tests/hyperv_stimer.flat w/ > CONFIG_PREEMPT and CONFIG_DEBUG_PREEMPT enabled. > > Safe access to per-CPU data requires a couple of constraints, though: the > thread working with the data cannot be preempted and it cannot be migrated > while it manipulates per-CPU variables. If the thread is preempted, the > thread that replaces it could try to work with the same variables; migration > to another CPU could also cause confusion. However there is no preemption > disable when reads host per-CPU tsc rate to calculate the current kvmclock > timestamp. > > This patch fixes it by utilizing get_cpu/put_cpu pair to guarantee both > __this_cpu_read() and rdtsc() are not preempted. > > Cc: Paolo Bonzini > Cc: Radim Krčmář > Signed-off-by: Wanpeng Li > --- > v2 -> v3: > * update patch description > v1 -> v2: > * utilize get_cpu/put_cpu pair to protect both __this_cpu_read and rdtsc() > > arch/x86/kvm/x86.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index b54125b..3b5fc7e 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -1763,6 +1763,7 @@ u64 get_kvmclock_ns(struct kvm *kvm) > { > struct kvm_arch *ka = &kvm->arch; > struct pvclock_vcpu_time_info hv_clock; > + u64 ret; > > spin_lock(&ka->pvclock_gtod_sync_lock); > if (!ka->use_master_clock) { > @@ -1774,10 +1775,17 @@ u64 get_kvmclock_ns(struct kvm *kvm) > hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset; > spin_unlock(&ka->pvclock_gtod_sync_lock); > > + /* both __this_cpu_read() and rdtsc() should be on the same cpu */ > + get_cpu(); > + > kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL, > &hv_clock.tsc_shift, > &hv_clock.tsc_to_system_mul); > - return __pvclock_read_cycles(&hv_clock, rdtsc()); > + ret = __pvclock_read_cycles(&hv_clock, rdtsc()); > + > + put_cpu(); > + > + return ret; > } > > static void kvm_setup_pvclock_page(struct kvm_vcpu *v) > Reviewed-by: Paolo Bonzini Cc: stable@vger.kernel.org