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[34.82.80.254]) by smtp.gmail.com with ESMTPSA id z15sm24291333pgu.71.2021.06.30.09.24.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Jun 2021 09:24:45 -0700 (PDT) Date: Wed, 30 Jun 2021 16:24:41 +0000 From: David Matlack To: Venkatesh Srinivas Cc: kvm@vger.kernel.org, jmattson@google.com, seanjc@google.com, elver@google.com, dvyukov@google.com Subject: Re: [PATCH] KVM: kvm_vcpu_kick: Do not read potentially-stale vcpu->cpu Message-ID: References: <20210630031037.584190-1-venkateshs@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210630031037.584190-1-venkateshs@chromium.org> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, Jun 29, 2021 at 08:10:37PM -0700, Venkatesh Srinivas wrote: > vcpu->cpu contains the last cpu a vcpu run/is running on; > kvm_vcpu_kick is used to 'kick' a guest vcpu by sending an IPI > to the last CPU; vcpu->cpu is updated on sched_in when a vcpu > moves to a new CPU, so it possible for the vcpu->cpu field to > be stale. > > kvm_vcpu_kick also read vcpu->cpu early with a plain read, > while vcpu->cpu could be concurrently written. This caused > a data race, found by kcsan: > > write to 0xffff8880274e8460 of 4 bytes by task 30718 on cpu 0: > kvm_arch_vcpu_load arch/x86/kvm/x86.c:4018 > kvm_sched_in virt/kvm/kvm_main.c:4864 > > vs > kvm_vcpu_kick virt/kvm/kvm_main.c:2909 > kvm_arch_async_page_present_queued arch/x86/kvm/x86.c:11287 > async_pf_execute virt/kvm/async_pf.c:79 > ... > > Use a READ_ONCE to atomically read vcpu->cpu and avoid the > data race. > > Found by kcsan & syzbot. > > Signed-off-by: Venkatesh Srinivas Reviewed-by: David Matlack > --- > virt/kvm/kvm_main.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 46fb042837d2..0525f42afb7d 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -3058,16 +3058,18 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_wake_up); > */ > void kvm_vcpu_kick(struct kvm_vcpu *vcpu) > { > - int me; > - int cpu = vcpu->cpu; > + int me, cpu; > > if (kvm_vcpu_wake_up(vcpu)) > return; > > me = get_cpu(); > - if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu)) > - if (kvm_arch_vcpu_should_kick(vcpu)) > + if (kvm_arch_vcpu_should_kick(vcpu)) { > + cpu = READ_ONCE(vcpu->cpu); > + WARN_ON_ONCE(cpu == me); nit: A comment here may be useful to explain the interaction with kvm_arch_vcpu_should_kick(). Took me a minute to understand why you added the warning. > + if ((unsigned)cpu < nr_cpu_ids && cpu_online(cpu)) > smp_send_reschedule(cpu); > + } > put_cpu(); > } > EXPORT_SYMBOL_GPL(kvm_vcpu_kick); > -- > 2.30.2 >