Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v2 3/3] KVM: Fix leak vCPU's VMCS value into other pCPU
To:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        stable@vger.kernel.org, Marc Zyngier <Marc.Zyngier@arm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>
References: <1564572438-15518-3-git-send-email-wanpengli@tencent.com>
 <1564573198-16219-1-git-send-email-wanpengli@tencent.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Openpgp: preference=signencrypt
Message-ID: <9240ada8-8e18-d2b2-006e-41ededb89efb@redhat.com>
Date:   Wed, 31 Jul 2019 14:55:33 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <1564573198-16219-1-git-send-email-wanpengli@tencent.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 31/07/19 13:39, Wanpeng Li wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ed061d8..12f2c91 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2506,7 +2506,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>  				continue;
>  			if (vcpu == me)
>  				continue;
> -			if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
> +			if (READ_ONCE(vcpu->preempted) && swait_active(&vcpu->wq))
>  				continue;
>  			if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
>  				!kvm_arch_vcpu_in_kernel(vcpu))
> 

This cannot work.  swait_active means you are waiting, so you cannot be
involuntarily preempted.

The problem here is simply that kvm_vcpu_has_events is being called
without holding the lock.  So kvm_arch_vcpu_runnable is okay, it's the
implementation that's wrong.

Just rename the existing function to just vcpu_runnable and make a new
arch callback kvm_arch_dy_runnable.   kvm_arch_dy_runnable can be
conservative and only returns true for a subset of events, in particular
for x86 it can check:

- vcpu->arch.pv.pv_unhalted

- KVM_REQ_NMI or KVM_REQ_SMI or KVM_REQ_EVENT

- PIR.ON if APICv is set

Ultimately, all variables accessed in kvm_arch_dy_runnable should be
accessed with READ_ONCE or atomic_read.

And for all architectures, kvm_vcpu_on_spin should check
list_empty_careful(&vcpu->async_pf.done)

It's okay if your patch renames the function in non-x86 architectures,
leaving the fix to maintainers.  So, let's CC Marc and Christian since
ARM and s390 have pretty complex kvm_arch_vcpu_runnable as well.

Paolo