linux-kernel - Re: [PATCH 4/4] KVM: X86: Potential 'index out of range' bug

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Message-ID: <87tuiy3qtc.fsf@vitty.brq.redhat.com>
Date:   Mon, 06 Sep 2021 13:07:43 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Jiang Jiasheng <jiasheng@...as.ac.cn>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org,
        pbonzini@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com, jarkko@...nel.org,
        dave.hansen@...ux.intel.com
Subject: Re: [PATCH 4/4] KVM: X86: Potential 'index out of range' bug

Sean Christopherson <seanjc@...gle.com> writes:

> On Fri, Sep 03, 2021, Vitaly Kuznetsov wrote:
>> Jiang Jiasheng <jiasheng@...as.ac.cn> writes:
>> 
>> > The kvm_get_vcpu() will call for the array_index_nospec()
>> > with the value of atomic_read(&(v->kvm)->online_vcpus) as size,
>> > and the value of constant '0' as index.
>> > If the size is also '0', it will be unreasonabe
>> > that the index is no less than the size.
>> >
>> 
>> Can this really happen?
>> 
>> 'online_vcpus' is never decreased, it is increased with every
>> kvm_vm_ioctl_create_vcpu() call when a new vCPU is created and is set to
>> 0 when all vCPUs are destroyed (kvm_free_vcpus()).
>> 
>> kvm_guest_time_update() takes a vcpu as a parameter, this means that at
>> least 1 vCPU is currently present so 'online_vcpus' just can't be zero.
>
> Agreed, but doing kvm_get_vcpu() is ugly and overkill.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 86539c1686fa..cc1cb9a401cd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2969,7 +2969,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>                                        offsetof(struct compat_vcpu_info, time));
>         if (vcpu->xen.vcpu_time_info_set)
>                 kvm_setup_pvclock_page(v, &vcpu->xen.vcpu_time_info_cache, 0);
> -       if (v == kvm_get_vcpu(v->kvm, 0))
> +       if (!kvm_vcpu_get_idx(v))

Do we really need to keep kvm_vcpu_get_idx() around though? It has only
3 users, all in arch/x86/kvm/hyperv.[ch], and the inline simpy returns
'vcpu->vcpu_idx'.

>                 kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock);
>         return 0;
>  }
>

-- 
Vitaly