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

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Message-ID: <87czppnasv.fsf@vitty.brq.redhat.com>
Date:   Fri, 03 Sep 2021 13:44:16 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jiang Jiasheng <jiasheng@...as.ac.cn>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-sgx@...r.kernel.org, Jiang Jiasheng <jiasheng@...as.ac.cn>,
        pbonzini@...hat.com, seanjc@...gle.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

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.

> Signed-off-by: Jiang Jiasheng <jiasheng@...as.ac.cn>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e0f4a46..c59013c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2871,7 +2871,7 @@ 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 (atomic_read(&(v->kvm)->online_vcpus) > 0 && v == kvm_get_vcpu(v->kvm, 0))
>  		kvm_hv_setup_tsc_page(v->kvm, &vcpu->hv_clock);
>  	return 0;
>  }

-- 
Vitaly