Re: [PATCH] KVM: remove buggy vcpu id check on vcpu creation

[James Hogan]

Hi Greg,

On Wed, Apr 20, 2016 at 05:07:58PM +0200, Greg Kurz wrote:
> Commit 338c7dbadd26 ("KVM: Improve create VCPU parameter (CVE-2013-4587)")
> introduced a check to prevent potential kernel memory corruption in case
> the vcpu id is too great.
> 
> Unfortunately this check assumes vcpu ids grow in sequence with a common
> difference of 1, which is wrong: archs are free to use vcpu id as they fit.
> For example, QEMU originated vcpu ids for PowerPC cpus running in boot3s_hv
> mode, can grow with a common difference of 2, 4 or 8: if KVM_MAX_VCPUS is
> 1024, guests may be limited down to 128 vcpus on POWER8.
> 
> This means the check does not belong here and should be moved to some arch
> specific function: kvm_arch_vcpu_create() looks like a good candidate.
> 
> ARM and s390 already have such a check.
> 
> I could not spot any path in the PowerPC or common KVM code where a vcpu
> id is used as described in the above commit: I believe PowerPC can live
> without this check.
> 
> In the end, this patch simply moves the check to MIPS and x86.
> 
> Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/mips/kvm/mips.c |    3 +++
>  arch/x86/kvm/x86.c   |    3 +++
>  virt/kvm/kvm_main.c  |    3 ---
>  3 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index 70ef1a43c114..ce3f1e8a8b3f 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -251,6 +251,9 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)
>  
>  	struct kvm_vcpu *vcpu = kzalloc(sizeof(struct kvm_vcpu), GFP_KERNEL);
>  
> +	if (id >= KVM_MAX_VCPUS)
> +		return -EINVAL;

This needs to go before the kzalloc above, otherwise you introduce a
memory leak.

Cheers
James

> +
>  	if (!vcpu) {
>  		err = -ENOMEM;
>  		goto out;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9b7798c7b210..f705d57b12ed 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7358,6 +7358,9 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
>  {
>  	struct kvm_vcpu *vcpu;
>  
> +	if (id >= KVM_MAX_VCPUS)
> +		return -EINVAL;
> +
>  	if (check_tsc_unstable() && atomic_read(&kvm->online_vcpus) != 0)
>  		printk_once(KERN_WARNING
>  		"kvm: SMP vm created on host with unstable TSC; "
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 4fd482fb9260..6b6cca3cb488 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2272,9 +2272,6 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
>  	int r;
>  	struct kvm_vcpu *vcpu;
>  
> -	if (id >= KVM_MAX_VCPUS)
> -		return -EINVAL;
> -
>  	vcpu = kvm_arch_vcpu_create(kvm, id);
>  	if (IS_ERR(vcpu))
>  		return PTR_ERR(vcpu);
>

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