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

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Wed, 20 Apr 2016 16:12:46 +0100
James Hogan <james.hogan@imgtec.com> wrote:

> 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@linux.vnet.ibm.com>
> > ---
> >  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.
> 

Oops you're right.. my bad. I'll post a v2 right away.

Thanks !

--
Greg

> 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);
> >