Re: [PATCH v3] KVM: remove buggy vcpu id check on vcpu creation

[James Hogan <james.hogan@imgtec.com>]

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On Wed, Apr 20, 2016 at 07:02:10PM +0200, Radim Krčmář wrote:
> 2016-04-20 17:44+0200, Greg Kurz:
> > 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>
> > ---
> > v3: use ERR_PTR()
> > 
> >  arch/mips/kvm/mips.c |    7 ++++++-
> >  arch/x86/kvm/x86.c   |    3 +++
> >  virt/kvm/kvm_main.c  |    3 ---
> >  3 files changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> > index 70ef1a43c114..0278ea146db5 100644
> > --- a/arch/mips/kvm/mips.c
> > +++ b/arch/mips/kvm/mips.c
> > @@ -248,9 +248,14 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)
> >  	int err, size, offset;
> >  	void *gebase;
> >  	int i;
> > +	struct kvm_vcpu *vcpu;
> >  
> > -	struct kvm_vcpu *vcpu = kzalloc(sizeof(struct kvm_vcpu), GFP_KERNEL);
> > +	if (id >= KVM_MAX_VCPUS) {
> > +		err = -EINVAL;
> > +		goto out;
> 
> 'vcpu' looks undefined at this point, so kfree in 'out:' may bug.

Thats out_free_cpu I think?

Cheers
James

> Just 'return ERR_PTR(-EINVAL)'?
> 
> > +	}
> >  
> > +	vcpu = kzalloc(sizeof(struct kvm_vcpu), GFP_KERNEL);
> >  	if (!vcpu) {
> >  		err = -ENOMEM;
> >  		goto out;

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