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

[David Hildenbrand]

> On Wed, 20 Apr 2016 20:29:09 +0200
> Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx> 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.    
> > 
> > The only problematic path I see is kvm_get_vcpu_by_id(), which returns
> > NULL for any id above KVM_MAX_VCPUS.  
> 
> Oops my bad, I started to work on a 4.4 tree and I missed this check brought
> by commit c896939f7cff (KVM: use heuristic for fast VCPU lookup by id).
> 
> But again, I believe the check is wrong there also: the changelog just mentions
> this is a fastpath for the usual case where "VCPU ids match the array index"...
> why does the patch add a NULL return path if id >= KVM_MAX_VCPUS ?
> 
> > kvm_vm_ioctl_create_vcpu() uses kvm_get_vcpu_by_id() to check for
> > duplicate ids, so PowerPC could end up with many VCPUs of the same id.
> > I'm not sure what could fail, but code doesn't expect this situation.
> > Patching kvm_get_vcpu_by_id() is easy, though.
> >   
> 
> Something like this ?
> 
> 	if (id < 0)
> 		return NULL;
> 	if (id < KVM_MAX_VCPUS)
> 		vcpu = kvm_get_vcpu(kvm, id);
> 
> In the same patch ?

So the heuristic would only trigger if id < KVM_MAX_VCPUS.
By initializing vcpu to NULL this would work.

David