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

[David Hildenbrand <dahi@linux.vnet.ibm.com>]

> On Wed, 20 Apr 2016 17:44:54 +0200
> Greg Kurz <gkurz@linux.vnet.ibm.com> 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>
> > ---
> > 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(-)
> >   
> 
> Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> 

The existing s390 check looks sane to me, too!

David