Re: [PATCH v4 2/2] KVM: move vcpu id checking to archs

[Greg Kurz]

On Thu, 21 Apr 2016 18:00:19 +0200
Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx> wrote:

> 2016-04-21 16:20+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. While here,
> > we also update the documentation to dissociate vcpu ids from the maximum
> > number of vcpus per virtual machine.
> > 
> > Acked-by: James Hogan <james.hogan@xxxxxxxxxx>
> > Acked-by: Cornelia Huck <cornelia.huck@xxxxxxxxxx>
> > Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
> > ---
> > v4: - updated subject for more clarity on what the patch does
> >     - added James's and Connie's A-b tags
> >     - updated documentation
> > 
> >  Documentation/virtual/kvm/api.txt |    7 +++----
> >  arch/mips/kvm/mips.c              |    7 ++++++-
> >  arch/x86/kvm/x86.c                |    3 +++
> >  virt/kvm/kvm_main.c               |    3 ---
> >  4 files changed, 12 insertions(+), 8 deletions(-)
> > 
> > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > index 4d0542c5206b..486a1d783b82 100644
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -199,11 +199,10 @@ Type: vm ioctl
> >  Parameters: vcpu id (apic id on x86)
> >  Returns: vcpu fd on success, -1 on error
> >  
> > -This API adds a vcpu to a virtual machine.  The vcpu id is a small integer
> > -in the range [0, max_vcpus).
> > +This API adds a vcpu to a virtual machine.  The vcpu id is a positive integer.  
> 
> Userspace won't be able to tell if KVM_CREATE_VCPU failed because it
> provided too high vcpu_id to an old KVM or because new KVM failed in
> other areas.  Not a huge problem (because I expect that userspace will
> die on both), but a new KVM_CAP would be able to disambiguate it.
> 
> Toggleable capability doesn't seem necessary and only PowerPC changes,
> so the capability could be arch specific ... I think that a generic one
> makes more sense, though.
>

I'm not sure userspace can disambiguate all the cases where KVM_CREATE_VCPU
returns EINVAL already... and, FWIW, QEMU simply exits if it gets an error.

So I understand your concern but would we have a user for this ?

> Userspace also doesn't know the vcpu id limit anymore, and it might
> care.  What do you think about returning the arch-specific limit (or the
> highest positive integer) as int in KVM_CAP_MAX_VCPU_ID?
> 

This is partly true: only arch agnostic code would be lost.

Moreover this is a problem for powerpc only at the moment and userspace code
can compute the vcpu_id limit out of KVM_CAP_MAX_VCPUS and KVM_CAP_PPC_SMT.

For other architectures, it is simply KVM_MAX_VCPUS.

> I think this would also clarify the connection between VCPU limit and
> VCPU_ID limit.  Or is a boolean cap better?
> 

Well, I'm not fan of adding a generic API to handle a corner case... maybe later
if we have other scenarios where vcpu ids need to cross the limit ?

> > -The recommended max_vcpus value can be retrieved using the KVM_CAP_NR_VCPUS of
> > -the KVM_CHECK_EXTENSION ioctl() at run-time.
> > +The recommended maximum number of vcpus (max_vcpus) can be retrieved using the
> > +KVM_CAP_NR_VCPUS of the KVM_CHECK_EXTENSION ioctl() at run-time.
> >  The maximum possible value for max_vcpus can be retrieved using the
> >  KVM_CAP_MAX_VCPUS of the KVM_CHECK_EXTENSION ioctl() at run-time.  
>