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

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Thu, 21 Apr 2016 13:29:58 +0200
Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:

> On Wed, 20 Apr 2016 20:29:09 +0200
> Radim Krčmář <rkrcmar@redhat.com> 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 ?

Probably because noone considered power :)

> 
> > 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 ?
> 
> > Second issue is that Documentation/virtual/kvm/api.txt says
> >   4.7 KVM_CREATE_VCPU
> >   [...]
> >   This API adds a vcpu to a virtual machine.  The vcpu id is a small
> >   integer in the range [0, max_vcpus).
> > 
> 
> Yeah and I find the meaning of max_vcpus is unclear.
> 
> Here it is considered as a limit for vcpu id, but if you look at the code,
> KVM_MAX_VCPUS is also used as a limit for the number of vcpus:
> 
> virt/kvm/kvm_main.c:    if (atomic_read(&kvm->online_vcpus) == KVM_MAX_VCPUS) {
> 
> > so we'd remove those two lines and change the API too.  The change would
> > be somewhat backward compatible, but doesn't PowerPC use high vcpu_id
> > just because KVM is lacking an API to set DT ID?
> 
> This is related to a limitation when running in book3s_hv mode with cpus
> that support SMT (multiple HW threads): all HW threads within a core
> cannot be running in different guests at the same time. 
> 
> We solve this by using a vcpu numbering scheme as follows:
> 
> vcpu_id[N] = (N * thread_per_core_guest) / threads_per_core_host + N % threads_per_core_guest
> 
> where N means "the Nth vcpu presented to the guest". This allows to have groups of vcpus
> that can be scheduled to run on the same real core.
> 
> So, in the "worst" case where we want to run a guest with 1 thread/core and the host
> has 8 threads/core, we will need the vcpu_id limit to be 8*KVM_MAX_VCPUS.
> 
> > x86 (APIC ID) is affected by this and ARM (MP ID) probably too.
> > 
> 
> x86 is limited to KVM_MAX_VCPUS (== 255) vcpus: it won't be affected if we also
> patch kvm_get_vcpu_by_id() like suggested above.
> 
> Depending on the platform, ARM can be limited to VGIC_V3_MAX_CPUS (== 255) or
> VGIC_V8_MAX_CPUS (== 8). I guess it won't be affected either.

For s390, it's either 64 (no esca) or 248 (esca).

> 
> > (Maybe it is time to decouple VCPU ID used in KVM interfaces from
> >  architecture dependent CPU ID that the guest uses ...
> 
> Maybe... I did not get that far.

It seems that the various architectures are more different than I
thought... wasn't aware of the complicated situation on power, for
example.