On Thu, 21 Apr 2016 13:29:58 +0200 Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx> wrote: > On Wed, 20 Apr 2016 20:29:09 +0200 > Radim Krčmář <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 ? 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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html