[PATCH v7 1/6] KVM: x86: Fix potential race in KVM_GET_CLOCK

[Oliver Upton]

On Thu, Aug 19, 2021 at 11:43 AM Marcelo Tosatti <mtosatti at redhat.com> wrote:
>
> On Mon, Aug 16, 2021 at 12:11:25AM +0000, Oliver Upton wrote:
> > Sean noticed that KVM_GET_CLOCK was checking kvm_arch.use_master_clock
> > outside of the pvclock sync lock. This is problematic, as the clock
> > value written to the user may or may not actually correspond to a stable
> > TSC.
> >
> > Fix the race by populating the entire kvm_clock_data structure behind
> > the pvclock_gtod_sync_lock.
>
> Oliver,
>
> Can you please describe the race in more detail?
>
> Is it about host TSC going unstable VS parallel KVM_GET_CLOCK ?
>

Yeah, pretty much any event that causes us to set use_master_clock =
false could interleave with the KVM_GET_CLOCK ioctl. A guest could
kick its TSCs out of sync, for example, to cause this too. AFAICT, KVM
serializes the write side (pvclock_update_vm_gtod_copy()) with
pvclock_gtod_sync_lock, as it should.

--
Thanks,
Oliver