Re: [PATCH v2 02/13] KVM: x86: Protect kvm_hv_msr_[get|set]_crash_data() from Spectre-v1/L1TF attacks

[Marios Pomonis <pomonis@google.com>]

On Thu, Dec 12, 2019 at 1:43 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Marios Pomonis <pomonis@google.com> writes:
>
> > This fixes Spectre-v1/L1TF vulnerabilities in kvm_hv_msr_get_crash_data()
> > and kvm_hv_msr_set_crash_data().
> > These functions contain index computations that use the
> > (attacker-controlled) MSR number.
>
> Just to educate myself,
>
> in both cases 'index' is equal to 'msr - HV_X64_MSR_CRASH_P0' where
> 'msr' is constrained:
>   case HV_X64_MSR_CRASH_P0 ... HV_X64_MSR_CRASH_P4:
>          ....
>
> and moreover, kvm_hv_{get,set}_msr_common() is only being called for a
> narrow set of MSRs. How can an atacker overcome these limitations?
>
This attack scenario relies on speculative execution. Practically, one
could train the branch predictors involved to speculatively execute
this path even if the adversary-supplied MSR number does not fall into
the legitimate range. The adversary-supplied MSR number however is
going to be used when -speculatively- computing the index of the array
thus allowing an attacker to load normally illegitimate memory values
in the L1 cache.
> >
> > Fixes: commit e7d9513b60e8 ("kvm/x86: added hyper-v crash msrs into kvm hyperv context")
> >
> > Signed-off-by: Nick Finco <nifi@google.com>
> > Signed-off-by: Marios Pomonis <pomonis@google.com>
> > Reviewed-by: Andrew Honig <ahonig@google.com>
> > Cc: stable@vger.kernel.org
> > ---
> >  arch/x86/kvm/hyperv.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> > index 23ff65504d7e..26408434b9bc 100644
> > --- a/arch/x86/kvm/hyperv.c
> > +++ b/arch/x86/kvm/hyperv.c
> > @@ -809,11 +809,12 @@ static int kvm_hv_msr_get_crash_data(struct kvm_vcpu *vcpu,
> >                                    u32 index, u64 *pdata)
> >  {
> >       struct kvm_hv *hv = &vcpu->kvm->arch.hyperv;
> > +     size_t size = ARRAY_SIZE(hv->hv_crash_param);
> >
> > -     if (WARN_ON_ONCE(index >= ARRAY_SIZE(hv->hv_crash_param)))
> > +     if (WARN_ON_ONCE(index >= size))
> >               return -EINVAL;
> >
> > -     *pdata = hv->hv_crash_param[index];
> > +     *pdata = hv->hv_crash_param[array_index_nospec(index, size)];
> >       return 0;
> >  }
> >
> > @@ -852,11 +853,12 @@ static int kvm_hv_msr_set_crash_data(struct kvm_vcpu *vcpu,
> >                                    u32 index, u64 data)
> >  {
> >       struct kvm_hv *hv = &vcpu->kvm->arch.hyperv;
> > +     size_t size = ARRAY_SIZE(hv->hv_crash_param);
> >
> > -     if (WARN_ON_ONCE(index >= ARRAY_SIZE(hv->hv_crash_param)))
> > +     if (WARN_ON_ONCE(index >= size))
> >               return -EINVAL;
> >
> > -     hv->hv_crash_param[index] = data;
> > +     hv->hv_crash_param[array_index_nospec(index, size)] = data;
> >       return 0;
> >  }
>
> --
> Vitaly
>


-- 
Marios Pomonis
Software Engineer, Security
GCP Platform Security
US-KIR-6THC