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

>
> 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

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