Re: [PULL 04/11] i386: expand Hyper-V features during CPU feature expansion time

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 13 Jul 2021 at 17:19, Eduardo Habkost <ehabkost@redhat.com> wrote:
>
> From: Vitaly Kuznetsov <vkuznets@redhat.com>
>
> To make Hyper-V features appear in e.g. QMP query-cpu-model-expansion we
> need to expand and set the corresponding CPUID leaves early. Modify
> x86_cpu_get_supported_feature_word() to call newly intoduced Hyper-V
> specific kvm_hv_get_supported_cpuid() instead of
> kvm_arch_get_supported_cpuid(). We can't use kvm_arch_get_supported_cpuid()
> as Hyper-V specific CPUID leaves intersect with KVM's.
>
> Note, early expansion will only happen when KVM supports system wide
> KVM_GET_SUPPORTED_HV_CPUID ioctl (KVM_CAP_SYS_HYPERV_CPUID).
>
> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> Message-Id: <20210608120817.1325125-6-vkuznets@redhat.com>
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

Hi; Coverity reports an issue in this code (CID 1458243):

> -static bool hyperv_expand_features(CPUState *cs, Error **errp)
> +bool kvm_hyperv_expand_features(X86CPU *cpu, Error **errp)
>  {
> -    X86CPU *cpu = X86_CPU(cs);
> +    CPUState *cs = CPU(cpu);
>
>      if (!hyperv_enabled(cpu))
>          return true;
>
> +    /*
> +     * When kvm_hyperv_expand_features is called at CPU feature expansion
> +     * time per-CPU kvm_state is not available yet so we can only proceed
> +     * when KVM_CAP_SYS_HYPERV_CPUID is supported.
> +     */
> +    if (!cs->kvm_state &&
> +        !kvm_check_extension(kvm_state, KVM_CAP_SYS_HYPERV_CPUID))
> +        return true;

Here we check whether cs->kvm_state is NULL, but even if it is
NULL we can still continue execution further through the function.

Later in the function we call hv_cpuid_get_host(), which in turn
can call get_supported_hv_cpuid_legacy(), which can dereference
cs->kvm_state without checking it.

So either the check on cs->kvm_state above is unnecessary, or we
need to handle it being NULL in some way other than falling through.

Side note: this change isn't in line with our coding style, which
requires braces around the body of the if().

thanks
-- PMM