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

From: Vitaly Kuznetsov
Subject: Re: [PULL 04/11] i386: expand Hyper-V features during CPU feature expansion time
Date: Fri, 16 Jul 2021 11:07:06 +0200

Peter Maydell <peter.maydell@linaro.org> writes:

> 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
>> 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.

get_supported_hv_cpuid_legacy() is only called when KVM_CAP_HYPERV_CPUID
is not supported and this is not possible with
KVM_CAP_SYS_HYPERV_CPUID. Coverity, of course, can't know that.

> 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.

It seems an assert(cs) before calling get_supported_hv_cpuid_legacy()
(with a proper comment) should do the job.

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

My bad, will fix.


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