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Subject: Re: [PATCH] KVM: X86: Fix dereference null cpufreq policy
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>
References: <1583133336-7832-1-git-send-email-wanpengli@tencent.com>
 <ab51f6c9-a67d-0107-772a-7fe57a2319cf@redhat.com>
 <20200302081207.3kogqwxbkujqgc7z@vireshk-i7>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <73a7db77-c4c7-029f-fd8a-080911fde41e@redhat.com>
Date:   Mon, 2 Mar 2020 09:39:24 +0100
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On 02/03/20 09:12, Viresh Kumar wrote:
> On 02-03-20, 08:55, Paolo Bonzini wrote:
>> On 02/03/20 08:15, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpengli@tencent.com>
>>>
>>> cpufreq policy which is get by cpufreq_cpu_get() can be NULL if it is failure,
>>> this patch takes care of it.
>>>
>>> Fixes: aaec7c03de (KVM: x86: avoid useless copy of cpufreq policy)
>>> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
>>> Cc: Naresh Kamboju <naresh.kamboju@linaro.org>
>>> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
>>
>> My bad, I checked kobject_put but didn't check that kobj is first in
>> struct cpufreq_policy.
>>
>> I think we should do this in cpufreq_cpu_put or, even better, move the
>> kobject struct first in struct cpufreq_policy.  Rafael, Viresh, any
>> objection?
>>
>> Paolo
>>
>>>  		policy = cpufreq_cpu_get(cpu);
>>> -		if (policy && policy->cpuinfo.max_freq)
>>> -			max_tsc_khz = policy->cpuinfo.max_freq;
>>> +		if (policy) {
>>> +			if (policy->cpuinfo.max_freq)
>>> +				max_tsc_khz = policy->cpuinfo.max_freq;
>>> +			cpufreq_cpu_put(policy);
>>> +		}
> 
> I think this change makes sense and I am not sure why should we even
> try to support cpufreq_cpu_put(NULL).

For the same reason why we support kfree(NULL) and kobject_put(NULL)?

Paolo