linux-kernel - Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check

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Message-ID: <87mtjih3ag.fsf@redhat.com>
Date:   Wed, 26 Jan 2022 17:25:59 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Joe Perches <joe@...ches.com>, Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check_equal()

Sean Christopherson <seanjc@...gle.com> writes:

> On Wed, Jan 26, 2022, Vitaly Kuznetsov wrote:
>> Joe Perches <joe@...ches.com> writes:
>> 
>> > On Mon, 2022-01-24 at 11:36 +0100, Vitaly Kuznetsov wrote:
>> >> kvm_cpuid_check_equal() should also check .flags equality but instead
>> >> of adding it to the existing check, just switch to using memcmp() for
>> >> the whole 'struct kvm_cpuid_entry2'.
>> >
>> > Is the struct padding guaranteed to be identical ?
>> >
>> 
>> Well, yes (or we're all doomeed):
>> - 'struct kvm_cpuid_entry2' is part of KVM userspace ABI, it is supposed
>> to be stable.
>> - Here we compare structs which come from the same userspace during one
>> session (vCPU fd stays open), I can't imagine how structure layout can
>> change on-the-fly.
>
> I'm pretty sure Joe was asking if the contents of the padding field would be
> identical, i.e. if KVM can guarnatee there won't be false positives on
> mismatches,

Ah, sorry, I thought about structure layout. Generally, there's no
guarantee the content of the padding will be the same in the
KVM_SET_CPUID2 case as we vmemdup_user() what userspace VMM gives us.

> which is the same reason Paolo passed on this patch.  Though I still think we
> should roll the dice :-)

Well, so far we've only identified CPU (re-)hotplug by reusing an
existing vCPU fd as a broken use-case and it may happen that QEMU is the
only VMM which does that (and memcmp() approach works for it) ... but
who know what's out there)

-- 
Vitaly