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Subject: Re: [PATCH RFC 03/39] KVM: x86/xen: register shared_info page
To:     David Woodhouse <dwmw2@infradead.org>,
        Joao Martins <joao.m.martins@oracle.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org
References: <20190220201609.28290-1-joao.m.martins@oracle.com>
 <20190220201609.28290-4-joao.m.martins@oracle.com>
 <b647bed6c75f8743b8afea251a88f00a5feaee29.camel@infradead.org>
From:   Ankur Arora <ankur.a.arora@oracle.com>
Message-ID: <2d4df59d-f945-32dc-6999-a6f711e972ea@oracle.com>
Date:   Tue, 1 Dec 2020 16:40:07 -0800
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On 2020-12-01 5:07 a.m., David Woodhouse wrote:
> On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote:
>> +static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn)
>> +{
>> +       struct shared_info *shared_info;
>> +       struct page *page;
>> +
>> +       page = gfn_to_page(kvm, gfn);
>> +       if (is_error_page(page))
>> +               return -EINVAL;
>> +
>> +       kvm->arch.xen.shinfo_addr = gfn;
>> +
>> +       shared_info = page_to_virt(page);
>> +       memset(shared_info, 0, sizeof(struct shared_info));
>> +       kvm->arch.xen.shinfo = shared_info;
>> +       return 0;
>> +}
>> +
> 
> Hm.
> 
> How come we get to pin the page and directly dereference it every time,
> while kvm_setup_pvclock_page() has to use kvm_write_guest_cached()
> instead?

So looking at my WIP trees from the time, this is something that
we went back and forth on as well with using just a pinned page or a
persistent kvm_vcpu_map().

I remember distinguishing shared_info/vcpu_info from kvm_setup_pvclock_page()
as shared_info is created early and is not expected to change during the
lifetime of the guest which didn't seem true for MSR_KVM_SYSTEM_TIME (or
MSR_KVM_STEAL_TIME) so that would either need to do a kvm_vcpu_map()
kvm_vcpu_unmap() dance or do some kind of synchronization.

That said, I don't think this code explicitly disallows any updates
to shared_info.

> 
> If that was allowed, wouldn't it have been a much simpler fix for
> CVE-2019-3016? What am I missing?

Agreed.

Perhaps, Paolo can chime in with why KVM never uses pinned page
and always prefers to do cached mappings instead?

> 
> Should I rework these to use kvm_write_guest_cached()?

kvm_vcpu_map() would be better. The event channel logic does RMW operations
on shared_info->vcpu_info.


Ankur

> 
>