linux-kernel - Re: [PATCH v3 0/7] KVM: few more SMM fixes

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Date:   Wed, 22 Sep 2021 20:17:02 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 0/7] KVM: few more SMM fixes

On 22/09/21 17:52, Sean Christopherson wrote:
> On Wed, Sep 22, 2021, Paolo Bonzini wrote:
>> On 22/09/21 16:46, Sean Christopherson wrote:
>>> On Wed, Sep 22, 2021, Paolo Bonzini wrote:
>>>> On 13/09/21 16:09, Maxim Levitsky wrote:
>>>>>      KVM: x86: nVMX: re-evaluate emulation_required on nested VM exit
>>>
>>> ...
>>>> Queued, thanks.  However, I'm keeping patch 1 for 5.16 only.
>>>
>>> I'm pretty sure the above patch is wrong, emulation_required can simply be
>>> cleared on emulated VM-Exit.
>>
>> Are you sure?
> 
> Pretty sure, but not 100% sure :-)
> 
>> I think you can at least set the host segment fields to a data segment that
>> requires emulation.  For example the DPL of the host DS is hardcoded to zero,
>> but the RPL comes from the selector field and the DS selector is not
>> validated.
> 
> HOST_DS_SEL is validated:
> 
>    In the selector field for each of CS, SS, DS, ES, FS, GS and TR, the RPL
>    (bits 1:0) and the TI flag (bit 2) must be 0.

Ah, I think that's a bug in the manual.  In "27.5.2 Loading Host Segment 
and Descriptor-Table Registers" the reference to 26.3.1.2 should be 
26.2.3 ("Checks on Host Segment and Descriptor-Table Registers").  That 
one does cover all segment registers.  Hmm, who do we ask now about 
fixing Intel manuals?

So yeah, a WARN_ON_ONCE might be in order.  But I don't feel super safe 
making it false when it is possible to make KVM do something that is at 
least sensible.

Paolo

>> Therefore a subsequent vmentry could fail the access rights tests of 26.3.1.2
>> Checks on Guest Segment Registers:
> 
> Yes, but this path is loading host state on VM-Exit.
> 
>> DS, ES, FS, GS. The DPL cannot be less than the RPL in the selector field if
>> (1) the “unrestricted guest” VM-execution control is 0; (2) the register is
>> usable; and (3) the Type in the access-rights field is in the range 0 – 11
>> (data segment or non-conforming code segment).
>>
>> Paolo
>>
>