linux-kernel - Re: [PATCH 03/28] KVM: nSVM: inject exceptions via svm_check_nested

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Date:   Mon, 8 Mar 2021 23:51:46 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, mlevitsk@...hat.com,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 03/28] KVM: nSVM: inject exceptions via
 svm_check_nested_events

On 08/03/21 21:43, Sean Christopherson wrote:
> On Mon, Mar 08, 2021, Paolo Bonzini wrote:
>> On 08/03/21 17:44, Sean Christopherson wrote:
>>> VMCALL is also probably ok
>>> in most scenarios, but patching L2's code from L0 KVM is sketchy.
>>
>> I agree that patching is sketchy and I'll send a patch.  However...
>>
>>>> The same is true for the VMware #GP interception case.
>>>
>>> I highly doubt that will ever work out as intended for the modified IO #GP
>>> behavior.  The only way emulating #GP in L2 is correct if L1 wants to pass
>>> through the capabilities to L2, i.e. the I/O access isn't intercepted by L1.
>>> That seems unlikely.
>>
>> ... not all hypervisors trap everything.  In particular in this case the
>> VMCS12 I/O permission bitmap should be consulted (which we do in
>> vmx_check_intercept_io), but if the I/O is not trapped by L1 it should
>> bypass the IOPL and TSS-bitmap checks in my opinion.
> 
> I agree, _if_ it's not trapped.  But bypassing the checks when it is trapped is
> clearly wrong.

You can still trap #GP unconditionally and run the emulator. The 
intercept check will (or should) handle it.

Paolo