Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
To:     Sean Christopherson <seanjc@google.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        vkuznets@redhat.com, mlevitsk@redhat.com,
        Jim Mattson <jmattson@google.com>
References: <YELdblXaKBTQ4LGf@google.com>
 <fc2b0085-eb0f-dbab-28c2-a244916c655f@redhat.com>
 <YEZUhbBtNjWh0Zka@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 03/28] KVM: nSVM: inject exceptions via
 svm_check_nested_events
Message-ID: <006be822-697e-56d5-84a7-fa51f5087a34@redhat.com>
Date:   Mon, 8 Mar 2021 18:28:24 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.7.0
MIME-Version: 1.0
In-Reply-To: <YEZUhbBtNjWh0Zka@google.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Precedence: bulk

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.

Paolo

> If the I/O is is intercepted by L1, bypassing the IOPL and
> TSS-bitmap checks is wrong and will cause L1 to emulate I/O for L2 userspace
> that should never be allowed.  Odds are there isn't a corresponding emulated
> port in L1, i.e. there's no major security flaw, but it's far from good
> behavior.