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

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Date:   Mon, 8 Mar 2021 18:28:24 +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 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.