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    SubjectRe: [PATCH 03/28] KVM: nSVM: inject exceptions via svm_check_nested_events
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    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.

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