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SubjectRe: [PATCH RFC] KVM: nSVM: Fix L1 state corruption upon return from SMM
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On 23/06/21 15:01, Maxim Levitsky wrote:
> I did some homework on this now and I would like to share few my thoughts on this:
>
> First of all my attention caught the way we intercept the #SMI
> (this isn't 100% related to the bug but still worth talking about IMHO)
>
> A. Bare metal: Looks like SVM allows to intercept SMI, with SVM_EXIT_SMI,
> with an intention of then entering the BIOS SMM handler manually using the SMM_CTL msr.

... or just using STGI, which is what happens for KVM. This is in the
manual: "The hypervisor may respond to the #VMEXIT(SMI) by executing the
STGI instruction, which causes the pending SMI to be taken immediately".

It *should* work for KVM to just not intercept SMI, but it adds more
complexity for no particular gain.

> On bare metal we do set the INTERCEPT_SMI but we emulate the exit as a nop.
> I guess on bare metal there are some undocumented bits that BIOS set which
> make the CPU to ignore that SMI intercept and still take the #SMI handler,
> normally but I wonder if we could still break some motherboard
> code due to that.
>
> B. Nested: If #SMI is intercepted, then it causes nested VMEXIT.
> Since KVM does enable SMI intercept, when it runs nested it means that all SMIs
> that nested KVM gets are emulated as NOP, and L1's SMI handler is not run.

No, this is incorrect. Note that svm_check_nested_events does not clear
smi_pending the way vmx_check_nested_events does it for nmi_pending. So
the interrupt is still there and will be injected on the next STGI.

Paolo

>
> About the issue that was fixed in this patch. Let me try to understand how
> it would work on bare metal:
>
> 1. A guest is entered. Host state is saved to VM_HSAVE_PA area (or stashed somewhere
> in the CPU)
>
> 2. #SMI (without intercept) happens
>
> 3. CPU has to exit SVM, and start running the host SMI handler, it loads the SMM
> state without touching the VM_HSAVE_PA runs the SMI handler, then once it RSMs,
> it restores the guest state from SMM area and continues the guest
>
> 4. Once a normal VMexit happens, the host state is restored from VM_HSAVE_PA
>
> So host state indeed can't be saved to VMC01.
>
> I to be honest think would prefer not to use the L1's hsave area but rather add back our
> 'hsave' in KVM and store there the L1 host state on the nested entry always.
>
> This way we will avoid touching the vmcb01 at all and both solve the issue and
> reduce code complexity.
> (copying of L1 host state to what basically is L1 guest state area and back
> even has a comment to explain why it (was) possible to do so.
> (before you discovered that this doesn't work with SMM).
>
> Thanks again for fixing this bug!
>
> Best regards,
> Maxim Levitsky
>

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