linux-kernel - Re: [PATCH v2 0/3] KVM: few more SMM fixes

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Date:   Mon, 23 Aug 2021 16:01:50 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jim Mattson <jmattson@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Wanpeng Li <wanpengli@...cent.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v2 0/3] KVM: few more SMM fixes

On Mon, 2021-08-23 at 14:46 +0300, Maxim Levitsky wrote:
> These are few SMM fixes I was working on last week.
> 
> * First patch fixes a minor issue that remained after
>   commit 37be407b2ce8 ("KVM: nSVM: Fix L1 state corruption upon return from SMM")
> 
>   While now, returns to guest mode from SMM work due to restored state from HSAVE
>   area, the guest entry still sees incorrect HSAVE state.
> 
>   This for example breaks return from SMM when the guest is 32 bit, due to PDPTRs
>   loading which are done using incorrect MMU state which is incorrect,
>   because it was setup with incorrect L1 HSAVE state.
> 
> * 2nd patch fixes a theoretical issue that I introduced with my SREGS2 patchset,
>   which Sean Christopherson pointed out.
> 
>   The issue is that KVM_REQ_GET_NESTED_STATE_PAGES request is not only used
>   for completing the load of the nested state, but it is also used to complete
>   exit from SMM to guest mode, and my compatibility hack of pdptrs_from_userspace
>   was done assuming that this is not done.
> 
>   While it is safe to just reset 'pdptrs_from_userspace' on each VM entry,
>   I don't want to slow down the common code for this very rare hack.
>   Instead I explicitly zero this variable when SMM exit to guest mode is done,
>   because in this case PDPTRs do need to be reloaded from memory always.
> 
>   Note that this is a theoretical issue only, because after 'vendor' return from
>   smm code (aka .leave_smm) is done, even when it returned to the guest mode,
>   which loads some of L2 CPU state, we still load again all of the L2 cpu state
>   captured in SMRAM which includes CR3, at which point guest PDPTRs are re-loaded
>   anyway.
> 
>   Also note that across SMI entries the CR3 seems not to be updated, and Intel's
>   SDM notes that it saved value in SMRAM isn't writable, thus it is possible
>   that if SMM handler didn't change CR3, the pdptrs would not be touched.
> 
>   I guess that means that a SMI handler can in theory preserve PDPTRs by never
>   touching CR3, but since recently we removed that code that didn't update PDPTRs
>   if CR3 didn't change, I guess it won't work.
> 
>   Anyway I don't think any OS bothers to have PDPTRs not synced with whatever
>   page CR3 points at, thus I didn't bother to try and test what the real hardware
>   does in this case.
> 
> * 3rd patch makes SVM SMM exit to be a bit more similar to how VMX does it
>   by also raising KVM_REQ_GET_NESTED_STATE_PAGES requests.
> 
>   I do have doubts about why we need to do this on VMX though. The initial
>   justification for this comes from
> 
>   7f7f1ba33cf2 ("KVM: x86: do not load vmcs12 pages while still in SMM")
> 
>   With all the MMU changes, I am not sure that we can still have a case
>   of not up to date MMU when we enter the nested guest from SMM.
>   On SVM it does seem to work anyway without this.
> 
> I still track another SMM issue, which I debugged a bit today but still
> no lead on what is going on:
> 
> When HyperV guest is running nested, and uses SMM enabled OVMF, it crashes and
> reboots during the boot process.
> 
> Best regards,
> 	Maxim Levitsky
> 
> Maxim Levitsky (3):
>   KVM: nSVM: restore the L1 host state prior to resuming a nested guest
>     on SMM exit
>   KVM: x86: force PDPTRs reload on SMM exit
>   KVM: nSVM: call KVM_REQ_GET_NESTED_STATE_PAGES on exit from SMM mode
> 
>  arch/x86/kvm/svm/nested.c |  9 ++++++---
>  arch/x86/kvm/svm/svm.c    | 27 ++++++++++++++++++---------
>  arch/x86/kvm/svm/svm.h    |  3 ++-
>  arch/x86/kvm/vmx/vmx.c    |  7 +++++++
>  4 files changed, 33 insertions(+), 13 deletions(-)
> 
> -- 
> 2.26.3
> 
> 

This is not really v2, mistake on my part in git-publish.

Best regards,
	Maxim Levitsky