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

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Date:   Thu, 8 Jul 2021 19:24:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 0/3] KVM: SMM fixes

On 07/07/21 14:50, Maxim Levitsky wrote:
> Hi!
> 
> I did first round of testing of SMM by flooding the guest with SMIs,
> and running nested guests in it, and I found out that SMM
> breaks nested KVM due to a refactoring change
> that was done in 5.12 kernel. Fix for this is in patch 1.
> 
> I also fixed another issue I noticed in this patch which is purely
> theoretical but nevertheless should be fixed. This is patch 2.
> 
> I also propose to add (mostly for debug for now) a module param
> that can make the KVM to avoid intercepting #SMIs on SVM.
> (Intel doesn't have such intercept I think)
> The default is still to intercept #SMI so nothing is changed by
> default.
> 
> This allows to test the case in which SMI are not intercepted,
> by L1 without running Windows (which doesn't intercept #SMI).
> 
> In addition to that I found out that on bare metal, at least
> on two Zen2 machines I have, the CPU ignores SMI interception and
> never VM exits when SMI is received. As I guessed earlier
> this must have been done for security reasons.
> 
> Note that bug that I fixed in patch 1, should crash VMs very soon
> on bare metal as well, if the CPU were to honour the SMI intercept.
> as long as there are some SMIs generated while the system is running.
> 
> I tested this on bare metal by using local APIC to send SMIs
> to all real CPUs, and also used ioport 0xB2 to send SMIs.
> In both cases my system slowed to a crawl but didn't show
> any SMI vmexits (SMI intercept was enabled).
> 
> In a VM I also used ioport 0xB2 to generate a flood of SMIs,
> which allowed me to reproduce this bug (and with intercept_smi=0
> module parameter I can reproduce the bug that Vitaly fixed in
> his series as well while just running nested KVM).
> 
> Note that while doing nested migration I am still able to cause
> severe hangs of the L1 when I run the SMI stress test in L1
> and a nested VM. VM isn't fully hung but its GUI stops responding,
> and I see lots of cpu lockups errors in dmesg.
> This seems to happen regardless of #SMI interception in the L1
> (with Vitaly's patches applied of course)
> 
> Best regards,
> 	Maxim Levitsky
> 
> Maxim Levitsky (3):
>    KVM: SVM: #SMI interception must not skip the instruction
>    KVM: SVM: remove INIT intercept handler
>    KVM: SVM: add module param to control the #SMI interception
> 
>   arch/x86/kvm/svm/nested.c |  4 ++++
>   arch/x86/kvm/svm/svm.c    | 18 +++++++++++++++---
>   arch/x86/kvm/svm/svm.h    |  1 +
>   3 files changed, 20 insertions(+), 3 deletions(-)
> 

Queued, thanks.

Paolo