Re: [PATCH 0/3] KVM: SMM fixes

[Paolo Bonzini]

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