linux-kernel - Re: [PATCH 03/28] KVM: nSVM: inject exceptions via svm_check_nested

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Date:   Sat, 6 Mar 2021 10:26:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, mlevitsk@...hat.com,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 03/28] KVM: nSVM: inject exceptions via
 svm_check_nested_events

On 06/03/21 02:39, Sean Christopherson wrote:
> Unless KVM (L0) knowingly wants to override L1, e.g. KVM_GUESTDBG_* cases, KVM
> shouldn't do a damn thing except forward the exception to L1 if L1 wants the
> exception.
> 
> ud_interception() and gp_interception() do quite a bit before forwarding the
> exception, and in the case of #UD, it's entirely possible the #UD will never get
> forwarded to L1.  #GP is even more problematic because it's a contributory
> exception, and kvm_multiple_exception() is not equipped to check and handle
> nested intercepts before vectoring the exception, which means KVM will
> incorrectly escalate a #GP->#DF and #GP->#DF->Triple Fault instead of exiting
> to L1.  That's a wee bit problematic since KVM also has a soon-to-be-fixed bug
> where it kills L1 on a Triple Fault in L2...

I agree with the #GP problem, but this is on purpose.  For example, if 
L1 CPUID has MOVBE and it is being emulated via #UD, L1 would be right 
to set MOVBE in L2's CPUID and expect it not to cause a #UD.  The same 
is true for the VMware #GP interception case.

Maxim is also working on this, the root cause is that 
kvm_multiple_exception()'s escalation of contributory exceptions to #DF 
and triple fault is incorrect in the case of nested virtualization.

Paolo