linux-kernel - Re: [RFC] KVM: SVM: do not drop VMCB CPL to 0 if SS is not present

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Date:   Tue, 30 May 2017 18:05:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Roman Penyaev <roman.penyaev@...fitbricks.com>
Cc:     Mikhail Sennikovskii <mikhail.sennikovskii@...fitbricks.com>,
        Gleb Natapov <gleb@...nel.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>
Subject: Re: [RFC] KVM: SVM: do not drop VMCB CPL to 0 if SS is not present

On 30/05/2017 17:58, Roman Penyaev wrote:
> Indeed, what is left is eventually take it from SS.RPL. J.

Ahah! :)  But I only suggested that in specific cases.

> But jokes aside,  with your last patch you seems fixed a race problem
> when "CS.RPL is not equal to the CPL in the few instructions between
> setting CR0.PE and reloading CS".

Yes, exactly.  The symptom was a crash (triple fault) when you kept
interrupting with "info cpus" a guest that repeatedly went to protected
mode and back to real mode.

> We will have CPL in var->dpl, and it seems ok.  All we need is not
> to lose it on the way kernel->userspace->kernel.

You're right.  So what do you think of the other suggestion (svm.c
doesn't clear attributes for unusable registers, QEMU only clears P for
unusable registers)?

Thanks,

Paolo