linux-kernel - Re: [PATCH 3/5] x86: KVM: svm: clear interrupt shadow on all paths in skip_emulated

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Date:   Thu, 01 Aug 2019 16:43:05 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 3/5] x86: KVM: svm: clear interrupt shadow on all paths in skip_emulated_instruction()

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> On Thu, Aug 01, 2019 at 07:14:16AM +0200, Vitaly Kuznetsov wrote:
>> Regardless of the way how we skip instruction, interrupt shadow needs to be
>> cleared.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> Reviewed-by: Jim Mattson <jmattson@...gle.com>
>> ---
>>  arch/x86/kvm/svm.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 80f576e05112..7c7dff3f461f 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -784,13 +784,15 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
>>  				EMULATE_DONE)
>>  			pr_err_once("KVM: %s: unable to skip instruction\n",
>>  				    __func__);
>> -		return;
>> +		goto clear_int_shadow;
>
> A better fix would be to clear the interrupt shadow in x86_emulate_instruction()
> after updating RIP for EMULTYPE_SKIP.  VMX has this same flaw when running
> nested as handle_ept_misconfig() also expects the interrupt shadow to be
> handled by kvm_emulate_instruction().  Clearing the shadow if and only if
> the skipping is successful also means KVM isn't incorrectly zapping the
> shadow when emulation fails.

Oh, nice catch actually! Will do in v2.

-- 
Vitaly