linux-kernel - Re: [PATCH] KVM: X86: Fix async pf caused null-ptr-deref

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Date:   Mon, 29 Jun 2020 08:42:12 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Vivek Goyal <vgoyal@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: X86: Fix async pf caused null-ptr-deref

On Mon, Jun 29, 2020 at 03:59:25PM +0200, Paolo Bonzini wrote:
> On 29/06/20 15:46, Vitaly Kuznetsov wrote:
> >> +	if (!lapic_in_kernel(vcpu))
> >> +		return 1;
> >> +
> > I'm not sure how much we care about !lapic_in_kernel() case but this
> > change should be accompanied with userspace changes to not expose
> > KVM_FEATURE_ASYNC_PF_INT or how would the guest know that writing a
> > legitimate value will result in #GP?
> 
> Almost any pv feature is broken with QEMU if kernel_irqchip=off.  I
> wouldn't bother and I am seriously thinking of dropping all support for
> that, including:

Heh, based on my limited testing, that could be "Almost everything is
broken with Qemu if kernel_irqchip=off".

> - just injecting #UD for MOV from/to CR8 unless lapic_in_kernel()
> 
> - make KVM_INTERRUPT fail unless irqchip_in_kernel(), so that
> KVM_INTERRUPT is only used to inject EXTINT with kernel_irqchip=split
> 
> Paolo
> 
> > Alternatively, we may just return '0' here: guest will be able to check
> > what's in the MSR to see if the feature was enabled. Normally, guests
> > shouldn't care about this but maybe there are cases when they do?
> > 
>