linux-kernel - Re: [PATCH 05/12] KVM: MMU: avoid NULL-pointer dereference on page freeing bugs

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Message-ID: <18042cd3-c1fe-a54a-8682-2b5d1523c3d9@redhat.com>
Date:   Fri, 11 Feb 2022 12:21:18 +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, dmatlack@...gle.com
Subject: Re: [PATCH 05/12] KVM: MMU: avoid NULL-pointer dereference on page
 freeing bugs

On 2/11/22 01:24, Sean Christopherson wrote:
>>   
>>   	sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
>> +	if (WARN_ON(!sp))
>
> Should this be KVM_BUG_ON()?  I.e. when you triggered these, would continuing on
> potentially corrupt guest data, or was it truly benign-ish?

It only triggered on the mode_switch SVM unit test (with npt=0); so, in 
a very small test which just hung after the bug.  The WARN however was 
the 10-minute difference between rmmod and reboot...

I didn't use KVM_BUG_ON because we're in a pretty deep call stack 
(kvm_mmu_new_pgd, itself called from nested vmentry) and all sort of 
stuff will happen before bailing out.  My mental model is to use 
KVM_BUG_ON in situations for which error propagation is possible and clean.

Paolo