linux-kernel - Re: [BUG] arch/x86/kvm/vmx/vmx_onhyperv.h:109:36: error: dereference of NULL ‘0’

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Message-ID: <87mslff728.fsf@redhat.com>
Date: Wed, 14 Aug 2024 17:38:39 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Mirsad Todorovac <mtodorovac69@...il.com>, kvm@...r.kernel.org, Paolo
 Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo
 Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin"
 <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [BUG] arch/x86/kvm/vmx/vmx_onhyperv.h:109:36: error:
 dereference of NULL ‘0’

Sean Christopherson <seanjc@...gle.com> writes:

> On Wed, Aug 14, 2024, Vitaly Kuznetsov wrote:
>> What I meant is something along these lines (untested):
>> 
>> diff --git a/arch/x86/kvm/vmx/vmx_onhyperv.h b/arch/x86/kvm/vmx/vmx_onhyperv.h
>> index eb48153bfd73..e2d8c67d0cad 100644
>> --- a/arch/x86/kvm/vmx/vmx_onhyperv.h
>> +++ b/arch/x86/kvm/vmx/vmx_onhyperv.h
>> @@ -104,6 +104,14 @@ static inline void evmcs_load(u64 phys_addr)
>>         struct hv_vp_assist_page *vp_ap =
>>                 hv_get_vp_assist_page(smp_processor_id());
>>  
>> +       /*
>> +        * When enabling eVMCS, KVM verifies that every CPU has a valid hv_vp_assist_page()
>> +        * and aborts enabling the feature otherwise. CPU onlining path is also checked in
>> +        * vmx_hardware_enable(). With this, it is impossible to reach here with vp_ap == NULL
>> +        * but compilers may still complain.
>> +        */
>> +       BUG_ON(!vp_ap);
>
> A full BUG_ON() is overkill, and easily avoided.  If we want to add a sanity
> check here and do more than just WARN, then it's easy enough to plumb in @vcpu
> and make this a KVM_BUG_ON() so that the VM dies, i.e. so that KVM doesn't risk
> corrupting the guest somehow.
>

I'm still acting under the impression this is an absolutely impossible
situation :-)

AFAICS, we only call evmcs_load() from vmcs_load() but this one doesn't
have @vcpu/@kvm either and I wasn't sure it's worth the effort to do the
plumbing (or am I missing an easy way to go back from @vmcs to
@vcpu?). On the other hand, vmcs_load() should not be called that ofter
so if we prefer to have @vcpu there for some other reason -- why not.

-- 
Vitaly