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

Mirsad Todorovac
2024-08-13T20:33:22Z

On 7/29/24 15:31, Vitaly Kuznetsov wrote:
> Mirsad Todorovac <mtodorovac69@gmail.com> writes:
>
>> On 7/19/24 20:53, Sean Christopherson wrote:
>>> On Fri, Jul 19, 2024, Mirsad Todorovac wrote:
>>>> Hi, all!
>>>>
>>>> Here is another potential NULL pointer dereference in kvm subsystem of linux
>>>> stable vanilla 6.10, as GCC 12.3.0 complains.
>>>>
>>>> (Please don't throw stuff at me, I think this is the last one for today :-)
>>>>
>>>> arch/x86/include/asm/mshyperv.h
>>>> -------------------------------
>>>> 242 static inline struct hv_vp_assist_page *hv_get_vp_assist_page(unsigned int cpu)
>>>> 243 {
>>>> 244 if (!hv_vp_assist_page)
>>>> 245 return NULL;
>>>> 246
>>>> 247 return hv_vp_assist_page[cpu];
>>>> 248 }
>>>>
>>>> arch/x86/kvm/vmx/vmx_onhyperv.h
>>>> -------------------------------
>>>> 102 static inline void evmcs_load(u64 phys_addr)
>>>> 103 {
>>>> 104 struct hv_vp_assist_page *vp_ap =
>>>> 105 hv_get_vp_assist_page(smp_processor_id());
>>>> 106
>>>> 107 if (current_evmcs->hv_enlightenments_control.nested_flush_hypercall)
>>>> 108 vp_ap->nested_control.features.directhypercall = 1;
>>>> 109 vp_ap->current_nested_vmcs = phys_addr;
>>>> 110 vp_ap->enlighten_vmentry = 1;
>>>> 111 }
>>>>
>
> ...
>
>>
>> GCC 12.3.0 appears unaware of this fact that evmcs_load() cannot be called with hv_vp_assist_page() == NULL.
>>
>> This, for example, silences the warning and also hardens the code against the "impossible" situations:
>>
>> -------------------><------------------------------------------------------------------
>> diff --git a/arch/x86/kvm/vmx/vmx_onhyperv.h b/arch/x86/kvm/vmx/vmx_onhyperv.h
>> index eb48153bfd73..8b0e3ffa7fc1 100644
>> --- a/arch/x86/kvm/vmx/vmx_onhyperv.h
>> +++ b/arch/x86/kvm/vmx/vmx_onhyperv.h
>> @@ -104,6 +104,11 @@ static inline void evmcs_load(u64 phys_addr)
>> struct hv_vp_assist_page *vp_ap =
>> hv_get_vp_assist_page(smp_processor_id());
>>
>> + if (!vp_ap) {
>> + pr_warn("BUG: hy_get_vp_assist_page(%d) returned NULL.\n", smp_processor_id());
>> + return;
>> + }
>> +
>> if (current_evmcs->hv_enlightenments_control.nested_flush_hypercall)
>> vp_ap->nested_control.features.directhypercall = 1;
>> vp_ap->current_nested_vmcs = phys_addr;
>
> As Sean said, this does not seem to be possible today but I uderstand
> why the compiler is not able to infer this. If we were to fix this, I'd
> suggest we do something like "BUG_ON(!vp_ap)" (with a comment why)
> instead of the suggested patch:

That sounds awesome, but I really dare not poke into KVM stuff at my level. :-/

> - pr_warn() is not ratelimited

This is indeed a problem. I did not see that coming.

> - 'return' from evmcs_load does not propagate the error so the VM is
> going to misbehave somewhere else.

Agreed. But, frankly, I do not see where to jump or return to in case of such bug.

I would just feel safer with a sentinel or a return value check, just as in userland some
people expect malloc() to always succeed - but the diligent check return value of this and
all syscalls. ;-)

Best regards,
Mirsad Todorovac